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<div class="section" id="source-level-debugging-with-llvm">
<h1>Source Level Debugging with LLVM<a class="headerlink" href="#source-level-debugging-with-llvm" title="Permalink to this headline">¶</a></h1>
<div class="contents local topic" id="contents">
<ul class="simple">
<li><a class="reference internal" href="#introduction" id="id5">Introduction</a><ul>
<li><a class="reference internal" href="#philosophy-behind-llvm-debugging-information" id="id6">Philosophy behind LLVM debugging information</a></li>
<li><a class="reference internal" href="#debug-information-consumers" id="id7">Debug information consumers</a></li>
<li><a class="reference internal" href="#debugging-optimized-code" id="id8">Debugging optimized code</a></li>
</ul>
</li>
<li><a class="reference internal" href="#debugging-information-format" id="id9">Debugging information format</a><ul>
<li><a class="reference internal" href="#debug-information-descriptors" id="id10">Debug information descriptors</a><ul>
<li><a class="reference internal" href="#compile-unit-descriptors" id="id11">Compile unit descriptors</a></li>
<li><a class="reference internal" href="#file-descriptors" id="id12">File descriptors</a></li>
<li><a class="reference internal" href="#global-variable-descriptors" id="id13">Global variable descriptors</a></li>
<li><a class="reference internal" href="#subprogram-descriptors" id="id14">Subprogram descriptors</a></li>
<li><a class="reference internal" href="#block-descriptors" id="id15">Block descriptors</a></li>
<li><a class="reference internal" href="#basic-type-descriptors" id="id16">Basic type descriptors</a></li>
<li><a class="reference internal" href="#derived-type-descriptors" id="id17">Derived type descriptors</a></li>
<li><a class="reference internal" href="#composite-type-descriptors" id="id18">Composite type descriptors</a></li>
<li><a class="reference internal" href="#subrange-descriptors" id="id19">Subrange descriptors</a></li>
<li><a class="reference internal" href="#enumerator-descriptors" id="id20">Enumerator descriptors</a></li>
<li><a class="reference internal" href="#local-variables" id="id21">Local variables</a></li>
<li><a class="reference internal" href="#debugger-intrinsic-functions" id="id22">Debugger intrinsic functions</a></li>
<li><a class="reference internal" href="#llvm-dbg-declare" id="id23"><tt class="docutils literal"><span class="pre">llvm.dbg.declare</span></tt></a></li>
<li><a class="reference internal" href="#llvm-dbg-value" id="id24"><tt class="docutils literal"><span class="pre">llvm.dbg.value</span></tt></a></li>
</ul>
</li>
</ul>
</li>
<li><a class="reference internal" href="#object-lifetimes-and-scoping" id="id25">Object lifetimes and scoping</a></li>
<li><a class="reference internal" href="#c-c-front-end-specific-debug-information" id="id26">C/C++ front-end specific debug information</a><ul>
<li><a class="reference internal" href="#c-c-source-file-information" id="id27">C/C++ source file information</a></li>
<li><a class="reference internal" href="#c-c-global-variable-information" id="id28">C/C++ global variable information</a></li>
<li><a class="reference internal" href="#c-c-function-information" id="id29">C/C++ function information</a></li>
<li><a class="reference internal" href="#c-c-basic-types" id="id30">C/C++ basic types</a><ul>
<li><a class="reference internal" href="#bool" id="id31">bool</a></li>
<li><a class="reference internal" href="#char" id="id32">char</a></li>
<li><a class="reference internal" href="#unsigned-char" id="id33">unsigned char</a></li>
<li><a class="reference internal" href="#short" id="id34">short</a></li>
<li><a class="reference internal" href="#unsigned-short" id="id35">unsigned short</a></li>
<li><a class="reference internal" href="#int" id="id36">int</a></li>
<li><a class="reference internal" href="#unsigned-int" id="id37">unsigned int</a></li>
<li><a class="reference internal" href="#long-long" id="id38">long long</a></li>
<li><a class="reference internal" href="#unsigned-long-long" id="id39">unsigned long long</a></li>
<li><a class="reference internal" href="#float" id="id40">float</a></li>
<li><a class="reference internal" href="#double" id="id41">double</a></li>
</ul>
</li>
<li><a class="reference internal" href="#c-c-derived-types" id="id42">C/C++ derived types</a></li>
<li><a class="reference internal" href="#c-c-struct-union-types" id="id43">C/C++ struct/union types</a></li>
<li><a class="reference internal" href="#c-c-enumeration-types" id="id44">C/C++ enumeration types</a></li>
</ul>
</li>
<li><a class="reference internal" href="#id1" id="id45">Debugging information format</a><ul>
<li><a class="reference internal" href="#debugging-information-extension-for-objective-c-properties" id="id46">Debugging Information Extension for Objective C Properties</a><ul>
<li><a class="reference internal" href="#id2" id="id47">Introduction</a></li>
<li><a class="reference internal" href="#proposal" id="id48">Proposal</a></li>
<li><a class="reference internal" href="#new-dwarf-tags" id="id49">New DWARF Tags</a></li>
<li><a class="reference internal" href="#new-dwarf-attributes" id="id50">New DWARF Attributes</a></li>
<li><a class="reference internal" href="#new-dwarf-constants" id="id51">New DWARF Constants</a></li>
</ul>
</li>
<li><a class="reference internal" href="#name-accelerator-tables" id="id52">Name Accelerator Tables</a><ul>
<li><a class="reference internal" href="#id3" id="id53">Introduction</a></li>
<li><a class="reference internal" href="#hash-tables" id="id54">Hash Tables</a><ul>
<li><a class="reference internal" href="#standard-hash-tables" id="id55">Standard Hash Tables</a></li>
<li><a class="reference internal" href="#name-hash-tables" id="id56">Name Hash Tables</a></li>
</ul>
</li>
<li><a class="reference internal" href="#details" id="id57">Details</a><ul>
<li><a class="reference internal" href="#header-layout" id="id58">Header Layout</a></li>
<li><a class="reference internal" href="#fixed-lookup" id="id59">Fixed Lookup</a></li>
</ul>
</li>
<li><a class="reference internal" href="#id4" id="id60">Contents</a></li>
<li><a class="reference internal" href="#language-extensions-and-file-format-changes" id="id61">Language Extensions and File Format Changes</a><ul>
<li><a class="reference internal" href="#objective-c-extensions" id="id62">Objective-C Extensions</a></li>
<li><a class="reference internal" href="#mach-o-changes" id="id63">Mach-O Changes</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
</ul>
</div>
<div class="section" id="introduction">
<h2><a class="toc-backref" href="#id5">Introduction</a><a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
<p>This document is the central repository for all information pertaining to debug
information in LLVM. It describes the <a class="reference internal" href="#format"><em>actual format that the LLVM debug
information takes</em></a>, which is useful for those interested in creating
front-ends or dealing directly with the information. Further, this document
provides specific examples of what debug information for C/C++ looks like.</p>
<div class="section" id="philosophy-behind-llvm-debugging-information">
<h3><a class="toc-backref" href="#id6">Philosophy behind LLVM debugging information</a><a class="headerlink" href="#philosophy-behind-llvm-debugging-information" title="Permalink to this headline">¶</a></h3>
<p>The idea of the LLVM debugging information is to capture how the important
pieces of the source-language’s Abstract Syntax Tree map onto LLVM code.
Several design aspects have shaped the solution that appears here. The
important ones are:</p>
<ul class="simple">
<li>Debugging information should have very little impact on the rest of the
compiler. No transformations, analyses, or code generators should need to
be modified because of debugging information.</li>
<li>LLVM optimizations should interact in <a class="reference internal" href="#intro-debugopt"><em>well-defined and easily described
ways</em></a> with the debugging information.</li>
<li>Because LLVM is designed to support arbitrary programming languages,
LLVM-to-LLVM tools should not need to know anything about the semantics of
the source-level-language.</li>
<li>Source-level languages are often <strong>widely</strong> different from one another.
LLVM should not put any restrictions of the flavor of the source-language,
and the debugging information should work with any language.</li>
<li>With code generator support, it should be possible to use an LLVM compiler
to compile a program to native machine code and standard debugging
formats. This allows compatibility with traditional machine-code level
debuggers, like GDB or DBX.</li>
</ul>
<p>The approach used by the LLVM implementation is to use a small set of
<a class="reference internal" href="#format-common-intrinsics"><em>intrinsic functions</em></a> to define a mapping
between LLVM program objects and the source-level objects. The description of
the source-level program is maintained in LLVM metadata in an
<a class="reference internal" href="#ccxx-frontend"><em>implementation-defined format</em></a> (the C/C++ front-end
currently uses working draft 7 of the <a class="reference external" href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3 standard</a>).</p>
<p>When a program is being debugged, a debugger interacts with the user and turns
the stored debug information into source-language specific information. As
such, a debugger must be aware of the source-language, and is thus tied to a
specific language or family of languages.</p>
</div>
<div class="section" id="debug-information-consumers">
<h3><a class="toc-backref" href="#id7">Debug information consumers</a><a class="headerlink" href="#debug-information-consumers" title="Permalink to this headline">¶</a></h3>
<p>The role of debug information is to provide meta information normally stripped
away during the compilation process. This meta information provides an LLVM
user a relationship between generated code and the original program source
code.</p>
<p>Currently, debug information is consumed by DwarfDebug to produce dwarf
information used by the gdb debugger. Other targets could use the same
information to produce stabs or other debug forms.</p>
<p>It would also be reasonable to use debug information to feed profiling tools
for analysis of generated code, or, tools for reconstructing the original
source from generated code.</p>
<p>TODO - expound a bit more.</p>
</div>
<div class="section" id="debugging-optimized-code">
<span id="intro-debugopt"></span><h3><a class="toc-backref" href="#id8">Debugging optimized code</a><a class="headerlink" href="#debugging-optimized-code" title="Permalink to this headline">¶</a></h3>
<p>An extremely high priority of LLVM debugging information is to make it interact
well with optimizations and analysis. In particular, the LLVM debug
information provides the following guarantees:</p>
<ul class="simple">
<li>LLVM debug information <strong>always provides information to accurately read
the source-level state of the program</strong>, regardless of which LLVM
optimizations have been run, and without any modification to the
optimizations themselves. However, some optimizations may impact the
ability to modify the current state of the program with a debugger, such
as setting program variables, or calling functions that have been
deleted.</li>
<li>As desired, LLVM optimizations can be upgraded to be aware of the LLVM
debugging information, allowing them to update the debugging information
as they perform aggressive optimizations. This means that, with effort,
the LLVM optimizers could optimize debug code just as well as non-debug
code.</li>
<li>LLVM debug information does not prevent optimizations from
happening (for example inlining, basic block reordering/merging/cleanup,
tail duplication, etc).</li>
<li>LLVM debug information is automatically optimized along with the rest of
the program, using existing facilities. For example, duplicate
information is automatically merged by the linker, and unused information
is automatically removed.</li>
</ul>
<p>Basically, the debug information allows you to compile a program with
“<tt class="docutils literal"><span class="pre">-O0</span> <span class="pre">-g</span></tt>” and get full debug information, allowing you to arbitrarily modify
the program as it executes from a debugger. Compiling a program with
“<tt class="docutils literal"><span class="pre">-O3</span> <span class="pre">-g</span></tt>” gives you full debug information that is always available and
accurate for reading (e.g., you get accurate stack traces despite tail call
elimination and inlining), but you might lose the ability to modify the program
and call functions where were optimized out of the program, or inlined away
completely.</p>
<p><a class="reference internal" href="TestingGuide.html#test-suite-quickstart"><em>LLVM test suite</em></a> provides a framework to test
optimizer’s handling of debugging information. It can be run like this:</p>
<div class="highlight-bash"><div class="highlight"><pre>% <span class="nb">cd </span>llvm/projects/test-suite/MultiSource/Benchmarks <span class="c"># or some other level</span>
% make <span class="nv">TEST</span><span class="o">=</span>dbgopt
</pre></div>
</div>
<p>This will test impact of debugging information on optimization passes. If
debugging information influences optimization passes then it will be reported
as a failure. See <a class="reference internal" href="TestingGuide.html"><em>LLVM Testing Infrastructure Guide</em></a> for more information on LLVM test
infrastructure and how to run various tests.</p>
</div>
</div>
<div class="section" id="debugging-information-format">
<span id="format"></span><h2><a class="toc-backref" href="#id9">Debugging information format</a><a class="headerlink" href="#debugging-information-format" title="Permalink to this headline">¶</a></h2>
<p>LLVM debugging information has been carefully designed to make it possible for
the optimizer to optimize the program and debugging information without
necessarily having to know anything about debugging information. In
particular, the use of metadata avoids duplicated debugging information from
the beginning, and the global dead code elimination pass automatically deletes
debugging information for a function if it decides to delete the function.</p>
<p>To do this, most of the debugging information (descriptors for types,
variables, functions, source files, etc) is inserted by the language front-end
in the form of LLVM metadata.</p>
<p>Debug information is designed to be agnostic about the target debugger and
debugging information representation (e.g. DWARF/Stabs/etc). It uses a generic
pass to decode the information that represents variables, types, functions,
namespaces, etc: this allows for arbitrary source-language semantics and
type-systems to be used, as long as there is a module written for the target
debugger to interpret the information.</p>
<p>To provide basic functionality, the LLVM debugger does have to make some
assumptions about the source-level language being debugged, though it keeps
these to a minimum. The only common features that the LLVM debugger assumes
exist are <a class="reference internal" href="#format-files"><em>source files</em></a>, and <a class="reference internal" href="#format-global-variables"><em>program objects</em></a>. These abstract objects are used by a debugger to
form stack traces, show information about local variables, etc.</p>
<p>This section of the documentation first describes the representation aspects
common to any source-language. <a class="reference internal" href="#ccxx-frontend"><em>C/C++ front-end specific debug information</em></a> describes the data layout
conventions used by the C and C++ front-ends.</p>
<div class="section" id="debug-information-descriptors">
<h3><a class="toc-backref" href="#id10">Debug information descriptors</a><a class="headerlink" href="#debug-information-descriptors" title="Permalink to this headline">¶</a></h3>
<p>In consideration of the complexity and volume of debug information, LLVM
provides a specification for well formed debug descriptors.</p>
<p>Consumers of LLVM debug information expect the descriptors for program objects
to start in a canonical format, but the descriptors can include additional
information appended at the end that is source-language specific. All LLVM
debugging information is versioned, allowing backwards compatibility in the
case that the core structures need to change in some way. Also, all debugging
information objects start with a tag to indicate what type of object it is.
The source-language is allowed to define its own objects, by using unreserved
tag numbers. We recommend using with tags in the range 0x1000 through 0x2000
(there is a defined <tt class="docutils literal"><span class="pre">enum</span> <span class="pre">DW_TAG_user_base</span> <span class="pre">=</span> <span class="pre">0x1000</span></tt>.)</p>
<p>The fields of debug descriptors used internally by LLVM are restricted to only
the simple data types <tt class="docutils literal"><span class="pre">i32</span></tt>, <tt class="docutils literal"><span class="pre">i1</span></tt>, <tt class="docutils literal"><span class="pre">float</span></tt>, <tt class="docutils literal"><span class="pre">double</span></tt>, <tt class="docutils literal"><span class="pre">mdstring</span></tt> and
<tt class="docutils literal"><span class="pre">mdnode</span></tt>.</p>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!1</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; A tag</span>
<span class="p">...</span>
<span class="p">}</span>
</pre></div>
</div>
<p><a name=”LLVMDebugVersion”>The first field of a descriptor is always an
<tt class="docutils literal"><span class="pre">i32</span></tt> containing a tag value identifying the content of the descriptor.
The remaining fields are specific to the descriptor. The values of tags are
loosely bound to the tag values of DWARF information entries. However, that
does not restrict the use of the information supplied to DWARF targets. To
facilitate versioning of debug information, the tag is augmented with the
current debug version (<tt class="docutils literal"><span class="pre">LLVMDebugVersion</span> <span class="pre">=</span> <span class="pre">8</span> <span class="pre"><<</span> <span class="pre">16</span></tt> or 0x80000 or
524288.)</p>
<p>The details of the various descriptors follow.</p>
<div class="section" id="compile-unit-descriptors">
<h4><a class="toc-backref" href="#id11">Compile unit descriptors</a><a class="headerlink" href="#compile-unit-descriptors" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!0</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Tag = 17 + LLVMDebugVersion (DW_TAG_compile_unit)</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Unused field.</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; DWARF language identifier (ex. DW_LANG_C89)</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Source file name</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Source file directory (includes trailing slash)</span>
<span class="kt">metadata</span> <span class="c">;; Producer (ex. "4.0.1 LLVM (LLVM research group)")</span>
<span class="k">i1</span><span class="p">,</span> <span class="c">;; True if this is a main compile unit.</span>
<span class="k">i1</span><span class="p">,</span> <span class="c">;; True if this is optimized.</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="k">i32</span> <span class="c">;; Runtime version</span>
<span class="kt">metadata</span> <span class="c">;; List of enums types</span>
<span class="kt">metadata</span> <span class="c">;; List of retained types</span>
<span class="kt">metadata</span> <span class="c">;; List of subprograms</span>
<span class="kt">metadata</span> <span class="c">;; List of global variables</span>
<span class="p">}</span>
</pre></div>
</div>
<p>These descriptors contain a source language ID for the file (we use the DWARF
3.0 ID numbers, such as <tt class="docutils literal"><span class="pre">DW_LANG_C89</span></tt>, <tt class="docutils literal"><span class="pre">DW_LANG_C_plus_plus</span></tt>,
<tt class="docutils literal"><span class="pre">DW_LANG_Cobol74</span></tt>, etc), three strings describing the filename, working
directory of the compiler, and an identifier string for the compiler that
produced it.</p>
<p>Compile unit descriptors provide the root context for objects declared in a
specific compilation unit. File descriptors are defined using this context.
These descriptors are collected by a named metadata <tt class="docutils literal"><span class="pre">!llvm.dbg.cu</span></tt>. They
keep track of subprograms, global variables and type information.</p>
</div>
<div class="section" id="file-descriptors">
<span id="format-files"></span><h4><a class="toc-backref" href="#id12">File descriptors</a><a class="headerlink" href="#file-descriptors" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!0</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Tag = 41 + LLVMDebugVersion (DW_TAG_file_type)</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Source file name</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Source file directory (includes trailing slash)</span>
<span class="kt">metadata</span> <span class="c">;; Unused</span>
<span class="p">}</span>
</pre></div>
</div>
<p>These descriptors contain information for a file. Global variables and top
level functions would be defined using this context. File descriptors also
provide context for source line correspondence.</p>
<p>Each input file is encoded as a separate file descriptor in LLVM debugging
information output.</p>
</div>
<div class="section" id="global-variable-descriptors">
<span id="format-global-variables"></span><h4><a class="toc-backref" href="#id13">Global variable descriptors</a><a class="headerlink" href="#global-variable-descriptors" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!1</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Tag = 52 + LLVMDebugVersion (DW_TAG_variable)</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Unused field.</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Reference to context descriptor</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Name</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Display name (fully qualified C++ name)</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; MIPS linkage name (for C++)</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Reference to file where defined</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Line number where defined</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Reference to type descriptor</span>
<span class="k">i1</span><span class="p">,</span> <span class="c">;; True if the global is local to compile unit (static)</span>
<span class="k">i1</span><span class="p">,</span> <span class="c">;; True if the global is defined in the compile unit (not extern)</span>
<span class="p">{}*</span> <span class="c">;; Reference to the global variable</span>
<span class="p">}</span>
</pre></div>
</div>
<p>These descriptors provide debug information about globals variables. They
provide details such as name, type and where the variable is defined. All
global variables are collected inside the named metadata <tt class="docutils literal"><span class="pre">!llvm.dbg.cu</span></tt>.</p>
</div>
<div class="section" id="subprogram-descriptors">
<span id="format-subprograms"></span><h4><a class="toc-backref" href="#id14">Subprogram descriptors</a><a class="headerlink" href="#subprogram-descriptors" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><pre>!2 = metadata !{
i32, ;; Tag = 46 + LLVMDebugVersion (DW_TAG_subprogram)
i32, ;; Unused field.
metadata, ;; Reference to context descriptor
metadata, ;; Name
metadata, ;; Display name (fully qualified C++ name)
metadata, ;; MIPS linkage name (for C++)
metadata, ;; Reference to file where defined
i32, ;; Line number where defined
metadata, ;; Reference to type descriptor
i1, ;; True if the global is local to compile unit (static)
i1, ;; True if the global is defined in the compile unit (not extern)
i32, ;; Virtuality, e.g. dwarf::DW_VIRTUALITY__virtual
i32, ;; Index into a virtual function
metadata, ;; indicates which base type contains the vtable pointer for the
;; derived class
i32, ;; Flags - Artifical, Private, Protected, Explicit, Prototyped.
i1, ;; isOptimized
Function * , ;; Pointer to LLVM function
metadata, ;; Lists function template parameters
metadata, ;; Function declaration descriptor
metadata, ;; List of function variables
i32 ;; Line number where the scope of the subprogram begins
}</pre>
</div>
<p>These descriptors provide debug information about functions, methods and
subprograms. They provide details such as name, return types and the source
location where the subprogram is defined.</p>
</div>
<div class="section" id="block-descriptors">
<h4><a class="toc-backref" href="#id15">Block descriptors</a><a class="headerlink" href="#block-descriptors" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!3</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Tag = 11 + LLVMDebugVersion (DW_TAG_lexical_block)</span>
<span class="kt">metadata</span><span class="p">,</span><span class="c">;; Reference to context descriptor</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Column number</span>
<span class="kt">metadata</span><span class="p">,</span><span class="c">;; Reference to source file</span>
<span class="k">i32</span> <span class="c">;; Unique ID to identify blocks from a template function</span>
<span class="p">}</span>
</pre></div>
</div>
<p>This descriptor provides debug information about nested blocks within a
subprogram. The line number and column numbers are used to dinstinguish two
lexical blocks at same depth.</p>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!3</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Tag = 11 + LLVMDebugVersion (DW_TAG_lexical_block)</span>
<span class="kt">metadata</span> <span class="c">;; Reference to the scope we're annotating with a file change</span>
<span class="kt">metadata</span><span class="p">,</span><span class="c">;; Reference to the file the scope is enclosed in.</span>
<span class="p">}</span>
</pre></div>
</div>
<p>This descriptor provides a wrapper around a lexical scope to handle file
changes in the middle of a lexical block.</p>
</div>
<div class="section" id="basic-type-descriptors">
<span id="format-basic-type"></span><h4><a class="toc-backref" href="#id16">Basic type descriptors</a><a class="headerlink" href="#basic-type-descriptors" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!4</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Tag = 36 + LLVMDebugVersion (DW_TAG_base_type)</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Reference to context</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Name (may be "" for anonymous types)</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Reference to file where defined (may be NULL)</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Line number where defined (may be 0)</span>
<span class="k">i64</span><span class="p">,</span> <span class="c">;; Size in bits</span>
<span class="k">i64</span><span class="p">,</span> <span class="c">;; Alignment in bits</span>
<span class="k">i64</span><span class="p">,</span> <span class="c">;; Offset in bits</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="k">i32</span> <span class="c">;; DWARF type encoding</span>
<span class="p">}</span>
</pre></div>
</div>
<p>These descriptors define primitive types used in the code. Example <tt class="docutils literal"><span class="pre">int</span></tt>,
<tt class="docutils literal"><span class="pre">bool</span></tt> and <tt class="docutils literal"><span class="pre">float</span></tt>. The context provides the scope of the type, which is
usually the top level. Since basic types are not usually user defined the
context and line number can be left as NULL and 0. The size, alignment and
offset are expressed in bits and can be 64 bit values. The alignment is used
to round the offset when embedded in a <a class="reference internal" href="#format-composite-type"><em>composite type</em></a> (example to keep float doubles on 64 bit boundaries).
The offset is the bit offset if embedded in a <a class="reference internal" href="#format-composite-type"><em>composite type</em></a>.</p>
<p>The type encoding provides the details of the type. The values are typically
one of the following:</p>
<div class="highlight-llvm"><pre>DW_ATE_address = 1
DW_ATE_boolean = 2
DW_ATE_float = 4
DW_ATE_signed = 5
DW_ATE_signed_char = 6
DW_ATE_unsigned = 7
DW_ATE_unsigned_char = 8</pre>
</div>
</div>
<div class="section" id="derived-type-descriptors">
<span id="format-derived-type"></span><h4><a class="toc-backref" href="#id17">Derived type descriptors</a><a class="headerlink" href="#derived-type-descriptors" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!5</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Tag (see below)</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Reference to context</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Name (may be "" for anonymous types)</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Reference to file where defined (may be NULL)</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Line number where defined (may be 0)</span>
<span class="k">i64</span><span class="p">,</span> <span class="c">;; Size in bits</span>
<span class="k">i64</span><span class="p">,</span> <span class="c">;; Alignment in bits</span>
<span class="k">i64</span><span class="p">,</span> <span class="c">;; Offset in bits</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Flags to encode attributes, e.g. private</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Reference to type derived from</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; (optional) Name of the Objective C property associated with</span>
<span class="c">;; Objective-C an ivar, or the type of which this</span>
<span class="c">;; pointer-to-member is pointing to members of.</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; (optional) Name of the Objective C property getter selector.</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; (optional) Name of the Objective C property setter selector.</span>
<span class="k">i32</span> <span class="c">;; (optional) Objective C property attributes.</span>
<span class="p">}</span>
</pre></div>
</div>
<p>These descriptors are used to define types derived from other types. The value
of the tag varies depending on the meaning. The following are possible tag
values:</p>
<div class="highlight-llvm"><pre>DW_TAG_formal_parameter = 5
DW_TAG_member = 13
DW_TAG_pointer_type = 15
DW_TAG_reference_type = 16
DW_TAG_typedef = 22
DW_TAG_ptr_to_member_type = 31
DW_TAG_const_type = 38
DW_TAG_volatile_type = 53
DW_TAG_restrict_type = 55</pre>
</div>
<p><tt class="docutils literal"><span class="pre">DW_TAG_member</span></tt> is used to define a member of a <a class="reference internal" href="#format-composite-type"><em>composite type</em></a> or <a class="reference internal" href="#format-subprograms"><em>subprogram</em></a>. The type
of the member is the <a class="reference internal" href="#format-derived-type"><em>derived type</em></a>.
<tt class="docutils literal"><span class="pre">DW_TAG_formal_parameter</span></tt> is used to define a member which is a formal
argument of a subprogram.</p>
<p><tt class="docutils literal"><span class="pre">DW_TAG_typedef</span></tt> is used to provide a name for the derived type.</p>
<p><tt class="docutils literal"><span class="pre">DW_TAG_pointer_type</span></tt>, <tt class="docutils literal"><span class="pre">DW_TAG_reference_type</span></tt>, <tt class="docutils literal"><span class="pre">DW_TAG_const_type</span></tt>,
<tt class="docutils literal"><span class="pre">DW_TAG_volatile_type</span></tt> and <tt class="docutils literal"><span class="pre">DW_TAG_restrict_type</span></tt> are used to qualify the
<a class="reference internal" href="#format-derived-type"><em>derived type</em></a>.</p>
<p><a class="reference internal" href="#format-derived-type"><em>Derived type</em></a> location can be determined from the
context and line number. The size, alignment and offset are expressed in bits
and can be 64 bit values. The alignment is used to round the offset when
embedded in a <a class="reference internal" href="#format-composite-type"><em>composite type</em></a> (example to keep
float doubles on 64 bit boundaries.) The offset is the bit offset if embedded
in a <a class="reference internal" href="#format-composite-type"><em>composite type</em></a>.</p>
<p>Note that the <tt class="docutils literal"><span class="pre">void</span> <span class="pre">*</span></tt> type is expressed as a type derived from NULL.</p>
</div>
<div class="section" id="composite-type-descriptors">
<span id="format-composite-type"></span><h4><a class="toc-backref" href="#id18">Composite type descriptors</a><a class="headerlink" href="#composite-type-descriptors" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!6</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Tag (see below)</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Reference to context</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Name (may be "" for anonymous types)</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Reference to file where defined (may be NULL)</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Line number where defined (may be 0)</span>
<span class="k">i64</span><span class="p">,</span> <span class="c">;; Size in bits</span>
<span class="k">i64</span><span class="p">,</span> <span class="c">;; Alignment in bits</span>
<span class="k">i64</span><span class="p">,</span> <span class="c">;; Offset in bits</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Reference to type derived from</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Reference to array of member descriptors</span>
<span class="k">i32</span> <span class="c">;; Runtime languages</span>
<span class="p">}</span>
</pre></div>
</div>
<p>These descriptors are used to define types that are composed of 0 or more
elements. The value of the tag varies depending on the meaning. The following
are possible tag values:</p>
<div class="highlight-llvm"><pre>DW_TAG_array_type = 1
DW_TAG_enumeration_type = 4
DW_TAG_structure_type = 19
DW_TAG_union_type = 23
DW_TAG_subroutine_type = 21
DW_TAG_inheritance = 28</pre>
</div>
<p>The vector flag indicates that an array type is a native packed vector.</p>
<p>The members of array types (tag = <tt class="docutils literal"><span class="pre">DW_TAG_array_type</span></tt>) are
<a class="reference internal" href="#format-subrange"><em>subrange descriptors</em></a>, each
representing the range of subscripts at that level of indexing.</p>
<p>The members of enumeration types (tag = <tt class="docutils literal"><span class="pre">DW_TAG_enumeration_type</span></tt>) are
<a class="reference internal" href="#format-enumerator"><em>enumerator descriptors</em></a>, each representing the
definition of enumeration value for the set. All enumeration type descriptors
are collected inside the named metadata <tt class="docutils literal"><span class="pre">!llvm.dbg.cu</span></tt>.</p>
<p>The members of structure (tag = <tt class="docutils literal"><span class="pre">DW_TAG_structure_type</span></tt>) or union (tag =
<tt class="docutils literal"><span class="pre">DW_TAG_union_type</span></tt>) types are any one of the <a class="reference internal" href="#format-basic-type"><em>basic</em></a>, <a class="reference internal" href="#format-derived-type"><em>derived</em></a> or <a class="reference internal" href="#format-composite-type"><em>composite</em></a> type descriptors, each representing a field member of
the structure or union.</p>
<p>For C++ classes (tag = <tt class="docutils literal"><span class="pre">DW_TAG_structure_type</span></tt>), member descriptors provide
information about base classes, static members and member functions. If a
member is a <a class="reference internal" href="#format-derived-type"><em>derived type descriptor</em></a> and has a tag
of <tt class="docutils literal"><span class="pre">DW_TAG_inheritance</span></tt>, then the type represents a base class. If the member
of is a <a class="reference internal" href="#format-global-variables"><em>global variable descriptor</em></a> then it
represents a static member. And, if the member is a <a class="reference internal" href="#format-subprograms"><em>subprogram
descriptor</em></a> then it represents a member function. For
static members and member functions, <tt class="docutils literal"><span class="pre">getName()</span></tt> returns the members link or
the C++ mangled name. <tt class="docutils literal"><span class="pre">getDisplayName()</span></tt> the simplied version of the name.</p>
<p>The first member of subroutine (tag = <tt class="docutils literal"><span class="pre">DW_TAG_subroutine_type</span></tt>) type elements
is the return type for the subroutine. The remaining elements are the formal
arguments to the subroutine.</p>
<p><a class="reference internal" href="#format-composite-type"><em>Composite type</em></a> location can be determined from
the context and line number. The size, alignment and offset are expressed in
bits and can be 64 bit values. The alignment is used to round the offset when
embedded in a <a class="reference internal" href="#format-composite-type"><em>composite type</em></a> (as an example, to
keep float doubles on 64 bit boundaries). The offset is the bit offset if
embedded in a <a class="reference internal" href="#format-composite-type"><em>composite type</em></a>.</p>
</div>
<div class="section" id="subrange-descriptors">
<span id="format-subrange"></span><h4><a class="toc-backref" href="#id19">Subrange descriptors</a><a class="headerlink" href="#subrange-descriptors" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!42</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Tag = 33 + LLVMDebugVersion (DW_TAG_subrange_type)</span>
<span class="k">i64</span><span class="p">,</span> <span class="c">;; Low value</span>
<span class="k">i64</span> <span class="c">;; High value</span>
<span class="p">}</span>
</pre></div>
</div>
<p>These descriptors are used to define ranges of array subscripts for an array
<a class="reference internal" href="#format-composite-type"><em>composite type</em></a>. The low value defines the lower
bounds typically zero for C/C++. The high value is the upper bounds. Values
are 64 bit. <tt class="docutils literal"><span class="pre">High</span> <span class="pre">-</span> <span class="pre">Low</span> <span class="pre">+</span> <span class="pre">1</span></tt> is the size of the array. If <tt class="docutils literal"><span class="pre">Low</span> <span class="pre">></span> <span class="pre">High</span></tt>
the array bounds are not included in generated debugging information.</p>
</div>
<div class="section" id="enumerator-descriptors">
<span id="format-enumerator"></span><h4><a class="toc-backref" href="#id20">Enumerator descriptors</a><a class="headerlink" href="#enumerator-descriptors" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!6</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Tag = 40 + LLVMDebugVersion (DW_TAG_enumerator)</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Name</span>
<span class="k">i64</span> <span class="c">;; Value</span>
<span class="p">}</span>
</pre></div>
</div>
<p>These descriptors are used to define members of an enumeration <a class="reference internal" href="#format-composite-type"><em>composite
type</em></a>, it associates the name to the value.</p>
</div>
<div class="section" id="local-variables">
<h4><a class="toc-backref" href="#id21">Local variables</a><a class="headerlink" href="#local-variables" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!7</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; Tag (see below)</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Name</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Reference to file where defined</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; 24 bit - Line number where defined</span>
<span class="c">;; 8 bit - Argument number. 1 indicates 1st argument.</span>
<span class="kt">metadata</span><span class="p">,</span> <span class="c">;; Type descriptor</span>
<span class="k">i32</span><span class="p">,</span> <span class="c">;; flags</span>
<span class="kt">metadata</span> <span class="c">;; (optional) Reference to inline location</span>
<span class="p">}</span>
</pre></div>
</div>
<p>These descriptors are used to define variables local to a sub program. The
value of the tag depends on the usage of the variable:</p>
<div class="highlight-llvm"><pre>DW_TAG_auto_variable = 256
DW_TAG_arg_variable = 257</pre>
</div>
<p>An auto variable is any variable declared in the body of the function. An
argument variable is any variable that appears as a formal argument to the
function.</p>
<p>The context is either the subprogram or block where the variable is defined.
Name the source variable name. Context and line indicate where the variable
was defined. Type descriptor defines the declared type of the variable.</p>
</div>
<div class="section" id="debugger-intrinsic-functions">
<span id="format-common-intrinsics"></span><h4><a class="toc-backref" href="#id22">Debugger intrinsic functions</a><a class="headerlink" href="#debugger-intrinsic-functions" title="Permalink to this headline">¶</a></h4>
<p>LLVM uses several intrinsic functions (name prefixed with “<tt class="docutils literal"><span class="pre">llvm.dbg</span></tt>”) to
provide debug information at various points in generated code.</p>
</div>
<div class="section" id="llvm-dbg-declare">
<h4><a class="toc-backref" href="#id23"><tt class="docutils literal"><span class="pre">llvm.dbg.declare</span></tt></a><a class="headerlink" href="#llvm-dbg-declare" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="kt">void</span> <span class="nv">%llvm.dbg.declare</span><span class="p">(</span><span class="kt">metadata</span><span class="p">,</span> <span class="kt">metadata</span><span class="p">)</span>
</pre></div>
</div>
<p>This intrinsic provides information about a local element (e.g., variable).
The first argument is metadata holding the alloca for the variable. The second
argument is metadata containing a description of the variable.</p>
</div>
<div class="section" id="llvm-dbg-value">
<h4><a class="toc-backref" href="#id24"><tt class="docutils literal"><span class="pre">llvm.dbg.value</span></tt></a><a class="headerlink" href="#llvm-dbg-value" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="kt">void</span> <span class="nv">%llvm.dbg.value</span><span class="p">(</span><span class="kt">metadata</span><span class="p">,</span> <span class="k">i64</span><span class="p">,</span> <span class="kt">metadata</span><span class="p">)</span>
</pre></div>
</div>
<p>This intrinsic provides information when a user source variable is set to a new
value. The first argument is the new value (wrapped as metadata). The second
argument is the offset in the user source variable where the new value is
written. The third argument is metadata containing a description of the user
source variable.</p>
</div>
</div>
</div>
<div class="section" id="object-lifetimes-and-scoping">
<h2><a class="toc-backref" href="#id25">Object lifetimes and scoping</a><a class="headerlink" href="#object-lifetimes-and-scoping" title="Permalink to this headline">¶</a></h2>
<p>In many languages, the local variables in functions can have their lifetimes or
scopes limited to a subset of a function. In the C family of languages, for
example, variables are only live (readable and writable) within the source
block that they are defined in. In functional languages, values are only
readable after they have been defined. Though this is a very obvious concept,
it is non-trivial to model in LLVM, because it has no notion of scoping in this
sense, and does not want to be tied to a language’s scoping rules.</p>
<p>In order to handle this, the LLVM debug format uses the metadata attached to
llvm instructions to encode line number and scoping information. Consider the
following C fragment, for example:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="mf">1.</span> <span class="kt">void</span> <span class="n">foo</span><span class="p">()</span> <span class="p">{</span>
<span class="mf">2.</span> <span class="kt">int</span> <span class="n">X</span> <span class="o">=</span> <span class="mi">21</span><span class="p">;</span>
<span class="mf">3.</span> <span class="kt">int</span> <span class="n">Y</span> <span class="o">=</span> <span class="mi">22</span><span class="p">;</span>
<span class="mf">4.</span> <span class="p">{</span>
<span class="mf">5.</span> <span class="kt">int</span> <span class="n">Z</span> <span class="o">=</span> <span class="mi">23</span><span class="p">;</span>
<span class="mf">6.</span> <span class="n">Z</span> <span class="o">=</span> <span class="n">X</span><span class="p">;</span>
<span class="mf">7.</span> <span class="p">}</span>
<span class="mf">8.</span> <span class="n">X</span> <span class="o">=</span> <span class="n">Y</span><span class="p">;</span>
<span class="mf">9.</span> <span class="p">}</span>
</pre></div>
</div>
<p>Compiled to LLVM, this function would be represented like this:</p>
<div class="highlight-llvm"><div class="highlight"><pre><span class="k">define</span> <span class="kt">void</span> <span class="vg">@foo</span><span class="p">()</span> <span class="k">nounwind</span> <span class="k">ssp</span> <span class="p">{</span>
<span class="nl">entry:</span>
<span class="nv">%X</span> <span class="p">=</span> <span class="k">alloca</span> <span class="k">i32</span><span class="p">,</span> <span class="k">align</span> <span class="m">4</span> <span class="c">; <i32*> [#uses=4]</span>
<span class="nv">%Y</span> <span class="p">=</span> <span class="k">alloca</span> <span class="k">i32</span><span class="p">,</span> <span class="k">align</span> <span class="m">4</span> <span class="c">; <i32*> [#uses=4]</span>
<span class="nv">%Z</span> <span class="p">=</span> <span class="k">alloca</span> <span class="k">i32</span><span class="p">,</span> <span class="k">align</span> <span class="m">4</span> <span class="c">; <i32*> [#uses=3]</span>
<span class="nv-Anonymous">%0</span> <span class="p">=</span> <span class="k">bitcast</span> <span class="k">i32</span><span class="p">*</span> <span class="nv">%X</span> <span class="k">to</span> <span class="p">{}*</span> <span class="c">; <{}*> [#uses=1]</span>
<span class="k">call</span> <span class="kt">void</span> <span class="vg">@llvm.dbg.declare</span><span class="p">(</span><span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="p">*</span> <span class="nv">%X</span><span class="p">},</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!0</span><span class="p">),</span> <span class="nv">!dbg</span> <span class="nv-Anonymous">!7</span>
<span class="k">store</span> <span class="k">i32</span> <span class="m">21</span><span class="p">,</span> <span class="k">i32</span><span class="p">*</span> <span class="nv">%X</span><span class="p">,</span> <span class="nv">!dbg</span> <span class="nv-Anonymous">!8</span>
<span class="nv-Anonymous">%1</span> <span class="p">=</span> <span class="k">bitcast</span> <span class="k">i32</span><span class="p">*</span> <span class="nv">%Y</span> <span class="k">to</span> <span class="p">{}*</span> <span class="c">; <{}*> [#uses=1]</span>
<span class="k">call</span> <span class="kt">void</span> <span class="vg">@llvm.dbg.declare</span><span class="p">(</span><span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="p">*</span> <span class="nv">%Y</span><span class="p">},</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!9</span><span class="p">),</span> <span class="nv">!dbg</span> <span class="nv-Anonymous">!10</span>
<span class="k">store</span> <span class="k">i32</span> <span class="m">22</span><span class="p">,</span> <span class="k">i32</span><span class="p">*</span> <span class="nv">%Y</span><span class="p">,</span> <span class="nv">!dbg</span> <span class="nv-Anonymous">!11</span>
<span class="nv-Anonymous">%2</span> <span class="p">=</span> <span class="k">bitcast</span> <span class="k">i32</span><span class="p">*</span> <span class="nv">%Z</span> <span class="k">to</span> <span class="p">{}*</span> <span class="c">; <{}*> [#uses=1]</span>
<span class="k">call</span> <span class="kt">void</span> <span class="vg">@llvm.dbg.declare</span><span class="p">(</span><span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="p">*</span> <span class="nv">%Z</span><span class="p">},</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!12</span><span class="p">),</span> <span class="nv">!dbg</span> <span class="nv-Anonymous">!14</span>
<span class="k">store</span> <span class="k">i32</span> <span class="m">23</span><span class="p">,</span> <span class="k">i32</span><span class="p">*</span> <span class="nv">%Z</span><span class="p">,</span> <span class="nv">!dbg</span> <span class="nv-Anonymous">!15</span>
<span class="nv">%tmp</span> <span class="p">=</span> <span class="k">load</span> <span class="k">i32</span><span class="p">*</span> <span class="nv">%X</span><span class="p">,</span> <span class="nv">!dbg</span> <span class="nv-Anonymous">!16</span> <span class="c">; <i32> [#uses=1]</span>
<span class="nv">%tmp1</span> <span class="p">=</span> <span class="k">load</span> <span class="k">i32</span><span class="p">*</span> <span class="nv">%Y</span><span class="p">,</span> <span class="nv">!dbg</span> <span class="nv-Anonymous">!16</span> <span class="c">; <i32> [#uses=1]</span>
<span class="nv">%add</span> <span class="p">=</span> <span class="k">add</span> <span class="k">nsw</span> <span class="k">i32</span> <span class="nv">%tmp</span><span class="p">,</span> <span class="nv">%tmp1</span><span class="p">,</span> <span class="nv">!dbg</span> <span class="nv-Anonymous">!16</span> <span class="c">; <i32> [#uses=1]</span>
<span class="k">store</span> <span class="k">i32</span> <span class="nv">%add</span><span class="p">,</span> <span class="k">i32</span><span class="p">*</span> <span class="nv">%Z</span><span class="p">,</span> <span class="nv">!dbg</span> <span class="nv-Anonymous">!16</span>
<span class="nv">%tmp2</span> <span class="p">=</span> <span class="k">load</span> <span class="k">i32</span><span class="p">*</span> <span class="nv">%Y</span><span class="p">,</span> <span class="nv">!dbg</span> <span class="nv-Anonymous">!17</span> <span class="c">; <i32> [#uses=1]</span>
<span class="k">store</span> <span class="k">i32</span> <span class="nv">%tmp2</span><span class="p">,</span> <span class="k">i32</span><span class="p">*</span> <span class="nv">%X</span><span class="p">,</span> <span class="nv">!dbg</span> <span class="nv-Anonymous">!17</span>
<span class="k">ret</span> <span class="kt">void</span><span class="p">,</span> <span class="nv">!dbg</span> <span class="nv-Anonymous">!18</span>
<span class="p">}</span>
<span class="k">declare</span> <span class="kt">void</span> <span class="vg">@llvm.dbg.declare</span><span class="p">(</span><span class="kt">metadata</span><span class="p">,</span> <span class="kt">metadata</span><span class="p">)</span> <span class="k">nounwind</span> <span class="k">readnone</span>
<span class="nv-Anonymous">!0</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">459008</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv">!"X"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!3</span><span class="p">,</span> <span class="k">i32</span> <span class="m">2</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!6</span><span class="p">}</span><span class="c">; [ DW_TAG_auto_variable ]</span>
<span class="nv-Anonymous">!1</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">458763</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!2</span><span class="p">}</span><span class="c">; [DW_TAG_lexical_block ]</span>
<span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">458798</span><span class="p">,</span> <span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!3</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv">!"foo"</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv">!"foo"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv">!"foo"</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!3</span><span class="p">,</span> <span class="k">i32</span> <span class="m">1</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!4</span><span class="p">,</span>
<span class="k">i1</span> <span class="k">false</span><span class="p">,</span> <span class="k">i1</span> <span class="k">true</span><span class="p">}</span><span class="c">; [DW_TAG_subprogram ]</span>
<span class="nv-Anonymous">!3</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">458769</span><span class="p">,</span> <span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="k">i32</span> <span class="m">12</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv">!"foo.c"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv">!"/private/tmp"</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv">!"clang 1.1"</span><span class="p">,</span> <span class="k">i1</span> <span class="k">true</span><span class="p">,</span>
<span class="k">i1</span> <span class="k">false</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv">!""</span><span class="p">,</span> <span class="k">i32</span> <span class="m">0</span><span class="p">}</span><span class="c">; [DW_TAG_compile_unit ]</span>
<span class="nv-Anonymous">!4</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">458773</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!3</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv">!""</span><span class="p">,</span> <span class="k">null</span><span class="p">,</span> <span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="k">i64</span> <span class="m">0</span><span class="p">,</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="k">null</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!5</span><span class="p">,</span> <span class="k">i32</span> <span class="m">0</span><span class="p">}</span><span class="c">; [DW_TAG_subroutine_type ]</span>
<span class="nv-Anonymous">!5</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">null</span><span class="p">}</span>
<span class="nv-Anonymous">!6</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">458788</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!3</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv">!"int"</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!3</span><span class="p">,</span> <span class="k">i32</span> <span class="m">0</span><span class="p">,</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="k">i32</span> <span class="m">5</span><span class="p">}</span><span class="c">; [DW_TAG_base_type ]</span>
<span class="nv-Anonymous">!7</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">2</span><span class="p">,</span> <span class="k">i32</span> <span class="m">7</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="k">null</span><span class="p">}</span>
<span class="nv-Anonymous">!8</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">2</span><span class="p">,</span> <span class="k">i32</span> <span class="m">3</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="k">null</span><span class="p">}</span>
<span class="nv-Anonymous">!9</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">459008</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv">!"Y"</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!3</span><span class="p">,</span> <span class="k">i32</span> <span class="m">3</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!6</span><span class="p">}</span><span class="c">; [ DW_TAG_auto_variable ]</span>
<span class="nv-Anonymous">!10</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">3</span><span class="p">,</span> <span class="k">i32</span> <span class="m">7</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="k">null</span><span class="p">}</span>
<span class="nv-Anonymous">!11</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">3</span><span class="p">,</span> <span class="k">i32</span> <span class="m">3</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="k">null</span><span class="p">}</span>
<span class="nv-Anonymous">!12</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">459008</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!13</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv">!"Z"</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!3</span><span class="p">,</span> <span class="k">i32</span> <span class="m">5</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!6</span><span class="p">}</span><span class="c">; [ DW_TAG_auto_variable ]</span>
<span class="nv-Anonymous">!13</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">458763</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">}</span><span class="c">; [DW_TAG_lexical_block ]</span>
<span class="nv-Anonymous">!14</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">5</span><span class="p">,</span> <span class="k">i32</span> <span class="m">9</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!13</span><span class="p">,</span> <span class="k">null</span><span class="p">}</span>
<span class="nv-Anonymous">!15</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">5</span><span class="p">,</span> <span class="k">i32</span> <span class="m">5</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!13</span><span class="p">,</span> <span class="k">null</span><span class="p">}</span>
<span class="nv-Anonymous">!16</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">6</span><span class="p">,</span> <span class="k">i32</span> <span class="m">5</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!13</span><span class="p">,</span> <span class="k">null</span><span class="p">}</span>
<span class="nv-Anonymous">!17</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">8</span><span class="p">,</span> <span class="k">i32</span> <span class="m">3</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="k">null</span><span class="p">}</span>
<span class="nv-Anonymous">!18</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">9</span><span class="p">,</span> <span class="k">i32</span> <span class="m">1</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!2</span><span class="p">,</span> <span class="k">null</span><span class="p">}</span>
</pre></div>
</div>
<p>This example illustrates a few important details about LLVM debugging
information. In particular, it shows how the <tt class="docutils literal"><span class="pre">llvm.dbg.declare</span></tt> intrinsic and
location information, which are attached to an instruction, are applied
together to allow a debugger to analyze the relationship between statements,
variable definitions, and the code used to implement the function.</p>
<div class="highlight-llvm"><div class="highlight"><pre><span class="k">call</span> <span class="kt">void</span> <span class="vg">@llvm.dbg.declare</span><span class="p">(</span><span class="kt">metadata</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!0</span><span class="p">),</span> <span class="nv">!dbg</span> <span class="nv-Anonymous">!7</span>
</pre></div>
</div>
<p>The first intrinsic <tt class="docutils literal"><span class="pre">%llvm.dbg.declare</span></tt> encodes debugging information for the
variable <tt class="docutils literal"><span class="pre">X</span></tt>. The metadata <tt class="docutils literal"><span class="pre">!dbg</span> <span class="pre">!7</span></tt> attached to the intrinsic provides
scope information for the variable <tt class="docutils literal"><span class="pre">X</span></tt>.</p>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!7</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">2</span><span class="p">,</span> <span class="k">i32</span> <span class="m">7</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="k">null</span><span class="p">}</span>
<span class="nv-Anonymous">!1</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">458763</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!2</span><span class="p">}</span><span class="c">; [DW_TAG_lexical_block ]</span>
<span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">458798</span><span class="p">,</span> <span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!3</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv">!"foo"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv">!"foo"</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv">!"foo"</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!3</span><span class="p">,</span> <span class="k">i32</span> <span class="m">1</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!4</span><span class="p">,</span> <span class="k">i1</span> <span class="k">false</span><span class="p">,</span> <span class="k">i1</span> <span class="k">true</span><span class="p">}</span><span class="c">; [DW_TAG_subprogram ]</span>
</pre></div>
</div>
<p>Here <tt class="docutils literal"><span class="pre">!7</span></tt> is metadata providing location information. It has four fields:
line number, column number, scope, and original scope. The original scope
represents inline location if this instruction is inlined inside a caller, and
is null otherwise. In this example, scope is encoded by <tt class="docutils literal"><span class="pre">!1</span></tt>. <tt class="docutils literal"><span class="pre">!1</span></tt>
represents a lexical block inside the scope <tt class="docutils literal"><span class="pre">!2</span></tt>, where <tt class="docutils literal"><span class="pre">!2</span></tt> is a
<a class="reference internal" href="#format-subprograms"><em>subprogram descriptor</em></a>. This way the location
information attached to the intrinsics indicates that the variable <tt class="docutils literal"><span class="pre">X</span></tt> is
declared at line number 2 at a function level scope in function <tt class="docutils literal"><span class="pre">foo</span></tt>.</p>
<p>Now lets take another example.</p>
<div class="highlight-llvm"><div class="highlight"><pre><span class="k">call</span> <span class="kt">void</span> <span class="vg">@llvm.dbg.declare</span><span class="p">(</span><span class="kt">metadata</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!12</span><span class="p">),</span> <span class="nv">!dbg</span> <span class="nv-Anonymous">!14</span>
</pre></div>
</div>
<p>The second intrinsic <tt class="docutils literal"><span class="pre">%llvm.dbg.declare</span></tt> encodes debugging information for
variable <tt class="docutils literal"><span class="pre">Z</span></tt>. The metadata <tt class="docutils literal"><span class="pre">!dbg</span> <span class="pre">!14</span></tt> attached to the intrinsic provides
scope information for the variable <tt class="docutils literal"><span class="pre">Z</span></tt>.</p>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!13</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">458763</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">}</span><span class="c">; [DW_TAG_lexical_block ]</span>
<span class="nv-Anonymous">!14</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">5</span><span class="p">,</span> <span class="k">i32</span> <span class="m">9</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!13</span><span class="p">,</span> <span class="k">null</span><span class="p">}</span>
</pre></div>
</div>
<p>Here <tt class="docutils literal"><span class="pre">!14</span></tt> indicates that <tt class="docutils literal"><span class="pre">Z</span></tt> is declared at line number 5 and
column number 9 inside of lexical scope <tt class="docutils literal"><span class="pre">!13</span></tt>. The lexical scope itself
resides inside of lexical scope <tt class="docutils literal"><span class="pre">!1</span></tt> described above.</p>
<p>The scope information attached with each instruction provides a straightforward
way to find instructions covered by a scope.</p>
</div>
<div class="section" id="c-c-front-end-specific-debug-information">
<span id="ccxx-frontend"></span><h2><a class="toc-backref" href="#id26">C/C++ front-end specific debug information</a><a class="headerlink" href="#c-c-front-end-specific-debug-information" title="Permalink to this headline">¶</a></h2>
<p>The C and C++ front-ends represent information about the program in a format
that is effectively identical to <a class="reference external" href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3.0</a> in terms of information
content. This allows code generators to trivially support native debuggers by
generating standard dwarf information, and contains enough information for
non-dwarf targets to translate it as needed.</p>
<p>This section describes the forms used to represent C and C++ programs. Other
languages could pattern themselves after this (which itself is tuned to
representing programs in the same way that DWARF 3 does), or they could choose
to provide completely different forms if they don’t fit into the DWARF model.
As support for debugging information gets added to the various LLVM
source-language front-ends, the information used should be documented here.</p>
<p>The following sections provide examples of various C/C++ constructs and the
debug information that would best describe those constructs.</p>
<div class="section" id="c-c-source-file-information">
<h3><a class="toc-backref" href="#id27">C/C++ source file information</a><a class="headerlink" href="#c-c-source-file-information" title="Permalink to this headline">¶</a></h3>
<p>Given the source files <tt class="docutils literal"><span class="pre">MySource.cpp</span></tt> and <tt class="docutils literal"><span class="pre">MyHeader.h</span></tt> located in the
directory <tt class="docutils literal"><span class="pre">/Users/mine/sources</span></tt>, the following code:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="cp">#include "MyHeader.h"</span>
<span class="kt">int</span> <span class="nf">main</span><span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">argv</span><span class="p">[])</span> <span class="p">{</span>
<span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>a C/C++ front-end would generate the following descriptors:</p>
<div class="highlight-llvm"><div class="highlight"><pre><span class="p">...</span>
<span class="c">;;</span>
<span class="c">;; Define the compile unit for the main source file "/Users/mine/sources/MySource.cpp".</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524305</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Unused</span>
<span class="k">i32</span> <span class="m">4</span><span class="p">,</span> <span class="c">;; Language Id</span>
<span class="kt">metadata</span> <span class="nv">!"MySource.cpp"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv">!"/Users/mine/sources"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv">!"4.2.1 (Based on Apple Inc. build 5649) (LLVM build 00)"</span><span class="p">,</span>
<span class="k">i1</span> <span class="k">true</span><span class="p">,</span> <span class="c">;; Main Compile Unit</span>
<span class="k">i1</span> <span class="k">false</span><span class="p">,</span> <span class="c">;; Optimized compile unit</span>
<span class="kt">metadata</span> <span class="nv">!""</span><span class="p">,</span> <span class="c">;; Compiler flags</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">}</span> <span class="c">;; Runtime version</span>
<span class="c">;;</span>
<span class="c">;; Define the file for the file "/Users/mine/sources/MySource.cpp".</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!1</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524329</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv">!"MySource.cpp"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv">!"/Users/mine/sources"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!2</span> <span class="c">;; Compile unit</span>
<span class="p">}</span>
<span class="c">;;</span>
<span class="c">;; Define the file for the file "/Users/mine/sources/Myheader.h"</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!3</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524329</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv">!"Myheader.h"</span>
<span class="kt">metadata</span> <span class="nv">!"/Users/mine/sources"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!2</span> <span class="c">;; Compile unit</span>
<span class="p">}</span>
<span class="p">...</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">llvm::Instruction</span></tt> provides easy access to metadata attached with an
instruction. One can extract line number information encoded in LLVM IR using
<tt class="docutils literal"><span class="pre">Instruction::getMetadata()</span></tt> and <tt class="docutils literal"><span class="pre">DILocation::getLineNumber()</span></tt>.</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="k">if</span> <span class="p">(</span><span class="n">MDNode</span> <span class="o">*</span><span class="n">N</span> <span class="o">=</span> <span class="n">I</span><span class="o">-></span><span class="n">getMetadata</span><span class="p">(</span><span class="s">"dbg"</span><span class="p">))</span> <span class="p">{</span> <span class="c1">// Here I is an LLVM instruction</span>
<span class="n">DILocation</span> <span class="n">Loc</span><span class="p">(</span><span class="n">N</span><span class="p">);</span> <span class="c1">// DILocation is in DebugInfo.h</span>
<span class="kt">unsigned</span> <span class="n">Line</span> <span class="o">=</span> <span class="n">Loc</span><span class="p">.</span><span class="n">getLineNumber</span><span class="p">();</span>
<span class="n">StringRef</span> <span class="n">File</span> <span class="o">=</span> <span class="n">Loc</span><span class="p">.</span><span class="n">getFilename</span><span class="p">();</span>
<span class="n">StringRef</span> <span class="n">Dir</span> <span class="o">=</span> <span class="n">Loc</span><span class="p">.</span><span class="n">getDirectory</span><span class="p">();</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="c-c-global-variable-information">
<h3><a class="toc-backref" href="#id28">C/C++ global variable information</a><a class="headerlink" href="#c-c-global-variable-information" title="Permalink to this headline">¶</a></h3>
<p>Given an integer global variable declared as follows:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="kt">int</span> <span class="n">MyGlobal</span> <span class="o">=</span> <span class="mi">100</span><span class="p">;</span>
</pre></div>
</div>
<p>a C/C++ front-end would generate the following descriptors:</p>
<div class="highlight-llvm"><pre>;;
;; Define the global itself.
;;
%MyGlobal = global int 100
...
;;
;; List of debug info of globals
;;
!llvm.dbg.cu = !{!0}
;; Define the compile unit.
!0 = metadata !{
i32 786449, ;; Tag
i32 0, ;; Context
i32 4, ;; Language
metadata !"foo.cpp", ;; File
metadata !"/Volumes/Data/tmp", ;; Directory
metadata !"clang version 3.1 ", ;; Producer
i1 true, ;; Deprecated field
i1 false, ;; "isOptimized"?
metadata !"", ;; Flags
i32 0, ;; Runtime Version
metadata !1, ;; Enum Types
metadata !1, ;; Retained Types
metadata !1, ;; Subprograms
metadata !3 ;; Global Variables
} ; [ DW_TAG_compile_unit ]
;; The Array of Global Variables
!3 = metadata !{
metadata !4
}
!4 = metadata !{
metadata !5
}
;;
;; Define the global variable itself.
;;
!5 = metadata !{
i32 786484, ;; Tag
i32 0, ;; Unused
null, ;; Unused
metadata !"MyGlobal", ;; Name
metadata !"MyGlobal", ;; Display Name
metadata !"", ;; Linkage Name
metadata !6, ;; File
i32 1, ;; Line
metadata !7, ;; Type
i32 0, ;; IsLocalToUnit
i32 1, ;; IsDefinition
i32* @MyGlobal ;; LLVM-IR Value
} ; [ DW_TAG_variable ]
;;
;; Define the file
;;
!6 = metadata !{
i32 786473, ;; Tag
metadata !"foo.cpp", ;; File
metadata !"/Volumes/Data/tmp", ;; Directory
null ;; Unused
} ; [ DW_TAG_file_type ]
;;
;; Define the type
;;
!7 = metadata !{
i32 786468, ;; Tag
null, ;; Unused
metadata !"int", ;; Name
null, ;; Unused
i32 0, ;; Line
i64 32, ;; Size in Bits
i64 32, ;; Align in Bits
i64 0, ;; Offset
i32 0, ;; Flags
i32 5 ;; Encoding
} ; [ DW_TAG_base_type ]</pre>
</div>
</div>
<div class="section" id="c-c-function-information">
<h3><a class="toc-backref" href="#id29">C/C++ function information</a><a class="headerlink" href="#c-c-function-information" title="Permalink to this headline">¶</a></h3>
<p>Given a function declared as follows:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="kt">int</span> <span class="nf">main</span><span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">argv</span><span class="p">[])</span> <span class="p">{</span>
<span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>a C/C++ front-end would generate the following descriptors:</p>
<div class="highlight-llvm"><pre>;;
;; Define the anchor for subprograms. Note that the second field of the
;; anchor is 46, which is the same as the tag for subprograms
;; (46 = DW_TAG_subprogram.)
;;
!6 = metadata !{
i32 524334, ;; Tag
i32 0, ;; Unused
metadata !1, ;; Context
metadata !"main", ;; Name
metadata !"main", ;; Display name
metadata !"main", ;; Linkage name
metadata !1, ;; File
i32 1, ;; Line number
metadata !4, ;; Type
i1 false, ;; Is local
i1 true, ;; Is definition
i32 0, ;; Virtuality attribute, e.g. pure virtual function
i32 0, ;; Index into virtual table for C++ methods
i32 0, ;; Type that holds virtual table.
i32 0, ;; Flags
i1 false, ;; True if this function is optimized
Function *, ;; Pointer to llvm::Function
null ;; Function template parameters
}
;;
;; Define the subprogram itself.
;;
define i32 @main(i32 %argc, i8** %argv) {
...
}</pre>
</div>
</div>
<div class="section" id="c-c-basic-types">
<h3><a class="toc-backref" href="#id30">C/C++ basic types</a><a class="headerlink" href="#c-c-basic-types" title="Permalink to this headline">¶</a></h3>
<p>The following are the basic type descriptors for C/C++ core types:</p>
<div class="section" id="bool">
<h4><a class="toc-backref" href="#id31">bool</a><a class="headerlink" href="#bool" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524324</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"bool"</span><span class="p">,</span> <span class="c">;; Name</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">8</span><span class="p">,</span> <span class="c">;; Size in Bits</span>
<span class="k">i64</span> <span class="m">8</span><span class="p">,</span> <span class="c">;; Align in Bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in Bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="k">i32</span> <span class="m">2</span> <span class="c">;; Encoding</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="char">
<h4><a class="toc-backref" href="#id32">char</a><a class="headerlink" href="#char" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524324</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"char"</span><span class="p">,</span> <span class="c">;; Name</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">8</span><span class="p">,</span> <span class="c">;; Size in Bits</span>
<span class="k">i64</span> <span class="m">8</span><span class="p">,</span> <span class="c">;; Align in Bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in Bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="k">i32</span> <span class="m">6</span> <span class="c">;; Encoding</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="unsigned-char">
<h4><a class="toc-backref" href="#id33">unsigned char</a><a class="headerlink" href="#unsigned-char" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524324</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"unsigned char"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">8</span><span class="p">,</span> <span class="c">;; Size in Bits</span>
<span class="k">i64</span> <span class="m">8</span><span class="p">,</span> <span class="c">;; Align in Bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in Bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="k">i32</span> <span class="m">8</span> <span class="c">;; Encoding</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="short">
<h4><a class="toc-backref" href="#id34">short</a><a class="headerlink" href="#short" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524324</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"short int"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">16</span><span class="p">,</span> <span class="c">;; Size in Bits</span>
<span class="k">i64</span> <span class="m">16</span><span class="p">,</span> <span class="c">;; Align in Bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in Bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="k">i32</span> <span class="m">5</span> <span class="c">;; Encoding</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="unsigned-short">
<h4><a class="toc-backref" href="#id35">unsigned short</a><a class="headerlink" href="#unsigned-short" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524324</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"short unsigned int"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">16</span><span class="p">,</span> <span class="c">;; Size in Bits</span>
<span class="k">i64</span> <span class="m">16</span><span class="p">,</span> <span class="c">;; Align in Bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in Bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="k">i32</span> <span class="m">7</span> <span class="c">;; Encoding</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="int">
<h4><a class="toc-backref" href="#id36">int</a><a class="headerlink" href="#int" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524324</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"int"</span><span class="p">,</span> <span class="c">;; Name</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Size in Bits</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Align in Bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in Bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="k">i32</span> <span class="m">5</span> <span class="c">;; Encoding</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="unsigned-int">
<h4><a class="toc-backref" href="#id37">unsigned int</a><a class="headerlink" href="#unsigned-int" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524324</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"unsigned int"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Size in Bits</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Align in Bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in Bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="k">i32</span> <span class="m">7</span> <span class="c">;; Encoding</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="long-long">
<h4><a class="toc-backref" href="#id38">long long</a><a class="headerlink" href="#long-long" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524324</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"long long int"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">64</span><span class="p">,</span> <span class="c">;; Size in Bits</span>
<span class="k">i64</span> <span class="m">64</span><span class="p">,</span> <span class="c">;; Align in Bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in Bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="k">i32</span> <span class="m">5</span> <span class="c">;; Encoding</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="unsigned-long-long">
<h4><a class="toc-backref" href="#id39">unsigned long long</a><a class="headerlink" href="#unsigned-long-long" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524324</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"long long unsigned int"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">64</span><span class="p">,</span> <span class="c">;; Size in Bits</span>
<span class="k">i64</span> <span class="m">64</span><span class="p">,</span> <span class="c">;; Align in Bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in Bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="k">i32</span> <span class="m">7</span> <span class="c">;; Encoding</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="float">
<h4><a class="toc-backref" href="#id40">float</a><a class="headerlink" href="#float" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524324</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"float"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Size in Bits</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Align in Bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in Bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="k">i32</span> <span class="m">4</span> <span class="c">;; Encoding</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="double">
<h4><a class="toc-backref" href="#id41">double</a><a class="headerlink" href="#double" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524324</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"double"</span><span class="p">,</span><span class="c">;; Name</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">64</span><span class="p">,</span> <span class="c">;; Size in Bits</span>
<span class="k">i64</span> <span class="m">64</span><span class="p">,</span> <span class="c">;; Align in Bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in Bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="k">i32</span> <span class="m">4</span> <span class="c">;; Encoding</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="c-c-derived-types">
<h3><a class="toc-backref" href="#id42">C/C++ derived types</a><a class="headerlink" href="#c-c-derived-types" title="Permalink to this headline">¶</a></h3>
<p>Given the following as an example of C/C++ derived type:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="k">typedef</span> <span class="k">const</span> <span class="kt">int</span> <span class="o">*</span><span class="n">IntPtr</span><span class="p">;</span>
</pre></div>
</div>
<p>a C/C++ front-end would generate the following descriptors:</p>
<div class="highlight-llvm"><div class="highlight"><pre><span class="c">;;</span>
<span class="c">;; Define the typedef "IntPtr".</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524310</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"IntPtr"</span><span class="p">,</span> <span class="c">;; Name</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!3</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Size in bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Align in bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!4</span> <span class="c">;; Derived From type</span>
<span class="p">}</span>
<span class="c">;;</span>
<span class="c">;; Define the pointer type.</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!4</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524303</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!""</span><span class="p">,</span> <span class="c">;; Name</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">64</span><span class="p">,</span> <span class="c">;; Size in bits</span>
<span class="k">i64</span> <span class="m">64</span><span class="p">,</span> <span class="c">;; Align in bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!5</span> <span class="c">;; Derived From type</span>
<span class="p">}</span>
<span class="c">;;</span>
<span class="c">;; Define the const type.</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!5</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524326</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!""</span><span class="p">,</span> <span class="c">;; Name</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Size in bits</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Align in bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!6</span> <span class="c">;; Derived From type</span>
<span class="p">}</span>
<span class="c">;;</span>
<span class="c">;; Define the int type.</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!6</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524324</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"int"</span><span class="p">,</span> <span class="c">;; Name</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Size in bits</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Align in bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="m">5</span> <span class="c">;; Encoding</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="c-c-struct-union-types">
<h3><a class="toc-backref" href="#id43">C/C++ struct/union types</a><a class="headerlink" href="#c-c-struct-union-types" title="Permalink to this headline">¶</a></h3>
<p>Given the following as an example of C/C++ struct type:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="k">struct</span> <span class="n">Color</span> <span class="p">{</span>
<span class="kt">unsigned</span> <span class="n">Red</span><span class="p">;</span>
<span class="kt">unsigned</span> <span class="n">Green</span><span class="p">;</span>
<span class="kt">unsigned</span> <span class="n">Blue</span><span class="p">;</span>
<span class="p">};</span>
</pre></div>
</div>
<p>a C/C++ front-end would generate the following descriptors:</p>
<div class="highlight-llvm"><div class="highlight"><pre><span class="c">;;</span>
<span class="c">;; Define basic type for unsigned int.</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!5</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524324</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"unsigned int"</span><span class="p">,</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Size in Bits</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Align in Bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in Bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="k">i32</span> <span class="m">7</span> <span class="c">;; Encoding</span>
<span class="p">}</span>
<span class="c">;;</span>
<span class="c">;; Define composite type for struct Color.</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524307</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"Color"</span><span class="p">,</span> <span class="c">;; Name</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Compile unit</span>
<span class="k">i32</span> <span class="m">1</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">96</span><span class="p">,</span> <span class="c">;; Size in bits</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Align in bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="k">null</span><span class="p">,</span> <span class="c">;; Derived From</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!3</span><span class="p">,</span> <span class="c">;; Elements</span>
<span class="k">i32</span> <span class="m">0</span> <span class="c">;; Runtime Language</span>
<span class="p">}</span>
<span class="c">;;</span>
<span class="c">;; Define the Red field.</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!4</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524301</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"Red"</span><span class="p">,</span> <span class="c">;; Name</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">2</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Size in bits</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Align in bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!5</span> <span class="c">;; Derived From type</span>
<span class="p">}</span>
<span class="c">;;</span>
<span class="c">;; Define the Green field.</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!6</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524301</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"Green"</span><span class="p">,</span> <span class="c">;; Name</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">3</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Size in bits</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Align in bits</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Offset in bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!5</span> <span class="c">;; Derived From type</span>
<span class="p">}</span>
<span class="c">;;</span>
<span class="c">;; Define the Blue field.</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!7</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524301</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"Blue"</span><span class="p">,</span> <span class="c">;; Name</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">4</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Size in bits</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Align in bits</span>
<span class="k">i64</span> <span class="m">64</span><span class="p">,</span> <span class="c">;; Offset in bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!5</span> <span class="c">;; Derived From type</span>
<span class="p">}</span>
<span class="c">;;</span>
<span class="c">;; Define the array of fields used by the composite type Color.</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!3</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="kt">metadata</span> <span class="nv-Anonymous">!4</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!6</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!7</span><span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="c-c-enumeration-types">
<h3><a class="toc-backref" href="#id44">C/C++ enumeration types</a><a class="headerlink" href="#c-c-enumeration-types" title="Permalink to this headline">¶</a></h3>
<p>Given the following as an example of C/C++ enumeration type:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="k">enum</span> <span class="n">Trees</span> <span class="p">{</span>
<span class="n">Spruce</span> <span class="o">=</span> <span class="mi">100</span><span class="p">,</span>
<span class="n">Oak</span> <span class="o">=</span> <span class="mi">200</span><span class="p">,</span>
<span class="n">Maple</span> <span class="o">=</span> <span class="mi">300</span>
<span class="p">};</span>
</pre></div>
</div>
<p>a C/C++ front-end would generate the following descriptors:</p>
<div class="highlight-llvm"><div class="highlight"><pre><span class="c">;;</span>
<span class="c">;; Define composite type for enum Trees</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!2</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span>
<span class="k">i32</span> <span class="m">524292</span><span class="p">,</span> <span class="c">;; Tag</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; Context</span>
<span class="kt">metadata</span> <span class="nv">!"Trees"</span><span class="p">,</span> <span class="c">;; Name</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!1</span><span class="p">,</span> <span class="c">;; File</span>
<span class="k">i32</span> <span class="m">1</span><span class="p">,</span> <span class="c">;; Line number</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Size in bits</span>
<span class="k">i64</span> <span class="m">32</span><span class="p">,</span> <span class="c">;; Align in bits</span>
<span class="k">i64</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Offset in bits</span>
<span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="c">;; Flags</span>
<span class="k">null</span><span class="p">,</span> <span class="c">;; Derived From type</span>
<span class="kt">metadata</span> <span class="nv-Anonymous">!3</span><span class="p">,</span> <span class="c">;; Elements</span>
<span class="k">i32</span> <span class="m">0</span> <span class="c">;; Runtime language</span>
<span class="p">}</span>
<span class="c">;;</span>
<span class="c">;; Define the array of enumerators used by composite type Trees.</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!3</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="kt">metadata</span> <span class="nv-Anonymous">!4</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!5</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv-Anonymous">!6</span><span class="p">}</span>
<span class="c">;;</span>
<span class="c">;; Define Spruce enumerator.</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!4</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">524328</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv">!"Spruce"</span><span class="p">,</span> <span class="k">i64</span> <span class="m">100</span><span class="p">}</span>
<span class="c">;;</span>
<span class="c">;; Define Oak enumerator.</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!5</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">524328</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv">!"Oak"</span><span class="p">,</span> <span class="k">i64</span> <span class="m">200</span><span class="p">}</span>
<span class="c">;;</span>
<span class="c">;; Define Maple enumerator.</span>
<span class="c">;;</span>
<span class="nv-Anonymous">!6</span> <span class="p">=</span> <span class="kt">metadata</span> <span class="p">!{</span><span class="k">i32</span> <span class="m">524328</span><span class="p">,</span> <span class="kt">metadata</span> <span class="nv">!"Maple"</span><span class="p">,</span> <span class="k">i64</span> <span class="m">300</span><span class="p">}</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="id1">
<h2><a class="toc-backref" href="#id45">Debugging information format</a><a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h2>
<div class="section" id="debugging-information-extension-for-objective-c-properties">
<h3><a class="toc-backref" href="#id46">Debugging Information Extension for Objective C Properties</a><a class="headerlink" href="#debugging-information-extension-for-objective-c-properties" title="Permalink to this headline">¶</a></h3>
<div class="section" id="id2">
<h4><a class="toc-backref" href="#id47">Introduction</a><a class="headerlink" href="#id2" title="Permalink to this headline">¶</a></h4>
<p>Objective C provides a simpler way to declare and define accessor methods using
declared properties. The language provides features to declare a property and
to let compiler synthesize accessor methods.</p>
<p>The debugger lets developer inspect Objective C interfaces and their instance
variables and class variables. However, the debugger does not know anything
about the properties defined in Objective C interfaces. The debugger consumes
information generated by compiler in DWARF format. The format does not support
encoding of Objective C properties. This proposal describes DWARF extensions to
encode Objective C properties, which the debugger can use to let developers
inspect Objective C properties.</p>
</div>
<div class="section" id="proposal">
<h4><a class="toc-backref" href="#id48">Proposal</a><a class="headerlink" href="#proposal" title="Permalink to this headline">¶</a></h4>
<p>Objective C properties exist separately from class members. A property can be
defined only by “setter” and “getter” selectors, and be calculated anew on each
access. Or a property can just be a direct access to some declared ivar.
Finally it can have an ivar “automatically synthesized” for it by the compiler,
in which case the property can be referred to in user code directly using the
standard C dereference syntax as well as through the property “dot” syntax, but
there is no entry in the <tt class="docutils literal"><span class="pre">@interface</span></tt> declaration corresponding to this ivar.</p>
<p>To facilitate debugging, these properties we will add a new DWARF TAG into the
<tt class="docutils literal"><span class="pre">DW_TAG_structure_type</span></tt> definition for the class to hold the description of a
given property, and a set of DWARF attributes that provide said description.
The property tag will also contain the name and declared type of the property.</p>
<p>If there is a related ivar, there will also be a DWARF property attribute placed
in the <tt class="docutils literal"><span class="pre">DW_TAG_member</span></tt> DIE for that ivar referring back to the property TAG
for that property. And in the case where the compiler synthesizes the ivar
directly, the compiler is expected to generate a <tt class="docutils literal"><span class="pre">DW_TAG_member</span></tt> for that
ivar (with the <tt class="docutils literal"><span class="pre">DW_AT_artificial</span></tt> set to 1), whose name will be the name used
to access this ivar directly in code, and with the property attribute pointing
back to the property it is backing.</p>
<p>The following examples will serve as illustration for our discussion:</p>
<div class="highlight-objc"><div class="highlight"><pre><span class="k">@interface</span> <span class="nc">I1</span> <span class="p">{</span>
<span class="kt">int</span> <span class="n">n2</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">@property</span> <span class="kt">int</span> <span class="n">p1</span><span class="p">;</span>
<span class="k">@property</span> <span class="kt">int</span> <span class="n">p2</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">I1</span>
<span class="k">@synthesize</span> <span class="n">p1</span><span class="p">;</span>
<span class="k">@synthesize</span> <span class="n">p2</span> <span class="o">=</span> <span class="n">n2</span><span class="p">;</span>
<span class="k">@end</span>
</pre></div>
</div>
<p>This produces the following DWARF (this is a “pseudo dwarfdump” output):</p>
<div class="highlight-none"><div class="highlight"><pre>0x00000100: TAG_structure_type [7] *
AT_APPLE_runtime_class( 0x10 )
AT_name( "I1" )
AT_decl_file( "Objc_Property.m" )
AT_decl_line( 3 )
0x00000110 TAG_APPLE_property
AT_name ( "p1" )
AT_type ( {0x00000150} ( int ) )
0x00000120: TAG_APPLE_property
AT_name ( "p2" )
AT_type ( {0x00000150} ( int ) )
0x00000130: TAG_member [8]
AT_name( "_p1" )
AT_APPLE_property ( {0x00000110} "p1" )
AT_type( {0x00000150} ( int ) )
AT_artificial ( 0x1 )
0x00000140: TAG_member [8]
AT_name( "n2" )
AT_APPLE_property ( {0x00000120} "p2" )
AT_type( {0x00000150} ( int ) )
0x00000150: AT_type( ( int ) )
</pre></div>
</div>
<p>Note, the current convention is that the name of the ivar for an
auto-synthesized property is the name of the property from which it derives
with an underscore prepended, as is shown in the example. But we actually
don’t need to know this convention, since we are given the name of the ivar
directly.</p>
<p>Also, it is common practice in ObjC to have different property declarations in
the @interface and @implementation - e.g. to provide a read-only property in
the interface,and a read-write interface in the implementation. In that case,
the compiler should emit whichever property declaration will be in force in the
current translation unit.</p>
<p>Developers can decorate a property with attributes which are encoded using
<tt class="docutils literal"><span class="pre">DW_AT_APPLE_property_attribute</span></tt>.</p>
<div class="highlight-objc"><div class="highlight"><pre><span class="k">@property</span> <span class="p">(</span><span class="n">readonly</span><span class="p">,</span> <span class="n">nonatomic</span><span class="p">)</span> <span class="kt">int</span> <span class="n">pr</span><span class="p">;</span>
</pre></div>
</div>
<div class="highlight-none"><div class="highlight"><pre>TAG_APPLE_property [8]
AT_name( "pr" )
AT_type ( {0x00000147} (int) )
AT_APPLE_property_attribute (DW_APPLE_PROPERTY_readonly, DW_APPLE_PROPERTY_nonatomic)
</pre></div>
</div>
<p>The setter and getter method names are attached to the property using
<tt class="docutils literal"><span class="pre">DW_AT_APPLE_property_setter</span></tt> and <tt class="docutils literal"><span class="pre">DW_AT_APPLE_property_getter</span></tt> attributes.</p>
<div class="highlight-objc"><div class="highlight"><pre><span class="k">@interface</span> <span class="nc">I1</span>
<span class="k">@property</span> <span class="p">(</span><span class="n">setter</span><span class="o">=</span><span class="n">myOwnP3Setter</span><span class="o">:</span><span class="p">)</span> <span class="kt">int</span> <span class="n">p3</span><span class="p">;</span>
<span class="k">-</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span><span class="nf">myOwnP3Setter:</span><span class="p">(</span><span class="kt">int</span><span class="p">)</span><span class="nv">a</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">I1</span>
<span class="k">@synthesize</span> <span class="n">p3</span><span class="p">;</span>
<span class="k">-</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span><span class="nf">myOwnP3Setter:</span><span class="p">(</span><span class="kt">int</span><span class="p">)</span><span class="nv">a</span><span class="p">{</span> <span class="p">}</span>
<span class="k">@end</span>
</pre></div>
</div>
<p>The DWARF for this would be:</p>
<div class="highlight-none"><div class="highlight"><pre>0x000003bd: TAG_structure_type [7] *
AT_APPLE_runtime_class( 0x10 )
AT_name( "I1" )
AT_decl_file( "Objc_Property.m" )
AT_decl_line( 3 )
0x000003cd TAG_APPLE_property
AT_name ( "p3" )
AT_APPLE_property_setter ( "myOwnP3Setter:" )
AT_type( {0x00000147} ( int ) )
0x000003f3: TAG_member [8]
AT_name( "_p3" )
AT_type ( {0x00000147} ( int ) )
AT_APPLE_property ( {0x000003cd} )
AT_artificial ( 0x1 )
</pre></div>
</div>
</div>
<div class="section" id="new-dwarf-tags">
<h4><a class="toc-backref" href="#id49">New DWARF Tags</a><a class="headerlink" href="#new-dwarf-tags" title="Permalink to this headline">¶</a></h4>
<table border="1" class="docutils">
<colgroup>
<col width="74%" />
<col width="26%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">TAG</th>
<th class="head">Value</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td>DW_TAG_APPLE_property</td>
<td>0x4200</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="new-dwarf-attributes">
<h4><a class="toc-backref" href="#id50">New DWARF Attributes</a><a class="headerlink" href="#new-dwarf-attributes" title="Permalink to this headline">¶</a></h4>
<table border="1" class="docutils">
<colgroup>
<col width="63%" />
<col width="16%" />
<col width="22%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Attribute</th>
<th class="head">Value</th>
<th class="head">Classes</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td>DW_AT_APPLE_property</td>
<td>0x3fed</td>
<td>Reference</td>
</tr>
<tr class="row-odd"><td>DW_AT_APPLE_property_getter</td>
<td>0x3fe9</td>
<td>String</td>
</tr>
<tr class="row-even"><td>DW_AT_APPLE_property_setter</td>
<td>0x3fea</td>
<td>String</td>
</tr>
<tr class="row-odd"><td>DW_AT_APPLE_property_attribute</td>
<td>0x3feb</td>
<td>Constant</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="new-dwarf-constants">
<h4><a class="toc-backref" href="#id51">New DWARF Constants</a><a class="headerlink" href="#new-dwarf-constants" title="Permalink to this headline">¶</a></h4>
<table border="1" class="docutils">
<colgroup>
<col width="82%" />
<col width="18%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Name</th>
<th class="head">Value</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td>DW_AT_APPLE_PROPERTY_readonly</td>
<td>0x1</td>
</tr>
<tr class="row-odd"><td>DW_AT_APPLE_PROPERTY_readwrite</td>
<td>0x2</td>
</tr>
<tr class="row-even"><td>DW_AT_APPLE_PROPERTY_assign</td>
<td>0x4</td>
</tr>
<tr class="row-odd"><td>DW_AT_APPLE_PROPERTY_retain</td>
<td>0x8</td>
</tr>
<tr class="row-even"><td>DW_AT_APPLE_PROPERTY_copy</td>
<td>0x10</td>
</tr>
<tr class="row-odd"><td>DW_AT_APPLE_PROPERTY_nonatomic</td>
<td>0x20</td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="section" id="name-accelerator-tables">
<h3><a class="toc-backref" href="#id52">Name Accelerator Tables</a><a class="headerlink" href="#name-accelerator-tables" title="Permalink to this headline">¶</a></h3>
<div class="section" id="id3">
<h4><a class="toc-backref" href="#id53">Introduction</a><a class="headerlink" href="#id3" title="Permalink to this headline">¶</a></h4>
<p>The “<tt class="docutils literal"><span class="pre">.debug_pubnames</span></tt>” and “<tt class="docutils literal"><span class="pre">.debug_pubtypes</span></tt>” formats are not what a
debugger needs. The “<tt class="docutils literal"><span class="pre">pub</span></tt>” in the section name indicates that the entries
in the table are publicly visible names only. This means no static or hidden
functions show up in the “<tt class="docutils literal"><span class="pre">.debug_pubnames</span></tt>”. No static variables or private
class variables are in the “<tt class="docutils literal"><span class="pre">.debug_pubtypes</span></tt>”. Many compilers add different
things to these tables, so we can’t rely upon the contents between gcc, icc, or
clang.</p>
<p>The typical query given by users tends not to match up with the contents of
these tables. For example, the DWARF spec states that “In the case of the name
of a function member or static data member of a C++ structure, class or union,
the name presented in the “<tt class="docutils literal"><span class="pre">.debug_pubnames</span></tt>” section is not the simple name
given by the <tt class="docutils literal"><span class="pre">DW_AT_name</span> <span class="pre">attribute</span></tt> of the referenced debugging information
entry, but rather the fully qualified name of the data or function member.”
So the only names in these tables for complex C++ entries is a fully
qualified name. Debugger users tend not to enter their search strings as
“<tt class="docutils literal"><span class="pre">a::b::c(int,const</span> <span class="pre">Foo&)</span> <span class="pre">const</span></tt>”, but rather as “<tt class="docutils literal"><span class="pre">c</span></tt>”, “<tt class="docutils literal"><span class="pre">b::c</span></tt>” , or
“<tt class="docutils literal"><span class="pre">a::b::c</span></tt>”. So the name entered in the name table must be demangled in
order to chop it up appropriately and additional names must be manually entered
into the table to make it effective as a name lookup table for debuggers to
se.</p>
<p>All debuggers currently ignore the “<tt class="docutils literal"><span class="pre">.debug_pubnames</span></tt>” table as a result of
its inconsistent and useless public-only name content making it a waste of
space in the object file. These tables, when they are written to disk, are not
sorted in any way, leaving every debugger to do its own parsing and sorting.
These tables also include an inlined copy of the string values in the table
itself making the tables much larger than they need to be on disk, especially
for large C++ programs.</p>
<p>Can’t we just fix the sections by adding all of the names we need to this
table? No, because that is not what the tables are defined to contain and we
won’t know the difference between the old bad tables and the new good tables.
At best we could make our own renamed sections that contain all of the data we
need.</p>
<p>These tables are also insufficient for what a debugger like LLDB needs. LLDB
uses clang for its expression parsing where LLDB acts as a PCH. LLDB is then
often asked to look for type “<tt class="docutils literal"><span class="pre">foo</span></tt>” or namespace “<tt class="docutils literal"><span class="pre">bar</span></tt>”, or list items in
namespace “<tt class="docutils literal"><span class="pre">baz</span></tt>”. Namespaces are not included in the pubnames or pubtypes
tables. Since clang asks a lot of questions when it is parsing an expression,
we need to be very fast when looking up names, as it happens a lot. Having new
accelerator tables that are optimized for very quick lookups will benefit this
type of debugging experience greatly.</p>
<p>We would like to generate name lookup tables that can be mapped into memory
from disk, and used as is, with little or no up-front parsing. We would also
be able to control the exact content of these different tables so they contain
exactly what we need. The Name Accelerator Tables were designed to fix these
issues. In order to solve these issues we need to:</p>
<ul class="simple">
<li>Have a format that can be mapped into memory from disk and used as is</li>
<li>Lookups should be very fast</li>
<li>Extensible table format so these tables can be made by many producers</li>
<li>Contain all of the names needed for typical lookups out of the box</li>
<li>Strict rules for the contents of tables</li>
</ul>
<p>Table size is important and the accelerator table format should allow the reuse
of strings from common string tables so the strings for the names are not
duplicated. We also want to make sure the table is ready to be used as-is by
simply mapping the table into memory with minimal header parsing.</p>
<p>The name lookups need to be fast and optimized for the kinds of lookups that
debuggers tend to do. Optimally we would like to touch as few parts of the
mapped table as possible when doing a name lookup and be able to quickly find
the name entry we are looking for, or discover there are no matches. In the
case of debuggers we optimized for lookups that fail most of the time.</p>
<p>Each table that is defined should have strict rules on exactly what is in the
accelerator tables and documented so clients can rely on the content.</p>
</div>
<div class="section" id="hash-tables">
<h4><a class="toc-backref" href="#id54">Hash Tables</a><a class="headerlink" href="#hash-tables" title="Permalink to this headline">¶</a></h4>
<div class="section" id="standard-hash-tables">
<h5><a class="toc-backref" href="#id55">Standard Hash Tables</a><a class="headerlink" href="#standard-hash-tables" title="Permalink to this headline">¶</a></h5>
<p>Typical hash tables have a header, buckets, and each bucket points to the
bucket contents:</p>
<div class="highlight-none"><div class="highlight"><pre>.------------.
| HEADER |
|------------|
| BUCKETS |
|------------|
| DATA |
`------------'
</pre></div>
</div>
<p>The BUCKETS are an array of offsets to DATA for each hash:</p>
<div class="highlight-none"><div class="highlight"><pre>.------------.
| 0x00001000 | BUCKETS[0]
| 0x00002000 | BUCKETS[1]
| 0x00002200 | BUCKETS[2]
| 0x000034f0 | BUCKETS[3]
| | ...
| 0xXXXXXXXX | BUCKETS[n_buckets]
'------------'
</pre></div>
</div>
<p>So for <tt class="docutils literal"><span class="pre">bucket[3]</span></tt> in the example above, we have an offset into the table
0x000034f0 which points to a chain of entries for the bucket. Each bucket must
contain a next pointer, full 32 bit hash value, the string itself, and the data
for the current string value.</p>
<div class="highlight-none"><div class="highlight"><pre> .------------.
0x000034f0: | 0x00003500 | next pointer
| 0x12345678 | 32 bit hash
| "erase" | string value
| data[n] | HashData for this bucket
|------------|
0x00003500: | 0x00003550 | next pointer
| 0x29273623 | 32 bit hash
| "dump" | string value
| data[n] | HashData for this bucket
|------------|
0x00003550: | 0x00000000 | next pointer
| 0x82638293 | 32 bit hash
| "main" | string value
| data[n] | HashData for this bucket
`------------'
</pre></div>
</div>
<p>The problem with this layout for debuggers is that we need to optimize for the
negative lookup case where the symbol we’re searching for is not present. So
if we were to lookup “<tt class="docutils literal"><span class="pre">printf</span></tt>” in the table above, we would make a 32 hash
for “<tt class="docutils literal"><span class="pre">printf</span></tt>”, it might match <tt class="docutils literal"><span class="pre">bucket[3]</span></tt>. We would need to go to the
offset 0x000034f0 and start looking to see if our 32 bit hash matches. To do
so, we need to read the next pointer, then read the hash, compare it, and skip
to the next bucket. Each time we are skipping many bytes in memory and
touching new cache pages just to do the compare on the full 32 bit hash. All
of these accesses then tell us that we didn’t have a match.</p>
</div>
<div class="section" id="name-hash-tables">
<h5><a class="toc-backref" href="#id56">Name Hash Tables</a><a class="headerlink" href="#name-hash-tables" title="Permalink to this headline">¶</a></h5>
<p>To solve the issues mentioned above we have structured the hash tables a bit
differently: a header, buckets, an array of all unique 32 bit hash values,
followed by an array of hash value data offsets, one for each hash value, then
the data for all hash values:</p>
<div class="highlight-none"><div class="highlight"><pre>.-------------.
| HEADER |
|-------------|
| BUCKETS |
|-------------|
| HASHES |
|-------------|
| OFFSETS |
|-------------|
| DATA |
`-------------'
</pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">BUCKETS</span></tt> in the name tables are an index into the <tt class="docutils literal"><span class="pre">HASHES</span></tt> array. By
making all of the full 32 bit hash values contiguous in memory, we allow
ourselves to efficiently check for a match while touching as little memory as
possible. Most often checking the 32 bit hash values is as far as the lookup
goes. If it does match, it usually is a match with no collisions. So for a
table with “<tt class="docutils literal"><span class="pre">n_buckets</span></tt>” buckets, and “<tt class="docutils literal"><span class="pre">n_hashes</span></tt>” unique 32 bit hash
values, we can clarify the contents of the <tt class="docutils literal"><span class="pre">BUCKETS</span></tt>, <tt class="docutils literal"><span class="pre">HASHES</span></tt> and
<tt class="docutils literal"><span class="pre">OFFSETS</span></tt> as:</p>
<div class="highlight-none"><div class="highlight"><pre>.-------------------------.
| HEADER.magic | uint32_t
| HEADER.version | uint16_t
| HEADER.hash_function | uint16_t
| HEADER.bucket_count | uint32_t
| HEADER.hashes_count | uint32_t
| HEADER.header_data_len | uint32_t
| HEADER_DATA | HeaderData
|-------------------------|
| BUCKETS | uint32_t[n_buckets] // 32 bit hash indexes
|-------------------------|
| HASHES | uint32_t[n_hashes] // 32 bit hash values
|-------------------------|
| OFFSETS | uint32_t[n_hashes] // 32 bit offsets to hash value data
|-------------------------|
| ALL HASH DATA |
`-------------------------'
</pre></div>
</div>
<p>So taking the exact same data from the standard hash example above we end up
with:</p>
<div class="highlight-none"><div class="highlight"><pre> .------------.
| HEADER |
|------------|
| 0 | BUCKETS[0]
| 2 | BUCKETS[1]
| 5 | BUCKETS[2]
| 6 | BUCKETS[3]
| | ...
| ... | BUCKETS[n_buckets]
|------------|
| 0x........ | HASHES[0]
| 0x........ | HASHES[1]
| 0x........ | HASHES[2]
| 0x........ | HASHES[3]
| 0x........ | HASHES[4]
| 0x........ | HASHES[5]
| 0x12345678 | HASHES[6] hash for BUCKETS[3]
| 0x29273623 | HASHES[7] hash for BUCKETS[3]
| 0x82638293 | HASHES[8] hash for BUCKETS[3]
| 0x........ | HASHES[9]
| 0x........ | HASHES[10]
| 0x........ | HASHES[11]
| 0x........ | HASHES[12]
| 0x........ | HASHES[13]
| 0x........ | HASHES[n_hashes]
|------------|
| 0x........ | OFFSETS[0]
| 0x........ | OFFSETS[1]
| 0x........ | OFFSETS[2]
| 0x........ | OFFSETS[3]
| 0x........ | OFFSETS[4]
| 0x........ | OFFSETS[5]
| 0x000034f0 | OFFSETS[6] offset for BUCKETS[3]
| 0x00003500 | OFFSETS[7] offset for BUCKETS[3]
| 0x00003550 | OFFSETS[8] offset for BUCKETS[3]
| 0x........ | OFFSETS[9]
| 0x........ | OFFSETS[10]
| 0x........ | OFFSETS[11]
| 0x........ | OFFSETS[12]
| 0x........ | OFFSETS[13]
| 0x........ | OFFSETS[n_hashes]
|------------|
| |
| |
| |
| |
| |
|------------|
0x000034f0: | 0x00001203 | .debug_str ("erase")
| 0x00000004 | A 32 bit array count - number of HashData with name "erase"
| 0x........ | HashData[0]
| 0x........ | HashData[1]
| 0x........ | HashData[2]
| 0x........ | HashData[3]
| 0x00000000 | String offset into .debug_str (terminate data for hash)
|------------|
0x00003500: | 0x00001203 | String offset into .debug_str ("collision")
| 0x00000002 | A 32 bit array count - number of HashData with name "collision"
| 0x........ | HashData[0]
| 0x........ | HashData[1]
| 0x00001203 | String offset into .debug_str ("dump")
| 0x00000003 | A 32 bit array count - number of HashData with name "dump"
| 0x........ | HashData[0]
| 0x........ | HashData[1]
| 0x........ | HashData[2]
| 0x00000000 | String offset into .debug_str (terminate data for hash)
|------------|
0x00003550: | 0x00001203 | String offset into .debug_str ("main")
| 0x00000009 | A 32 bit array count - number of HashData with name "main"
| 0x........ | HashData[0]
| 0x........ | HashData[1]
| 0x........ | HashData[2]
| 0x........ | HashData[3]
| 0x........ | HashData[4]
| 0x........ | HashData[5]
| 0x........ | HashData[6]
| 0x........ | HashData[7]
| 0x........ | HashData[8]
| 0x00000000 | String offset into .debug_str (terminate data for hash)
`------------'
</pre></div>
</div>
<p>So we still have all of the same data, we just organize it more efficiently for
debugger lookup. If we repeat the same “<tt class="docutils literal"><span class="pre">printf</span></tt>” lookup from above, we
would hash “<tt class="docutils literal"><span class="pre">printf</span></tt>” and find it matches <tt class="docutils literal"><span class="pre">BUCKETS[3]</span></tt> by taking the 32 bit
hash value and modulo it by <tt class="docutils literal"><span class="pre">n_buckets</span></tt>. <tt class="docutils literal"><span class="pre">BUCKETS[3]</span></tt> contains “6” which
is the index into the <tt class="docutils literal"><span class="pre">HASHES</span></tt> table. We would then compare any consecutive
32 bit hashes values in the <tt class="docutils literal"><span class="pre">HASHES</span></tt> array as long as the hashes would be in
<tt class="docutils literal"><span class="pre">BUCKETS[3]</span></tt>. We do this by verifying that each subsequent hash value modulo
<tt class="docutils literal"><span class="pre">n_buckets</span></tt> is still 3. In the case of a failed lookup we would access the
memory for <tt class="docutils literal"><span class="pre">BUCKETS[3]</span></tt>, and then compare a few consecutive 32 bit hashes
before we know that we have no match. We don’t end up marching through
multiple words of memory and we really keep the number of processor data cache
lines being accessed as small as possible.</p>
<p>The string hash that is used for these lookup tables is the Daniel J.
Bernstein hash which is also used in the ELF <tt class="docutils literal"><span class="pre">GNU_HASH</span></tt> sections. It is a
very good hash for all kinds of names in programs with very few hash
collisions.</p>
<p>Empty buckets are designated by using an invalid hash index of <tt class="docutils literal"><span class="pre">UINT32_MAX</span></tt>.</p>
</div>
</div>
<div class="section" id="details">
<h4><a class="toc-backref" href="#id57">Details</a><a class="headerlink" href="#details" title="Permalink to this headline">¶</a></h4>
<p>These name hash tables are designed to be generic where specializations of the
table get to define additional data that goes into the header (“<tt class="docutils literal"><span class="pre">HeaderData</span></tt>”),
how the string value is stored (“<tt class="docutils literal"><span class="pre">KeyType</span></tt>”) and the content of the data for each
hash value.</p>
<div class="section" id="header-layout">
<h5><a class="toc-backref" href="#id58">Header Layout</a><a class="headerlink" href="#header-layout" title="Permalink to this headline">¶</a></h5>
<p>The header has a fixed part, and the specialized part. The exact format of the
header is:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="k">struct</span> <span class="n">Header</span>
<span class="p">{</span>
<span class="kt">uint32_t</span> <span class="n">magic</span><span class="p">;</span> <span class="c1">// 'HASH' magic value to allow endian detection</span>
<span class="kt">uint16_t</span> <span class="n">version</span><span class="p">;</span> <span class="c1">// Version number</span>
<span class="kt">uint16_t</span> <span class="n">hash_function</span><span class="p">;</span> <span class="c1">// The hash function enumeration that was used</span>
<span class="kt">uint32_t</span> <span class="n">bucket_count</span><span class="p">;</span> <span class="c1">// The number of buckets in this hash table</span>
<span class="kt">uint32_t</span> <span class="n">hashes_count</span><span class="p">;</span> <span class="c1">// The total number of unique hash values and hash data offsets in this table</span>
<span class="kt">uint32_t</span> <span class="n">header_data_len</span><span class="p">;</span> <span class="c1">// The bytes to skip to get to the hash indexes (buckets) for correct alignment</span>
<span class="c1">// Specifically the length of the following HeaderData field - this does not</span>
<span class="c1">// include the size of the preceding fields</span>
<span class="n">HeaderData</span> <span class="n">header_data</span><span class="p">;</span> <span class="c1">// Implementation specific header data</span>
<span class="p">};</span>
</pre></div>
</div>
<p>The header starts with a 32 bit “<tt class="docutils literal"><span class="pre">magic</span></tt>” value which must be <tt class="docutils literal"><span class="pre">'HASH'</span></tt>
encoded as an ASCII integer. This allows the detection of the start of the
hash table and also allows the table’s byte order to be determined so the table
can be correctly extracted. The “<tt class="docutils literal"><span class="pre">magic</span></tt>” value is followed by a 16 bit
<tt class="docutils literal"><span class="pre">version</span></tt> number which allows the table to be revised and modified in the
future. The current version number is 1. <tt class="docutils literal"><span class="pre">hash_function</span></tt> is a <tt class="docutils literal"><span class="pre">uint16_t</span></tt>
enumeration that specifies which hash function was used to produce this table.
The current values for the hash function enumerations include:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="k">enum</span> <span class="n">HashFunctionType</span>
<span class="p">{</span>
<span class="n">eHashFunctionDJB</span> <span class="o">=</span> <span class="mi">0u</span><span class="p">,</span> <span class="c1">// Daniel J Bernstein hash function</span>
<span class="p">};</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">bucket_count</span></tt> is a 32 bit unsigned integer that represents how many buckets
are in the <tt class="docutils literal"><span class="pre">BUCKETS</span></tt> array. <tt class="docutils literal"><span class="pre">hashes_count</span></tt> is the number of unique 32 bit
hash values that are in the <tt class="docutils literal"><span class="pre">HASHES</span></tt> array, and is the same number of offsets
are contained in the <tt class="docutils literal"><span class="pre">OFFSETS</span></tt> array. <tt class="docutils literal"><span class="pre">header_data_len</span></tt> specifies the size
in bytes of the <tt class="docutils literal"><span class="pre">HeaderData</span></tt> that is filled in by specialized versions of
this table.</p>
</div>
<div class="section" id="fixed-lookup">
<h5><a class="toc-backref" href="#id59">Fixed Lookup</a><a class="headerlink" href="#fixed-lookup" title="Permalink to this headline">¶</a></h5>
<p>The header is followed by the buckets, hashes, offsets, and hash value data.</p>
<div class="highlight-c"><div class="highlight"><pre><span class="k">struct</span> <span class="n">FixedTable</span>
<span class="p">{</span>
<span class="kt">uint32_t</span> <span class="n">buckets</span><span class="p">[</span><span class="n">Header</span><span class="p">.</span><span class="n">bucket_count</span><span class="p">];</span> <span class="c1">// An array of hash indexes into the "hashes[]" array below</span>
<span class="kt">uint32_t</span> <span class="n">hashes</span> <span class="p">[</span><span class="n">Header</span><span class="p">.</span><span class="n">hashes_count</span><span class="p">];</span> <span class="c1">// Every unique 32 bit hash for the entire table is in this table</span>
<span class="kt">uint32_t</span> <span class="n">offsets</span><span class="p">[</span><span class="n">Header</span><span class="p">.</span><span class="n">hashes_count</span><span class="p">];</span> <span class="c1">// An offset that corresponds to each item in the "hashes[]" array above</span>
<span class="p">};</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">buckets</span></tt> is an array of 32 bit indexes into the <tt class="docutils literal"><span class="pre">hashes</span></tt> array. The
<tt class="docutils literal"><span class="pre">hashes</span></tt> array contains all of the 32 bit hash values for all names in the
hash table. Each hash in the <tt class="docutils literal"><span class="pre">hashes</span></tt> table has an offset in the <tt class="docutils literal"><span class="pre">offsets</span></tt>
array that points to the data for the hash value.</p>
<p>This table setup makes it very easy to repurpose these tables to contain
different data, while keeping the lookup mechanism the same for all tables.
This layout also makes it possible to save the table to disk and map it in
later and do very efficient name lookups with little or no parsing.</p>
<p>DWARF lookup tables can be implemented in a variety of ways and can store a lot
of information for each name. We want to make the DWARF tables extensible and
able to store the data efficiently so we have used some of the DWARF features
that enable efficient data storage to define exactly what kind of data we store
for each name.</p>
<p>The <tt class="docutils literal"><span class="pre">HeaderData</span></tt> contains a definition of the contents of each HashData chunk.
We might want to store an offset to all of the debug information entries (DIEs)
for each name. To keep things extensible, we create a list of items, or
Atoms, that are contained in the data for each name. First comes the type of
the data in each atom:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="k">enum</span> <span class="n">AtomType</span>
<span class="p">{</span>
<span class="n">eAtomTypeNULL</span> <span class="o">=</span> <span class="mi">0u</span><span class="p">,</span>
<span class="n">eAtomTypeDIEOffset</span> <span class="o">=</span> <span class="mi">1u</span><span class="p">,</span> <span class="c1">// DIE offset, check form for encoding</span>
<span class="n">eAtomTypeCUOffset</span> <span class="o">=</span> <span class="mi">2u</span><span class="p">,</span> <span class="c1">// DIE offset of the compiler unit header that contains the item in question</span>
<span class="n">eAtomTypeTag</span> <span class="o">=</span> <span class="mi">3u</span><span class="p">,</span> <span class="c1">// DW_TAG_xxx value, should be encoded as DW_FORM_data1 (if no tags exceed 255) or DW_FORM_data2</span>
<span class="n">eAtomTypeNameFlags</span> <span class="o">=</span> <span class="mi">4u</span><span class="p">,</span> <span class="c1">// Flags from enum NameFlags</span>
<span class="n">eAtomTypeTypeFlags</span> <span class="o">=</span> <span class="mi">5u</span><span class="p">,</span> <span class="c1">// Flags from enum TypeFlags</span>
<span class="p">};</span>
</pre></div>
</div>
<p>The enumeration values and their meanings are:</p>
<div class="highlight-none"><div class="highlight"><pre>eAtomTypeNULL - a termination atom that specifies the end of the atom list
eAtomTypeDIEOffset - an offset into the .debug_info section for the DWARF DIE for this name
eAtomTypeCUOffset - an offset into the .debug_info section for the CU that contains the DIE
eAtomTypeDIETag - The DW_TAG_XXX enumeration value so you don't have to parse the DWARF to see what it is
eAtomTypeNameFlags - Flags for functions and global variables (isFunction, isInlined, isExternal...)
eAtomTypeTypeFlags - Flags for types (isCXXClass, isObjCClass, ...)
</pre></div>
</div>
<p>Then we allow each atom type to define the atom type and how the data for each
atom type data is encoded:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="k">struct</span> <span class="n">Atom</span>
<span class="p">{</span>
<span class="kt">uint16_t</span> <span class="n">type</span><span class="p">;</span> <span class="c1">// AtomType enum value</span>
<span class="kt">uint16_t</span> <span class="n">form</span><span class="p">;</span> <span class="c1">// DWARF DW_FORM_XXX defines</span>
<span class="p">};</span>
</pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">form</span></tt> type above is from the DWARF specification and defines the exact
encoding of the data for the Atom type. See the DWARF specification for the
<tt class="docutils literal"><span class="pre">DW_FORM_</span></tt> definitions.</p>
<div class="highlight-c"><div class="highlight"><pre><span class="k">struct</span> <span class="n">HeaderData</span>
<span class="p">{</span>
<span class="kt">uint32_t</span> <span class="n">die_offset_base</span><span class="p">;</span>
<span class="kt">uint32_t</span> <span class="n">atom_count</span><span class="p">;</span>
<span class="n">Atoms</span> <span class="n">atoms</span><span class="p">[</span><span class="n">atom_count0</span><span class="p">];</span>
<span class="p">};</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">HeaderData</span></tt> defines the base DIE offset that should be added to any atoms
that are encoded using the <tt class="docutils literal"><span class="pre">DW_FORM_ref1</span></tt>, <tt class="docutils literal"><span class="pre">DW_FORM_ref2</span></tt>,
<tt class="docutils literal"><span class="pre">DW_FORM_ref4</span></tt>, <tt class="docutils literal"><span class="pre">DW_FORM_ref8</span></tt> or <tt class="docutils literal"><span class="pre">DW_FORM_ref_udata</span></tt>. It also defines
what is contained in each <tt class="docutils literal"><span class="pre">HashData</span></tt> object – <tt class="docutils literal"><span class="pre">Atom.form</span></tt> tells us how large
each field will be in the <tt class="docutils literal"><span class="pre">HashData</span></tt> and the <tt class="docutils literal"><span class="pre">Atom.type</span></tt> tells us how this data
should be interpreted.</p>
<p>For the current implementations of the “<tt class="docutils literal"><span class="pre">.apple_names</span></tt>” (all functions +
globals), the “<tt class="docutils literal"><span class="pre">.apple_types</span></tt>” (names of all types that are defined), and
the “<tt class="docutils literal"><span class="pre">.apple_namespaces</span></tt>” (all namespaces), we currently set the <tt class="docutils literal"><span class="pre">Atom</span></tt>
array to be:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="n">HeaderData</span><span class="p">.</span><span class="n">atom_count</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span>
<span class="n">HeaderData</span><span class="p">.</span><span class="n">atoms</span><span class="p">[</span><span class="mi">0</span><span class="p">].</span><span class="n">type</span> <span class="o">=</span> <span class="n">eAtomTypeDIEOffset</span><span class="p">;</span>
<span class="n">HeaderData</span><span class="p">.</span><span class="n">atoms</span><span class="p">[</span><span class="mi">0</span><span class="p">].</span><span class="n">form</span> <span class="o">=</span> <span class="n">DW_FORM_data4</span><span class="p">;</span>
</pre></div>
</div>
<p>This defines the contents to be the DIE offset (eAtomTypeDIEOffset) that is
encoded as a 32 bit value (DW_FORM_data4). This allows a single name to have
multiple matching DIEs in a single file, which could come up with an inlined
function for instance. Future tables could include more information about the
DIE such as flags indicating if the DIE is a function, method, block,
or inlined.</p>
<p>The KeyType for the DWARF table is a 32 bit string table offset into the
”.debug_str” table. The ”.debug_str” is the string table for the DWARF which
may already contain copies of all of the strings. This helps make sure, with
help from the compiler, that we reuse the strings between all of the DWARF
sections and keeps the hash table size down. Another benefit to having the
compiler generate all strings as DW_FORM_strp in the debug info, is that
DWARF parsing can be made much faster.</p>
<p>After a lookup is made, we get an offset into the hash data. The hash data
needs to be able to deal with 32 bit hash collisions, so the chunk of data
at the offset in the hash data consists of a triple:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="kt">uint32_t</span> <span class="n">str_offset</span>
<span class="kt">uint32_t</span> <span class="n">hash_data_count</span>
<span class="n">HashData</span><span class="p">[</span><span class="n">hash_data_count</span><span class="p">]</span>
</pre></div>
</div>
<p>If “str_offset” is zero, then the bucket contents are done. 99.9% of the
hash data chunks contain a single item (no 32 bit hash collision):</p>
<div class="highlight-none"><div class="highlight"><pre>.------------.
| 0x00001023 | uint32_t KeyType (.debug_str[0x0001023] => "main")
| 0x00000004 | uint32_t HashData count
| 0x........ | uint32_t HashData[0] DIE offset
| 0x........ | uint32_t HashData[1] DIE offset
| 0x........ | uint32_t HashData[2] DIE offset
| 0x........ | uint32_t HashData[3] DIE offset
| 0x00000000 | uint32_t KeyType (end of hash chain)
`------------'
</pre></div>
</div>
<p>If there are collisions, you will have multiple valid string offsets:</p>
<div class="highlight-none"><div class="highlight"><pre>.------------.
| 0x00001023 | uint32_t KeyType (.debug_str[0x0001023] => "main")
| 0x00000004 | uint32_t HashData count
| 0x........ | uint32_t HashData[0] DIE offset
| 0x........ | uint32_t HashData[1] DIE offset
| 0x........ | uint32_t HashData[2] DIE offset
| 0x........ | uint32_t HashData[3] DIE offset
| 0x00002023 | uint32_t KeyType (.debug_str[0x0002023] => "print")
| 0x00000002 | uint32_t HashData count
| 0x........ | uint32_t HashData[0] DIE offset
| 0x........ | uint32_t HashData[1] DIE offset
| 0x00000000 | uint32_t KeyType (end of hash chain)
`------------'
</pre></div>
</div>
<p>Current testing with real world C++ binaries has shown that there is around 1
32 bit hash collision per 100,000 name entries.</p>
</div>
</div>
<div class="section" id="id4">
<h4><a class="toc-backref" href="#id60">Contents</a><a class="headerlink" href="#id4" title="Permalink to this headline">¶</a></h4>
<p>As we said, we want to strictly define exactly what is included in the
different tables. For DWARF, we have 3 tables: “<tt class="docutils literal"><span class="pre">.apple_names</span></tt>”,
“<tt class="docutils literal"><span class="pre">.apple_types</span></tt>”, and “<tt class="docutils literal"><span class="pre">.apple_namespaces</span></tt>”.</p>
<p>“<tt class="docutils literal"><span class="pre">.apple_names</span></tt>” sections should contain an entry for each DWARF DIE whose
<tt class="docutils literal"><span class="pre">DW_TAG</span></tt> is a <tt class="docutils literal"><span class="pre">DW_TAG_label</span></tt>, <tt class="docutils literal"><span class="pre">DW_TAG_inlined_subroutine</span></tt>, or
<tt class="docutils literal"><span class="pre">DW_TAG_subprogram</span></tt> that has address attributes: <tt class="docutils literal"><span class="pre">DW_AT_low_pc</span></tt>,
<tt class="docutils literal"><span class="pre">DW_AT_high_pc</span></tt>, <tt class="docutils literal"><span class="pre">DW_AT_ranges</span></tt> or <tt class="docutils literal"><span class="pre">DW_AT_entry_pc</span></tt>. It also contains
<tt class="docutils literal"><span class="pre">DW_TAG_variable</span></tt> DIEs that have a <tt class="docutils literal"><span class="pre">DW_OP_addr</span></tt> in the location (global and
static variables). All global and static variables should be included,
including those scoped within functions and classes. For example using the
following code:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="k">static</span> <span class="kt">int</span> <span class="n">var</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
<span class="kt">void</span> <span class="nf">f</span> <span class="p">()</span>
<span class="p">{</span>
<span class="k">static</span> <span class="kt">int</span> <span class="n">var</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Both of the static <tt class="docutils literal"><span class="pre">var</span></tt> variables would be included in the table. All
functions should emit both their full names and their basenames. For C or C++,
the full name is the mangled name (if available) which is usually in the
<tt class="docutils literal"><span class="pre">DW_AT_MIPS_linkage_name</span></tt> attribute, and the <tt class="docutils literal"><span class="pre">DW_AT_name</span></tt> contains the
function basename. If global or static variables have a mangled name in a
<tt class="docutils literal"><span class="pre">DW_AT_MIPS_linkage_name</span></tt> attribute, this should be emitted along with the
simple name found in the <tt class="docutils literal"><span class="pre">DW_AT_name</span></tt> attribute.</p>
<p>“<tt class="docutils literal"><span class="pre">.apple_types</span></tt>” sections should contain an entry for each DWARF DIE whose
tag is one of:</p>
<ul class="simple">
<li>DW_TAG_array_type</li>
<li>DW_TAG_class_type</li>
<li>DW_TAG_enumeration_type</li>
<li>DW_TAG_pointer_type</li>
<li>DW_TAG_reference_type</li>
<li>DW_TAG_string_type</li>
<li>DW_TAG_structure_type</li>
<li>DW_TAG_subroutine_type</li>
<li>DW_TAG_typedef</li>
<li>DW_TAG_union_type</li>
<li>DW_TAG_ptr_to_member_type</li>
<li>DW_TAG_set_type</li>
<li>DW_TAG_subrange_type</li>
<li>DW_TAG_base_type</li>
<li>DW_TAG_const_type</li>
<li>DW_TAG_constant</li>
<li>DW_TAG_file_type</li>
<li>DW_TAG_namelist</li>
<li>DW_TAG_packed_type</li>
<li>DW_TAG_volatile_type</li>
<li>DW_TAG_restrict_type</li>
<li>DW_TAG_interface_type</li>
<li>DW_TAG_unspecified_type</li>
<li>DW_TAG_shared_type</li>
</ul>
<p>Only entries with a <tt class="docutils literal"><span class="pre">DW_AT_name</span></tt> attribute are included, and the entry must
not be a forward declaration (<tt class="docutils literal"><span class="pre">DW_AT_declaration</span></tt> attribute with a non-zero
value). For example, using the following code:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="kt">int</span> <span class="nf">main</span> <span class="p">()</span>
<span class="p">{</span>
<span class="kt">int</span> <span class="o">*</span><span class="n">b</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
<span class="k">return</span> <span class="o">*</span><span class="n">b</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>We get a few type DIEs:</p>
<div class="highlight-none"><div class="highlight"><pre>0x00000067: TAG_base_type [5]
AT_encoding( DW_ATE_signed )
AT_name( "int" )
AT_byte_size( 0x04 )
0x0000006e: TAG_pointer_type [6]
AT_type( {0x00000067} ( int ) )
AT_byte_size( 0x08 )
</pre></div>
</div>
<p>The DW_TAG_pointer_type is not included because it does not have a <tt class="docutils literal"><span class="pre">DW_AT_name</span></tt>.</p>
<p>“<tt class="docutils literal"><span class="pre">.apple_namespaces</span></tt>” section should contain all <tt class="docutils literal"><span class="pre">DW_TAG_namespace</span></tt> DIEs.
If we run into a namespace that has no name this is an anonymous namespace, and
the name should be output as “<tt class="docutils literal"><span class="pre">(anonymous</span> <span class="pre">namespace)</span></tt>” (without the quotes).
Why? This matches the output of the <tt class="docutils literal"><span class="pre">abi::cxa_demangle()</span></tt> that is in the
standard C++ library that demangles mangled names.</p>
</div>
<div class="section" id="language-extensions-and-file-format-changes">
<h4><a class="toc-backref" href="#id61">Language Extensions and File Format Changes</a><a class="headerlink" href="#language-extensions-and-file-format-changes" title="Permalink to this headline">¶</a></h4>
<div class="section" id="objective-c-extensions">
<h5><a class="toc-backref" href="#id62">Objective-C Extensions</a><a class="headerlink" href="#objective-c-extensions" title="Permalink to this headline">¶</a></h5>
<p>“<tt class="docutils literal"><span class="pre">.apple_objc</span></tt>” section should contain all <tt class="docutils literal"><span class="pre">DW_TAG_subprogram</span></tt> DIEs for an
Objective-C class. The name used in the hash table is the name of the
Objective-C class itself. If the Objective-C class has a category, then an
entry is made for both the class name without the category, and for the class
name with the category. So if we have a DIE at offset 0x1234 with a name of
method “<tt class="docutils literal"><span class="pre">-[NSString(my_additions)</span> <span class="pre">stringWithSpecialString:]</span></tt>”, we would add
an entry for “<tt class="docutils literal"><span class="pre">NSString</span></tt>” that points to DIE 0x1234, and an entry for
“<tt class="docutils literal"><span class="pre">NSString(my_additions)</span></tt>” that points to 0x1234. This allows us to quickly
track down all Objective-C methods for an Objective-C class when doing
expressions. It is needed because of the dynamic nature of Objective-C where
anyone can add methods to a class. The DWARF for Objective-C methods is also
emitted differently from C++ classes where the methods are not usually
contained in the class definition, they are scattered about across one or more
compile units. Categories can also be defined in different shared libraries.
So we need to be able to quickly find all of the methods and class functions
given the Objective-C class name, or quickly find all methods and class
functions for a class + category name. This table does not contain any
selector names, it just maps Objective-C class names (or class names +
category) to all of the methods and class functions. The selectors are added
as function basenames in the “<tt class="docutils literal"><span class="pre">.debug_names</span></tt>” section.</p>
<p>In the “<tt class="docutils literal"><span class="pre">.apple_names</span></tt>” section for Objective-C functions, the full name is
the entire function name with the brackets (“<tt class="docutils literal"><span class="pre">-[NSString</span>
<span class="pre">stringWithCString:]</span></tt>”) and the basename is the selector only
(“<tt class="docutils literal"><span class="pre">stringWithCString:</span></tt>”).</p>
</div>
<div class="section" id="mach-o-changes">
<h5><a class="toc-backref" href="#id63">Mach-O Changes</a><a class="headerlink" href="#mach-o-changes" title="Permalink to this headline">¶</a></h5>
<p>The sections names for the apple hash tables are for non mach-o files. For
mach-o files, the sections should be contained in the <tt class="docutils literal"><span class="pre">__DWARF</span></tt> segment with
names as follows:</p>
<ul class="simple">
<li>“<tt class="docutils literal"><span class="pre">.apple_names</span></tt>” -> “<tt class="docutils literal"><span class="pre">__apple_names</span></tt>“</li>
<li>“<tt class="docutils literal"><span class="pre">.apple_types</span></tt>” -> “<tt class="docutils literal"><span class="pre">__apple_types</span></tt>“</li>
<li>“<tt class="docutils literal"><span class="pre">.apple_namespaces</span></tt>” -> “<tt class="docutils literal"><span class="pre">__apple_namespac</span></tt>” (16 character limit)</li>
<li>“<tt class="docutils literal"><span class="pre">.apple_objc</span></tt>” -> “<tt class="docutils literal"><span class="pre">__apple_objc</span></tt>“</li>
</ul>
</div>
</div>
</div>
</div>
</div>
</div>
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