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GCC 5 Release Series &mdash; Changes, New Features, and Fixes
- GNU Project - Free Software Foundation (FSF)</title>
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<!-- GCC maintainers, please do not hesitate to update/contribute entries
     concerning those part of GCC you maintain!  2002-03-23, Gerald.
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<h1>GCC 5 Release Series<br />Changes, New Features, and Fixes</h1>


<h2>Caveats</h2>
  <ul>
    <li>The default mode for C is now <code>-std=gnu11</code> instead of
        <code>-std=gnu89</code>.</li>
    <li>The C++ runtime library (libstdc++) uses a new ABI by default
        (see <a href="#libstdcxx">below</a>).</li>
    <li>The Graphite framework for loop optimizations no longer requires the
	CLooG library, only ISL version 0.14 (recommended) or 0.12.2.  The
        installation manual contains more information about requirements to
        build GCC.</li>
    <li>The non-standard C++0x type traits
        <code>has_trivial_default_constructor</code>,
        <code>has_trivial_copy_constructor</code> and
        <code>has_trivial_copy_assign</code> have been deprecated and will
        be removed in a future version. The standard C++11 traits
        <code>is_trivially_default_constructible</code>,
        <code>is_trivially_copy_constructible</code> and
        <code>is_trivially_copy_assignable</code> should be used instead.</li>
  </ul>

<h2 id="general">General Optimizer Improvements</h2>

  <ul>
    <li>Inter-procedural optimization improvements:
    <ul>
     <li>An Identical Code Folding (ICF) pass (controlled via
	 <code>-fipa-icf</code>) has been added.  Compared to the identical
	 code folding performed by the Gold linker this
	 pass does not require function sections.  It also performs merging
	 before inlining, so inter-procedural optimizations are aware of the
	 code re-use. On the other hand not all unifications performed
	 by a linker are doable by GCC which must honor
	 aliasing information. During link-time optimization of Firefox,
	 this pass unifies about 31000 functions, that is 14% overall.</li>
     <li>The devirtualization pass was significantly improved by adding
	 better support for speculative devirtualization and dynamic type
	 detection. About 50% of virtual calls in Firefox are now
	 speculatively devirtualized during link-time optimization.</li>
     <li>A new comdat localization pass allows the linker to eliminate more
         dead code in presence of C++ inline functions.</li>
     <li>Virtual tables are now optimized. Local aliases are used to reduce
	 dynamic linking time of C++ virtual tables on ELF targets and
	 data alignment has been reduced to limit data segment bloat.</li>
     <li>A new <code>-fno-semantic-interposition</code> option can be used
	 to improve code quality of shared libraries where interposition of
	 exported symbols is not allowed.</li>
     <li>Write-only variables are now detected and optimized out.</li>
     <li>With profile feedback the function inliner can now bypass
	 <code>--param inline-insns-auto</code> and <code>--param
	 inline-insns-single</code> limits for hot calls.</li>
     <li>The IPA reference pass was significantly sped up making it feasible
	 to enable <code>-fipa-reference</code> with
	 <code>-fprofile-generate</code>. This also solves a bottleneck
	 seen when building Chromium with link-time optimization.</li>
     <li>The symbol table and call-graph API was reworked to C++ and
	 simplified.</li>
     <li>The interprocedural propagation of constants now also propagates
         alignments of pointer parameters.  This for example means that the
         vectorizer often does not need to generate loop prologues and epilogues
         to make up for potential misalignments.</li>
    </ul></li>
    <li>Link-time optimization improvements:
    <ul>
      <li>One Definition Rule based merging of C++ types has been implemented.
	  Type merging enables better devirtualization and alias analysis.
	  Streaming extra information needed to merge types adds about 2-6% of
	  memory size and object size increase. This can be controlled by
	  <code>-flto-odr-type-merging</code>.</li>
       <li><p>Command-line optimization and target options are now streamed on
	  a per-function basis and honored by the link-time optimizer.
	  This change makes link-time optimization a more transparent
	  replacement of per-file optimizations.
	  It is now possible to build projects that require
	  different optimization
	  settings for different translation units (such as
	  <code>-ffast-math</code>, <code>-mavx</code>, or
	  <code>-finline</code>).
	  Contrary to earlier GCC releases, the optimization and target
	  options passed on the link command line are ignored.</p>
	  <p>Note that this applies only to those command-line options
	  that can be passed to <code>optimize</code> and
	  <code>target</code> attributes.
	  Command-line options affecting global code generation
	  (such as <code>-fpic</code>), warnings
	  (such as <code>-Wodr</code>),
	  optimizations affecting the way static variables
	  are optimized (such as <code>-fcommon</code>), debug output (such as
	  <code>-g</code>),
	  and <code>--param</code> parameters can be applied only
	  to the whole link-time optimization unit. 
	  In these cases, it is recommended to consistently use the same
	  options at both compile time and link time.</p></li>
      <li>GCC bootstrap now uses slim LTO object files.</li>
      <li>Memory usage and link times were improved.  Tree merging was sped up,
	  memory usage of GIMPLE declarations and types was reduced, and,
	  support for on-demand streaming of variable constructors was added.</li>
    </ul></li>
    <li>Feedback directed optimization improvements:
    <ul>
      <li>A new auto-FDO mode uses profiles collected by low overhead
	  profiling tools (perf) instead of more expensive program
	  instrumentation (via <code>-fprofile-generate</code>). SPEC2006
	  benchmarks on x86-64 improve by 4.7% with auto-FDO and by 7.3% with
	  traditional feedback directed optimization.</li>
      <li>Profile precision was improved in presence of C++ inline and extern
	  inline functions.</li>
      <li>The new <code>gcov-tool</code> utility allows manipulating
          profiles.</li>
      <li>Profiles are now more tolerant to source file changes (this can be
	  controlled by <code>--param profile-func-internal-id</code>).</li>
    </ul></li>
    <li>Register allocation improvements:
    <ul>
      <li>A new local register allocator (LRA) sub-pass, controlled by
          <code>-flra-remat</code>, implements control-flow sensitive
          global register rematerialization.  Instead of spilling and
          restoring a register value, it is recalculated if it is
          profitable.  The sub-pass improved SPEC2000 generated code
          by 1% and 0.5% correspondingly on ARM and x86-64.</li>
      <li>Reuse of the PIC hard register, instead of using a fixed
          register, was implemented on x86/x86-64 targets.  This
          improves generated PIC code performance as more hard
          registers can be used.  Shared libraries can significantly
          benefit from this optimization.  Currently it is switched on
          only for x86/x86-64 targets.  As RA infrastructure is
          already implemented for PIC register reuse, other targets
          might follow this in the future.</li>
      <li>A simple form of inter-procedural RA was implemented.  When
          it is known that a called function does not use caller-saved
          registers, save/restore code is not generated around the
          call for such registers. This optimization can be controlled
          by <code>-fipa-ra</code></li>
      <li>LRA is now much more effective at generating spills of
          general registers into vector registers instead of memory on
          architectures (e.g., modern Intel processors) where this is
          profitable.</li>
    </ul></li>
    <li>UndefinedBehaviorSanitizer gained a few new sanitization options:
    <ul>
      <li><code>-fsanitize=float-divide-by-zero</code>: detect floating-point
	   division by zero;</li>
      <li><code>-fsanitize=float-cast-overflow</code>: check that the result
	   of floating-point type to integer conversions do not overflow;</li>
      <li><code>-fsanitize=bounds</code>: enable instrumentation of array
	  bounds and detect out-of-bounds accesses;</li>
      <li><code>-fsanitize=alignment</code>: enable alignment checking, detect
	  various misaligned objects;</li>
      <li><code>-fsanitize=object-size</code>: enable object size checking, detect
	  various out-of-bounds accesses.</li>
      <li><code>-fsanitize=vptr</code>: enable checking of C++ member function
	  calls, member accesses and some conversions between pointers to base
	  and derived classes, detect if the referenced object does not have
	  the correct dynamic type.</li>
    </ul>
    </li>
    <li>Pointer Bounds Checker, a bounds violation detector, has been added and
      can be enabled via <code>-fcheck-pointer-bounds</code>.  Memory accesses are
      instrumented with run-time checks of used pointers against their bounds to
      detect pointer bounds violations (overflows).  The Pointer Bounds Checker
      is available on x86/x86-64 GNU/Linux targets with a new ISA extension
      Intel MPX support.  See the Pointer Bounds Checker
      <a href="https://gcc.gnu.org/wiki/Intel%20MPX%20support%20in%20the%20GCC%20compiler">
	Wiki page</a> for more details.
    </li>
  </ul>

<h2 id="languages">New Languages and Language specific improvements</h2>
  <ul>
    <li id="offload"><a href="http://www.openmp.org/mp-documents/OpenMP4.0.0.pdf">
      OpenMP 4.0 specification</a> offloading features are now supported by the C, C++,
	and Fortran compilers. Generic changes:
	<ul>
	  <li>Infrastructure (suitable for any vendor).</li>
	  <li>Testsuite which covers offloading from the
	  <a href="http://openmp.org/mp-documents/OpenMP4.0.0.Examples.pdf">
	  OpenMP 4.0 Examples</a> document.</li>
	</ul>
	Specific for upcoming Intel Xeon Phi products:
	<ul>
	  <li>Run-time library.</li>
	  <li>Card emulator.</li>
	</ul>
    </li>
    <li id="openacc">
      GCC 5 includes a preliminary implementation of the OpenACC 2.0a
      specification.  OpenACC is intended for programming accelerator devices
      such as GPUs.  See <a href="https://gcc.gnu.org/wiki/OpenACC">the OpenACC
      wiki page</a> for more information.
    </li>
  </ul>


<!-- h3 id="ada">Ada</h3 -->

<h3 id="c-family">C family</h3>
  <ul>
    <li>The default setting of the <code>-fdiagnostics-color=</code>
        command-line option is now
        <a href="https://gcc.gnu.org/install/configure.html">configurable
        when building GCC</a> using configuration option
        <code>--with-diagnostics-color=</code>. The possible values
        are: <code>never</code>, <code>always</code>, <code>auto</code>
        and <code>auto-if-env</code>.  The new
        default <code>auto</code> uses color only when the
        standard error is a terminal.  The default in GCC 4.9
        was <code>auto-if-env</code>, which is equivalent to
        <code>auto</code> if there is a
        non-empty <code>GCC_COLORS</code> environment variable,
        and <code>never</code> otherwise.  As in GCC 4.9, an empty
        <code>GCC_COLORS</code> variable in the environment will
        always disable colors, no matter what the default is or what
        command-line options are used.</li>
    <li>A new command-line option <code>-Wswitch-bool</code> has been added for
	the C and C++ compilers, which warns whenever a <code>switch</code>
	statement has an index of boolean type.</li>
    <li>A new command-line option <code>-Wlogical-not-parentheses</code> has
	been added for the C and C++ compilers, which warns about "logical not"
	used on the left hand side operand of a comparison.</li>
    <li>A new command-line option <code>-Wsizeof-array-argument</code> has been
	added for the C and C++ compilers, which warns when the
	<code>sizeof</code> operator is applied to a parameter that has been
	declared as an array in a function definition.</li>
    <li>A new command-line option <code>-Wbool-compare</code> has been added
	for the C and C++ compilers, which warns about boolean expressions
	compared with an integer value different from
	<code>true</code>/<code>false</code>.</li>
    <li>Full support for <a href="https://www.cilkplus.org/">Cilk Plus</a>
	has been added to the GCC compiler. Cilk Plus is an extension to
	the C and C++ languages to support data and task parallelism.</li>
    <li>A new attribute <code>no_reorder</code> prevents reordering of
        selected symbols against other such symbols or inline assembler.
        This enables to link-time optimize the Linux kernel without having
        to resort to <code>-fno-toplevel-reorder</code> that disables
        several optimizations.</li>
    <li>New preprocessor constructs, <code>__has_include</code>
        and <code>__has_include_next</code>, to test the availability of headers
        have been added.<br/>
        This demonstrates a way to include the header <code>&lt;optional&gt;</code>
        only if it is available:<br/>
<blockquote><pre>
#ifdef __has_include
#  if __has_include(&lt;optional&gt;)
#    include &lt;optional&gt;
#    define have_optional 1
#  elif __has_include(&lt;experimental/optional&gt;)
#    include &lt;experimental/optional&gt;
#    define have_optional 1
#    define experimental_optional
#  else
#    define have_optional 0
#  endif
#endif
</pre></blockquote>
        The header search paths for <code>__has_include</code>
        and <code>__has_include_next</code> are equivalent to those
        of the standard directive <code>#include</code>
        and the extension <code>#include_next</code> respectively.
    </li>
    <li>A new built-in function-like macro to determine the existence of an
	attribute, <code>__has_attribute</code>, has been added.
	The equivalent built-in macro <code>__has_cpp_attribute</code> was
	added to C++ to support
	<a href="http://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations">
	Feature-testing recommendations for C++</a>.
	The macro <code>__has_attribute</code> is added to all C-like
	languages as an extension:
<blockquote><pre>
int
#ifdef __has_attribute
#  if __has_attribute(__noinline__)
  __attribute__((__noinline__))
#  endif
#endif
foo(int x);
</pre></blockquote>
	If an attribute exists, a nonzero constant integer is returned.
	For standardized C++ attributes a date is returned, otherwise the
	constant returned is 1.
	Both <code>__has_attribute</code> and
	<code>__has_cpp_attribute</code> will add underscores to an
	attribute name if necessary to resolve the name.
	For C++11 and onwards the attribute may be scoped.
    </li>
    <li>A new set of built-in functions for arithmetics with overflow checking
	has been added: <code>__builtin_add_overflow</code>,
	<code>__builtin_sub_overflow</code> and <code>__builtin_mul_overflow</code>
	and for compatibility with clang also other variants.
	These builtins have two integral arguments (which don't need to have
	the same type), the arguments are extended to infinite precision
	signed type, <code>+</code>, <code>-</code> or <code>*</code>
	is performed on those, and the result is stored in an integer
	variable pointed to by the last argument.  If the stored value is
	equal to the infinite precision result, the built-in functions return
	<code>false</code>, otherwise <code>true</code>.  The type of
	the integer variable that will hold the result can be different from
	the types of the first two arguments.  The following snippet
	demonstrates how this can be used in computing the size for the
	<code>calloc</code> function:
<blockquote><pre>
void *
calloc (size_t x, size_t y)
{
  size_t sz;
  if (__builtin_mul_overflow (x, y, &amp;sz))
    return NULL;
  void *ret = malloc (sz);
  if (ret) memset (res, 0, sz);
  return ret;
}
</pre></blockquote>
	On e.g. i?86 or x86-64 the above will result in a <code>mul</code>
	instruction followed by a jump on overflow.
    </li>
    <li>The option <code>-fextended-identifiers</code> is now enabled
	by default for C++, and for C99 and later C versions.  Various
	bugs in the implementation of extended identifiers have been
	fixed.</li>
  </ul>

<h3 id="c">C</h3>
  <ul>
    <li>The default mode has been changed to <code>-std=gnu11</code>.</li>
    <li>A new command-line option <code>-Wc90-c99-compat</code> has been added
	to warn about features not present in ISO C90, but present in ISO
	C99.</li>
    <li>A new command-line option <code>-Wc99-c11-compat</code> has been added
	to warn about features not present in ISO C99, but present in ISO
	C11.</li>
    <li>It is possible to disable warnings about conversions between pointers
	that have incompatible types via a new warning option
	<code>-Wno-incompatible-pointer-types</code>; warnings about implicit
	incompatible integer to pointer and pointer to integer conversions via
	a new warning option <code>-Wno-int-conversion</code>; and warnings
	about qualifiers on pointers being discarded via a new warning option
	<code>-Wno-discarded-qualifiers</code>.</li>
    <li>To allow proper use of const qualifiers with multidimensional arrays,
	GCC will not warn about incompatible pointer types anymore for
	conversions between pointers to arrays with and without const qualifier
	(except when using <code>-pedantic</code>). Instead, a new warning is
	emitted only if the const qualifier is lost. This can be controlled with
	a new warning option <code>-Wno-discarded-array-qualifiers</code>.</li> 
    <li>The C front end now generates more precise caret diagnostics.</li>
    <li>The <code>-pg</code> command-line option now only affects the current
        file in an LTO build.</li>
  </ul>

<h3 id="cxx">C++</h3>
<ul>
  <li>G++ now supports <a href="../projects/cxx1y.html">C++14</a> variable
  templates.</li>
  <li><code>-Wnon-virtual-dtor</code> doesn't warn anymore
  for <code>final</code> classes.</li>
  <li>Excessive template instantiation depth is now a fatal error. This
  prevents excessive diagnostics that usually do not help to identify the
  problem.</li>
  <li>G++ and libstdc++ now implement the feature-testing macros from
    <a href="http://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations">Feature-testing
    recommendations for C++</a>.</li>
  <li>G++ now allows <code>typename</code> in a template template parameter.
    <blockquote><pre>
template&lt;template&lt;typename&gt; <b>typename</b> X&gt; struct D; // OK</pre></blockquote>
  </li>
  <li>G++ now supports <a href="../projects/cxx1y.html">C++14</a> aggregates with non-static data member initializers.
    <blockquote><pre>
struct A { int i, j = i; };
A a = { 42 }; // a.j is also 42</pre></blockquote>
  </li>
  <li>G++ now supports <a href="../projects/cxx1y.html">C++14</a> extended <code>constexpr</code>.
    <blockquote><pre>
constexpr int f (int i)
{
  int j = 0;
  for (; i > 0; --i)
    ++j;
  return j;
}

constexpr int i = f(42); // i is 42</pre></blockquote>
  </li>
  <li>G++ now supports the <a href="../projects/cxx1y.html">C++14</a> sized
  deallocation functions.
    <blockquote><pre>
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;</pre></blockquote>
  </li>
  <li>A new One Definition Rule violation warning (controlled by <code>-Wodr</code>)
      detects mismatches in type definitions and virtual table contents
      during link-time optimization.</li>
  <li>New warnings <code>-Wsuggest-final-types</code> and
      <code>-Wsuggest-final-methods</code> help developers
      to annotate programs with  <code>final</code> specifiers (or anonymous
      namespaces) to improve code generation.
      These warnings can be used at compile time, but they are more
      useful in combination with link-time optimization.</li>
<li>G++ no longer supports
  <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3639.html">N3639</a>
  variable length arrays, as they were removed from the C++14 working paper
  prior to ratification.  GNU VLAs are still supported, so VLA support is
  now the same in C++14 mode as in C++98 and C++11 modes.</li>
<li>G++ now allows passing a non-trivially-copyable class via C varargs,
  which is conditionally-supported with implementation-defined semantics in
  the standard.  This uses the same calling convention as a normal value
  parameter.</li>
<li>G++ now defaults to <code>-fabi-version=0</code>
and <code>-fabi-compat-version=2</code>.  So various mangling bugs are
fixed, but G++ will still emit aliases with the old, wrong mangling where
feasible.  <code>-Wabi</code> continues to warn about differences.</li>
</ul>

  <h4 id="libstdcxx">Runtime Library (libstdc++)</h4>
  <ul>
    <li>A <a href="/onlinedocs/gcc-5.1.0/libstdc++/manual/using_dual_abi.html">Dual
        ABI</a> is provided by the library. A new ABI is enabled by default.
        The old ABI is still supported and can be used by defining the macro
        <code>_GLIBCXX_USE_CXX11_ABI</code> to <code>0</code> before
        including any C++ standard library headers. </li>
    <li>A new implementation of <code>std::string</code> is enabled by default,
        using the <em>small string optimization</em> instead of
        <em>copy-on-write</em> reference counting.</li>
    <li> A new implementation of <code>std::list</code> is enabled by default,
        with an O(1) <code>size()</code> function; </li>
    <li><a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/libstdc++/manual/status.html#status.iso.2011">
      Full support for C++11</a>, including the following new features:
      <ul>
        <li> <code>std::deque</code> and <code>std::vector&lt;bool&gt;</code>
             meet the allocator-aware container requirements;</li>
        <li> movable and swappable iostream classes;</li>
        <li> support for <code>std::align</code> and
             <code>std::aligned_union</code>;</li>
        <li> type traits <code>std::is_trivially_copyable</code>,
             <code>std::is_trivially_constructible</code>,
             <code>std::is_trivially_assignable</code> etc.;
        </li>
        <li> I/O manipulators <code>std::put_time</code>, <code>std::get_time</code>,
             <code>std::hexfloat</code> and <code>std::defaultfloat</code>;
        </li>
        <li> generic locale-aware <code>std::isblank</code>; </li>
        <li> locale facets for Unicode conversion; </li>
        <li> atomic operations for <code>std::shared_ptr</code>; </li>
        <li> <code>std::notify_all_at_thread_exit()</code> and functions
          for making futures ready at thread exit.</li>
      </ul>
    </li>
    <li> Support for the C++11 <code>hexfloat</code> manipulator changes how
      the <code>num_put</code> facet formats floating point types when
      <code>ios_base::fixed|ios_base::scientific</code> is set in a stream's
      <code>fmtflags</code>. This change affects all language modes, even
      though the C++98 standard gave no special meaning to that combination
      of flags. To prevent the use of hexadecimal notation for floating point
      types use <code>str.unsetf(std::ios_base::floatfield)</code> to clear
      the relevant bits in <code>str.flags()</code>.
    </li>
    <li><a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/libstdc++/manual/status.html#status.iso.2014">
      Full experimental support for C++14</a>, including the following
      new features:
      <ul>
        <li> <code>std::is_final</code> type trait; </li>
        <li> heterogeneous comparison lookup in associative containers. </li>
        <li> global functions <code>cbegin</code>, <code>cend</code>, <code>rbegin</code>,
        <code>rend</code>, <code>crbegin</code>, and <code>crend</code> for
        range access to containers, arrays and initializer lists. </li>
      </ul>
    </li>
    <li><a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/libstdc++/manual/status.html#status.iso.2014">
      Improved experimental support for the Library Fundamentals TS</a>, including:
      <ul>
        <li> class <code>std::experimental::any</code>; </li>
        <li> function template <code>std::experimental::apply</code>; </li>
        <li> function template <code>std::experimental::sample</code>; </li>
        <li> function template <code>std::experimental::search</code> and
          related searcher types; </li>
        <li> variable templates for type traits; </li>
        <li> function template <code>std::experimental::not_fn</code>.</li>
      </ul>
    </li>
    <li>New random number distributions <code>logistic_distribution</code> and
      <code>uniform_on_sphere_distribution</code> as extensions.</li>
    <li><a href="https://sourceware.org/gdb/current/onlinedocs/gcc-5.1.0/gdb/Xmethods-In-Python.html">GDB
      Xmethods</a> for containers and <code>std::unique_ptr</code>.</li>
  </ul>

<h3 id="fortran">Fortran</h3>
  <ul>
    <li>Compatibility notice:<ul>
      <li>The version of the module files (.mod) has been incremented.</li>
      <li>For free-form source files,
        <a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/gfortran/Error-and-Warning-Options.html"><code>-Werror=line-truncation</code></a>
        is now enabled by default; note that comments exceeding the line length
        are not diagnosed.  (For fixed-form source code, the same warning is
        available but turned off by default, such that excess characters are
        ignored. <code>-ffree-line-length-<em>n</em></code> and
        <code>-ffixed-line-length-<em>n</em></code> can be used to modify the
        default line lengths of 132 and 72 columns, respectively.)</li>
      <li>The <code>-Wtabs</code> option is now more sensible: with
        <code>-Wtabs</code> the compiler warns if it encounters tabs and with
        <code>-Wno-tabs</code> this warning is turned off.  Before,
        <code>-Wno-tabs</code> warned and <code>-Wtabs</code> turned the warning
        off.  As before, the warning is also enabled by <code>-Wall</code>,
        <code>-pedantic</code> and the <code>f95</code>, <code>f2003</code>,
        <code>f2008</code> and <code>f2008ts</code> options of <code>-std=</code>.</li>
      </ul></li>
    <li>Incomplete support for colorizing diagnostics emitted by
      gfortran has been added. The
      option <code><a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/gcc/Language-Independent-Options.html"
      >-fdiagnostics-color</a></code> controls when color is used in
      diagnostics. The default value of this option can
      be <a href="https://gcc.gnu.org/install/configure.html">configured
      when building GCC</a>. The <code>GCC_COLORS</code> environment
      variable can be used to customize the colors or disable coloring
      completely. Sample diagnostics output:<br/>
      <pre>
      $ gfortran -fdiagnostics-color=always -Wuse-without-only test.f90
      <b>test.f90:6:1:</b>

       0 continue
       <b style='color:lime'>1</b>
      <b style='color:red'>Error:</b> Zero is not a valid statement label at (1)    
      <b>test.f90:9:6:</b>

         USE foo
            <b style='color:lime'>1</b>
      <b style='color:magenta'>Warning:</b> USE statement at (1) has no ONLY qualifier [-Wuse-without-only]
    </pre></li>

    <li>The <code>-Wuse-without-only</code> option has been added to warn when a
      <code>USE</code> statement has no <code>ONLY</code> qualifier and, thus,
      implicitly imports all public entities of the used module.</li>
    <li>Formatted READ and WRITE statements now work correctly in locale-aware
      programs.  For more information and potential caveats, see
      <a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/gfortran/Thread-safety-of-the-runtime-library.html">Section
      5.3 Thread-safety of the runtime library in the manual</a>.</li>
    <li><a href="https://gcc.gnu.org/wiki/Fortran2003Status">Fortran 2003</a>:
    <ul>
      <li>The intrinsic IEEE modules (<code>IEEE_FEATURES</code>,
	<code>IEEE_EXCEPTIONS</code> and <code>IEEE_ARITHMETIC</code>) are
	now supported.</li>
    </ul></li>
    <li><a href="https://gcc.gnu.org/wiki/Fortran2008Status">Fortran 2008</a>:
    <ul>
      <li><a href="https://gcc.gnu.org/wiki/Coarray">Coarrays</a>: Full
	experimental support of Fortran 2008's coarrays with
	<code>-fcoarray=lib</code> except for allocatable/pointer
	components of derived-type coarrays.  GCC currently only ships with a
	single-image library (<code>libcaf_single</code>), but
	multi-image support based on MPI and GASNet is provided by the libraries
	of the <a href="http://www.opencoarrays.org/">OpenCoarrays project</a>. 
	</li>
    </ul></li>
    <li>TS18508 Additional Parallel Features in Fortran:
    <ul>
      <li>Support for the collective intrinsic subroutines <code>CO_MAX</code>,
	<code>CO_MIN</code>, <code>CO_SUM</code>, <code>CO_BROADCAST</code> and
	<code>CO_REDUCE</code> has been added, including
	<code>-fcoarray=lib</code> support.</li>
      <li>Support for the new atomic intrinsics has been added, including
	<code>-fcoarray=lib</code> support.</li>
    </ul></li>
    <li>Fortran 2015:
    <ul>
      <li>Support for <code>IMPLICIT NONE (external, type)</code>.</li>
      <li><code>ERROR STOP</code> is now permitted in pure procedures.</li>
    </ul></li>
  </ul>

<h3 id="go">Go</h3>
  <ul>
    <li>GCC 5 provides a complete implementation of the Go 1.4.2
    release.</li>
    <li>Building GCC 5 with Go enabled will install two new
      programs: <a href="http://golang.org/cmd/go">go</a>
      and <a href="http://golang.org/cmd/gofmt">gofmt</a>.</li>
  </ul>

<!--h3 id="java">Java (GCJ)</h3-->

<h2 id="jit">libgccjit</h2>
<p>New in GCC 5 is the ability to build GCC as a shared library for embedding
in other processes (such as interpreters), suitable for Just-In-Time
compilation to machine code.</p>

<p>The shared library has a <a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/jit/intro/index.html">C API</a>
and a
<a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/jit/cp/index.html">C++ wrapper API</a>
providing some "syntactic sugar".
There are also bindings available from 3rd parties for
<a href="https://github.com/davidmalcolm/pygccjit">Python</a> and for
<a href="https://github.com/ibuclaw/gccjitd">D</a>.</p>

<p>For example, this library can be used by interpreters for
<a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/jit/intro/tutorial04.html">compiling
functions from bytecode to machine code</a>.</p>

<p>The library can also be used for ahead-of-time compilation, enabling
GCC to be plugged into a pre-existing frontend.  An example of using
this to build a compiler for an esoteric language we'll refer to as "brainf"
can be seen <a href="https://gcc.gnu.org/onlinedocs/gcc-5.1.0/jit/intro/tutorial05.html">
here</a>.</p>

<p>libgccjit is licensed under the GPLv3 (or at your option, any later version)</p>

<p>It should be regarded as experimental at this time.</p>

<h2 id="targets">New Targets and Target Specific Improvements</h2>

<h3 id="aarch64">AArch64</h3>
   <ul>
     <li>Code generation for the ARM Cortex-A57 processor has been improved.
       A more accurate instruction scheduling model for the processor is
       now used, and a number of compiler tuning parameters have been set
       to offer increased performance when compiling with
       <code>-mcpu=cortex-a57</code> or <code>-mtune=cortex-a57</code>.
     </li>
     <li> A workaround for the ARM Cortex-A53 erratum 835769 has been
       added and can be enabled by giving the
       <code>-mfix-cortex-a53-835769</code> option.
       Alternatively it can be enabled by default by configuring GCC with the
       <code>--enable-fix-cortex-a53-835769</code> option.
     </li>
     <li> The optional cryptographic extensions to the ARMv8-A architecture
       are no longer enabled by default when specifying the
       <code>-mcpu=cortex-a53</code>, <code>-mcpu=cortex-a57</code> or
       <code>-mcpu=cortex-a57.cortex-a53</code> options.  To enable these
       extensions add <code>+crypto</code> to the value of
       <code>-mcpu</code> or <code>-march</code> e.g.
       <code>-mcpu=cortex-a53+crypto</code>.
     </li>
     <li> Support has been added for the following processors
      (GCC identifiers in parentheses): ARM Cortex-A72
      (<code>cortex-a72</code>) and initial support for its big.LITTLE
      combination with the ARM Cortex-A53 (<code>cortex-a72.cortex-a53</code>),
      Cavium ThunderX (<code>thunderx</code>), Applied Micro X-Gene 1
      (<code>xgene1</code>), and Samsung Exynos M1 (<code>exynos-m1</code>).
      The GCC identifiers can be used
      as arguments to the <code>-mcpu</code> or <code>-mtune</code> options,
      for example: <code>-mcpu=xgene1</code> or
      <code>-mtune=cortex-a72.cortex-a53</code>.
      Using <code>-mcpu=cortex-a72</code> requires a version of GNU binutils
      that has support for the Cortex-A72.
     </li>
     <li>The transitional options <code>-mlra</code> and <code>-mno-lra</code>
       have been removed. The AArch64 backend now uses the local register
       allocator (LRA) only.
     </li>
   </ul>

<h3 id="arm">ARM</h3>
     <ul>
      <li>Thumb-1 assembly code is now generated in unified syntax. The new option
        <code>-masm-syntax-unified</code> specifies whether inline assembly
        code is using unified syntax. By default the option is off which means
        non-unified syntax is used. However this is subject to change in future releases.
        Eventually the non-unified syntax will be deprecated.
      </li>
      <li> It is now a configure-time error to use the <code>--with-cpu</code>
      configure option with either of <code>--with-tune</code> or
      <code>--with-arch</code>.
      </li>
      <li>Code generation for the ARM Cortex-A57 processor has been improved.
       A more accurate instruction scheduling model for the processor is
       now used, and a number of compiler tuning parameters have been set
       to offer increased performance when compiling with
       <code>-mcpu=cortex-a57</code> or <code>-mtune=cortex-a57</code>.
      </li>
      <li> Support has been added for the following processors
       (GCC identifiers in parentheses): ARM Cortex-A17 (<code>cortex-a17</code>) and
       initial support for its big.LITTLE combination with the ARM Cortex-A7
       (<code>cortex-a17.cortex-a7</code>), ARM Cortex-A72
       (<code>cortex-a72</code>) and initial support for its big.LITTLE
       combination with the ARM Cortex-A53 (<code>cortex-a72.cortex-a53</code>),
       ARM Cortex-M7 (<code>cortex-m7</code>), Applied Micro X-Gene 1
       (<code>xgene1</code>), and Samsung Exynos M1 (<code>exynos-m1</code>).
       The GCC identifiers can be used
       as arguments to the <code>-mcpu</code> or <code>-mtune</code> options,
       for example: <code>-mcpu=xgene1</code> or
       <code>-mtune=cortex-a72.cortex-a53</code>.
       Using <code>-mcpu=cortex-a72</code> requires a version of GNU binutils
       that has support for the Cortex-A72.
      </li>
      <li> The deprecated option <code>-mwords-little-endian</code>
       has been removed.
      </li>
      <li> The options <code>-mapcs</code>, <code>-mapcs-frame</code>,
      <code>-mtpcs-frame</code> and <code>-mtpcs-leaf-frame</code>
      which are only applicable to the old ABI have been deprecated.
      </li>
      <li>The transitional options <code>-mlra</code> and <code>-mno-lra</code>
       have been removed. The ARM backend now uses the local register allocator
       (LRA) only.
     </li>
     </ul>

<h3 id="x86">IA-32/x86-64</h3>
  <ul>
    <li>New ISA extensions support
	<a href="https://software.intel.com/sites/default/files/managed/0d/53/319433-022.pdf">
	AVX-512{BW,DQ,VL,IFMA,VBMI}</a> of Intel's CPU
	codenamed Skylake Server was added to GCC.  That includes inline
	assembly support, new intrinsics, and basic autovectorization.  These
	new AVX-512 extensions are available via
	the following GCC switches: AVX-512 Vector Length EVEX feature:
	<code>-mavx512vl</code>, AVX-512 Byte and Word instructions:
	<code>-mavx512bw</code>, AVX-512 Dword and Qword instructions:
	<code>-mavx512dq</code>, AVX-512 FMA-52 instructions:
	<code>-mavx512ifma</code> and for AVX-512 Vector Bit Manipulation
	Instructions: <code>-mavx512vbmi</code>.</li>
	<li>New ISA extensions support
	<a href="https://software.intel.com/sites/default/files/managed/0d/53/319433-022.pdf">
	Intel MPX</a> was added to GCC.  This new extension is available via the
	<code>-mmpx</code> compiler switch.  Intel MPX is a set of processor features which,
	with compiler, run-time library and OS support, brings increased robustness to
	software by run-time checking pointer references against their bounds.
	In GCC Intel MPX is supported by Pointer Bounds Checker and libmpx run-time
	libraries.</li>
	<li>The new <code>-mrecord-mcount</code> option for <code>-pg</code>
	generates a Linux kernel style table of pointers to
	<code>mcount</code> or <code>__fentry__</code> calls at the beginning
	of functions. The new <code>-mnop-mcount</code> option in addition
	also generates nops in place of the <code>__fentry__</code> or
	<code>mcount</code> call, so that a call per function can be later
	patched in. This can be used for low overhead tracing or hot code
	patching.</li>
	<li>The new <code>-malign-data</code> option controls how
	GCC aligns variables.  <code>-malign-data=compat</code> uses
	increased alignment compatible with GCC 4.8 and earlier,
	<code>-malign-data=abi</code> uses alignment as specified by
	the psABI, and <code>-malign-data=cacheline</code> uses increased
	alignment to match the cache line size.
	<code>-malign-data=compat</code> is the default.</li>
	<li>The new <code>-mskip-rax-setup</code> option skips setting
	up the RAX register when SSE is disabled and there are no variable
	arguments passed in vector registers.  This can be used to
	optimize the Linux kernel.</li>
  </ul>

<h3 id="mips">MIPS</h3>
  <ul>
    <li>MIPS Releases 3 and 5 are now directly supported.  Use the
      command-line options <code>-mips32r3</code>, <code>-mips64r3</code>,
      <code>-mips32r5</code> and <code>-mips64r5</code> to enable
      code-generation for these processors.</li>
    <li>The Imagination P5600 processor is now supported using the
      <code>-march=p5600</code> command-line option.</li>
    <li>The Cavium Octeon3 processor is now supported using the
      <code>-march=octeon3</code> command-line option.</li>
    <li>MIPS Release 6 is now supported using the <code>-mips32r6</code>
       and <code>-mips64r6</code> command-line options.</li>
    <li>The o32 ABI has been modified and extended.  The o32 64-bit
      floating-point register support is now obsolete and has been removed.
      It has been replaced by three ABI extensions FPXX, FP64A, and FP64.
      The meaning of the <code>-mfp64</code> command-line option has
      changed.  It is now used to enable the FP64A and FP64 ABI extensions.
      <ul>
	<li>The FPXX extension requires that code generated to access
	    double-precision values use even-numbered registers.  Code that
	    adheres to this extension is link-compatible with all other o32
	    double-precision ABI variants and will execute correctly in all
	    hardware FPU modes.  The command-line options <code>-mabi=32 
	    -mfpxx</code> can be used to enable this extension.  MIPS II is
	    the minimum processor required.</li>
	<li>The o32 FP64A extension requires that floating-point registers be
	    64-bit and odd-numbered single-precision registers are not
	    allowed.  Code that adheres to the o32 FP64A variant is
	    link-compatible with all other o32 double-precision ABI variants.
	    The command-line options <code>-mabi=32 -mfp64 -mno-odd-spreg
	    </code> can be used to enable this extension.  MIPS32R2 is the
	    minimum processor required.</li>
	<li>The o32 FP64 extension also requires that floating-point registers
	    be 64-bit, but permits the use of single-precision registers.
	    Code that adheres to the o32 FP64 variant is link-compatible with
	    o32 FPXX and o32 FP64A variants only, i.e. it is not compatible
	    with the original o32 double-precision ABI. The command-line
	    options <code>-mabi=32 -mfp64 -modd-spreg</code> can be used to
	    enable this extension.  MIPS32R2 is the minimum processor required.
	    </li>
      </ul>
      The new ABI variants can be enabled by default using the configure time
      options <code>--with-fp-32=[32|xx|64]</code> and
      <code>--with(out)-odd-sp-reg-32</code>.  It is strongly recommended that
      all vendors begin to set o32 FPXX as the default ABI.  This will be
      required to run the generated code on MIPSR5 cores in conjunction with
      future MIPS SIMD (MSA) code and MIPSR6 cores.</li>
    <li>GCC will now pass all floating-point options to the assembler if GNU
      binutils 2.25 is used.  As a result, any inline assembly code that
      uses hard-float instructions should be amended to include a
      <code>.set</code> directive to override the global assembler options
      when compiling for soft-float targets.</li>
  </ul>

<h3 id="nds32">NDS32</h3>
  <ul>
    <li>The variadic function ABI implementation is now compatible with
    past Andes toolchains where the caller uses registers to pass arguments
    and the callee is in charge of pushing them on stack.</li>
    <li>The options <code>-mforce-fp-as-gp</code>, <code>-mforbid-fp-as-gp</code>,
    and <code>-mex9</code> have been removed since they are not yet available
    in the nds32 port of GNU binutils.</li>
    <li>A new option <code>-mcmodel=[small|medium|large]</code> supports
    varied code models on code generation.  The <code>-mgp-direct</code>
    option became meaningless and can be discarded.</li>
  </ul>

<h3 id="rx">RX</h3>
  <ul>
    <li>A new command line option <code>-mno-allow-string-insns</code> can be
    used to disable the generation of the <code>SCMPU</code>, <code>SMOVU</code>,
    <code>SMOVB</code>, <code>SMOVF</code>, <code>SUNTIL</code>, <code>SWHILE</code>
    and <code>RMPA</code> instructions.  An erratum released by Renesas shows
    that it is unsafe to use these instructions on addresses within the I/O
    space of the processor.  The new option can be used when the programmer is
    concerned that the I/O space might be accessed.  The default is still to
    enable these instructions.</li>
  </ul>

<h3 id="sh">SH</h3>
  <ul>
    <li>The compiler will now pass the appropriate <code>--isa=</code> option
    to the assembler.</li>

    <li>The default handling for the <code>GBR</code> has been changed from
    call clobbered to call preserved.  The old behavior can be reinstated by
    specifying the option <code>-fcall-used-gbr</code>.</li>

    <li>Support for the SH4A <code>fpchg</code> instruction has been added which
    will be utilized when switching between single and double precision FPU
    modes.</li>

    <li>The compiler no longer uses the <code>__fpscr_values</code> array for
    switching between single and double FPU precision modes on non-SH4A targets.
    Instead mode switching will now be performed by storing, modifying and
    reloading the <code>FPSCR</code>, so that other <code>FPSCR</code> bits are
    preserved across mode switches.   The <code>__fpscr_values</code> array that
    is defined in libgcc is still present for backwards compatibility, but it
    will not be referenced by compiler generated code anymore.</li>

    <li>New builtin functions <code>__builtin_sh_get_fpscr</code> and
    <code>__builtin_sh_set_fpscr</code> have been added.  The
    <code>__builtin_sh_set_fpscr</code> function will mask the specified bits
    in such a way that the <code>SZ</code>, <code>PR</code> and <code>FR</code>
    mode bits will be preserved, while changing the other bits.  These new
    functions do not reference the <code>__fpscr_values</code> array.  The old
    functions <code>__set_fpscr</code> and <code>__get_fpscr</code> in libgcc
    which access the <code>__fpscr_values</code> array are still present for
    backwards compatibility, but their usage is highly discouraged.</li>

    <li>Some improvements to code generated for <code>__atomic</code> built-in
    functions.</li>

    <li>When compiling for SH2E the compiler will no longer force the usage of
    delay slots for conditional branch instructions <code>bt</code> and
    <code>bf</code>.  The old behavior can be reinstated (e.g. to work around a
    hardware bug in the original SH7055) by specifying the new option
    <code>-mcbranch-force-delay-slot</code>.</li>
  </ul>

<h2 id="os">Operating Systems</h2>

  <h3 id="dragonfly">DragonFly BSD</h3>

  <ul>
    <li>GCC now supports the DragonFly BSD operating system.</li>
  </ul>

  <h3 id="freebsd">FreeBSD</h3>

  <ul>
    <li>GCC now supports the FreeBSD operating system for the arm port
    through the <code>arm*-*-freebsd*</code> target triplets.</li>
  </ul>

  <h3 id="vxmils">VxWorks MILS</h3>

  <ul>
    <li>GCC now supports the MILS (Multiple Independent Levels
    of Security) variant of WindRiver's VxWorks operating system
    for PowerPC targets.</li>
  </ul>

<!-- h2>Documentation improvements</h2-->
<h2>Other significant improvements</h2>
  <h3 id="gcc-ar"></h3>
  <ul>
    <li>The <code>gcc-ar</code>, <code>gcc-nm</code>, <code>gcc-ranlib</code>
      wrappers now understand a <code>-B</code> option to set the compiler
      to use.</li>
  </ul>
  <h3 id="driver"></h3>
  <ul>
    <li>When the new command-line option <code>-freport-bug</code> is
      used, GCC automatically generates a developer-friendly reproducer
      whenever an internal compiler error is encountered.</li>
  </ul>



<!-- ==================================================================== -->

<div class="copyright">

<address style="margin-top:0;">For questions related to the use of GCC,
please consult these web pages and the
<a href="https://gcc.gnu.org/onlinedocs/">GCC manuals</a>. If that fails,
the <a href="mailto:gcc-help@gcc.gnu.org">gcc-help@gcc.gnu.org</a>
mailing list might help.
Comments on these web pages and the development of GCC are welcome on our
developer list at <a href="mailto:gcc@gcc.gnu.org">gcc@gcc.gnu.org</a>.
All of <a href="https://gcc.gnu.org/lists.html">our lists</a>
have public archives.
</address>

<p>Copyright (C)
<a href="http://www.fsf.org">Free Software Foundation, Inc.</a>
Verbatim copying and distribution of this entire article is
permitted in any medium, provided this notice is preserved.</p>

<p style="margin-bottom:0;">These pages are
<a href="https://gcc.gnu.org/about.html">maintained by the GCC team</a>.
Last modified 2015-04-22<!-- IGNORE DIFF
--><a href="http://validator.w3.org/check/referer">.</a></p>

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gcc-5 5.3.1-14ubuntu2 / usr / share / doc / gcc-5-base / NEWS.html
