/usr/share/doc/llvm-3.7-doc/html/BranchWeightMetadata.html is in llvm-3.7-doc 1:3.7.1-5ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 | <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>LLVM Branch Weight Metadata — LLVM 3.7 documentation</title>
<link rel="stylesheet" href="_static/llvm-theme.css" type="text/css" />
<link rel="stylesheet" href="_static/pygments.css" type="text/css" />
<script type="text/javascript">
var DOCUMENTATION_OPTIONS = {
URL_ROOT: './',
VERSION: '3.7',
COLLAPSE_INDEX: false,
FILE_SUFFIX: '.html',
HAS_SOURCE: true,
SOURCELINK_SUFFIX: '.txt'
};
</script>
<script type="text/javascript" src="_static/jquery.js"></script>
<script type="text/javascript" src="_static/underscore.js"></script>
<script type="text/javascript" src="_static/doctools.js"></script>
<link rel="index" title="Index" href="genindex.html" />
<link rel="search" title="Search" href="search.html" />
<link rel="next" title="LLVM bugpoint tool: design and usage" href="Bugpoint.html" />
<link rel="prev" title="LLVM Block Frequency Terminology" href="BlockFrequencyTerminology.html" />
<style type="text/css">
table.right { float: right; margin-left: 20px; }
table.right td { border: 1px solid #ccc; }
</style>
</head>
<body role="document">
<div class="logo">
<a href="index.html">
<img src="_static/logo.png"
alt="LLVM Logo" width="250" height="88"/></a>
</div>
<div class="related" role="navigation" aria-label="related navigation">
<h3>Navigation</h3>
<ul>
<li class="right" style="margin-right: 10px">
<a href="genindex.html" title="General Index"
accesskey="I">index</a></li>
<li class="right" >
<a href="Bugpoint.html" title="LLVM bugpoint tool: design and usage"
accesskey="N">next</a> |</li>
<li class="right" >
<a href="BlockFrequencyTerminology.html" title="LLVM Block Frequency Terminology"
accesskey="P">previous</a> |</li>
<li><a href="http://llvm.org/">LLVM Home</a> | </li>
<li><a href="index.html">Documentation</a>»</li>
</ul>
</div>
<div class="document">
<div class="documentwrapper">
<div class="body" role="main">
<div class="section" id="llvm-branch-weight-metadata">
<h1>LLVM Branch Weight Metadata<a class="headerlink" href="#llvm-branch-weight-metadata" 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="id1">Introduction</a></li>
<li><a class="reference internal" href="#supported-instructions" id="id2">Supported Instructions</a><ul>
<li><a class="reference internal" href="#branchinst" id="id3"><code class="docutils literal"><span class="pre">BranchInst</span></code></a></li>
<li><a class="reference internal" href="#switchinst" id="id4"><code class="docutils literal"><span class="pre">SwitchInst</span></code></a></li>
<li><a class="reference internal" href="#indirectbrinst" id="id5"><code class="docutils literal"><span class="pre">IndirectBrInst</span></code></a></li>
<li><a class="reference internal" href="#other" id="id6">Other</a></li>
</ul>
</li>
<li><a class="reference internal" href="#built-in-expect-instructions" id="id7">Built-in <code class="docutils literal"><span class="pre">expect</span></code> Instructions</a><ul>
<li><a class="reference internal" href="#if-statement" id="id8"><code class="docutils literal"><span class="pre">if</span></code> statement</a></li>
<li><a class="reference internal" href="#switch-statement" id="id9"><code class="docutils literal"><span class="pre">switch</span></code> statement</a></li>
</ul>
</li>
<li><a class="reference internal" href="#cfg-modifications" id="id10">CFG Modifications</a></li>
<li><a class="reference internal" href="#function-entry-counts" id="id11">Function Entry Counts</a></li>
</ul>
</div>
<div class="section" id="introduction">
<h2><a class="toc-backref" href="#id1">Introduction</a><a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
<p>Branch Weight Metadata represents branch weights as its likeliness to be taken
(see <a class="reference internal" href="BlockFrequencyTerminology.html"><span class="doc">LLVM Block Frequency Terminology</span></a>). Metadata is assigned to the
<code class="docutils literal"><span class="pre">TerminatorInst</span></code> as a <code class="docutils literal"><span class="pre">MDNode</span></code> of the <code class="docutils literal"><span class="pre">MD_prof</span></code> kind. The first operator
is always a <code class="docutils literal"><span class="pre">MDString</span></code> node with the string “branch_weights”. Number of
operators depends on the terminator type.</p>
<p>Branch weights might be fetch from the profiling file, or generated based on
<a class="reference internal" href="#builtin-expect">__builtin_expect</a> instruction.</p>
<p>All weights are represented as an unsigned 32-bit values, where higher value
indicates greater chance to be taken.</p>
</div>
<div class="section" id="supported-instructions">
<h2><a class="toc-backref" href="#id2">Supported Instructions</a><a class="headerlink" href="#supported-instructions" title="Permalink to this headline">¶</a></h2>
<div class="section" id="branchinst">
<h3><a class="toc-backref" href="#id3"><code class="docutils literal"><span class="pre">BranchInst</span></code></a><a class="headerlink" href="#branchinst" title="Permalink to this headline">¶</a></h3>
<p>Metadata is only assigned to the conditional branches. There are two extra
operarands for the true and the false branch.</p>
<div class="highlight-llvm"><div class="highlight"><pre><span></span>!0 = metadata !{
metadata !"branch_weights",
i32 <TRUE_BRANCH_WEIGHT>,
i32 <FALSE_BRANCH_WEIGHT>
}
</pre></div>
</div>
</div>
<div class="section" id="switchinst">
<h3><a class="toc-backref" href="#id4"><code class="docutils literal"><span class="pre">SwitchInst</span></code></a><a class="headerlink" href="#switchinst" title="Permalink to this headline">¶</a></h3>
<p>Branch weights are assigned to every case (including the <code class="docutils literal"><span class="pre">default</span></code> case which
is always case #0).</p>
<div class="highlight-llvm"><div class="highlight"><pre><span></span>!0 = metadata !{
metadata !"branch_weights",
i32 <DEFAULT_BRANCH_WEIGHT>
[ , i32 <CASE_BRANCH_WEIGHT> ... ]
}
</pre></div>
</div>
</div>
<div class="section" id="indirectbrinst">
<h3><a class="toc-backref" href="#id5"><code class="docutils literal"><span class="pre">IndirectBrInst</span></code></a><a class="headerlink" href="#indirectbrinst" title="Permalink to this headline">¶</a></h3>
<p>Branch weights are assigned to every destination.</p>
<div class="highlight-llvm"><div class="highlight"><pre><span></span>!0 = metadata !{
metadata !"branch_weights",
i32 <LABEL_BRANCH_WEIGHT>
[ , i32 <LABEL_BRANCH_WEIGHT> ... ]
}
</pre></div>
</div>
</div>
<div class="section" id="other">
<h3><a class="toc-backref" href="#id6">Other</a><a class="headerlink" href="#other" title="Permalink to this headline">¶</a></h3>
<p>Other terminator instructions are not allowed to contain Branch Weight Metadata.</p>
</div>
</div>
<div class="section" id="built-in-expect-instructions">
<span id="builtin-expect"></span><h2><a class="toc-backref" href="#id7">Built-in <code class="docutils literal"><span class="pre">expect</span></code> Instructions</a><a class="headerlink" href="#built-in-expect-instructions" title="Permalink to this headline">¶</a></h2>
<p><code class="docutils literal"><span class="pre">__builtin_expect(long</span> <span class="pre">exp,</span> <span class="pre">long</span> <span class="pre">c)</span></code> instruction provides branch prediction
information. The return value is the value of <code class="docutils literal"><span class="pre">exp</span></code>.</p>
<p>It is especially useful in conditional statements. Currently Clang supports two
conditional statements:</p>
<div class="section" id="if-statement">
<h3><a class="toc-backref" href="#id8"><code class="docutils literal"><span class="pre">if</span></code> statement</a><a class="headerlink" href="#if-statement" title="Permalink to this headline">¶</a></h3>
<p>The <code class="docutils literal"><span class="pre">exp</span></code> parameter is the condition. The <code class="docutils literal"><span class="pre">c</span></code> parameter is the expected
comparison value. If it is equal to 1 (true), the condition is likely to be
true, in other case condition is likely to be false. For example:</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="p">(</span><span class="n">__builtin_expect</span><span class="p">(</span><span class="n">x</span> <span class="o">></span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">))</span> <span class="p">{</span>
<span class="c1">// This block is likely to be taken.</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="switch-statement">
<h3><a class="toc-backref" href="#id9"><code class="docutils literal"><span class="pre">switch</span></code> statement</a><a class="headerlink" href="#switch-statement" title="Permalink to this headline">¶</a></h3>
<p>The <code class="docutils literal"><span class="pre">exp</span></code> parameter is the value. The <code class="docutils literal"><span class="pre">c</span></code> parameter is the expected
value. If the expected value doesn’t show on the cases list, the <code class="docutils literal"><span class="pre">default</span></code>
case is assumed to be likely taken.</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="k">switch</span> <span class="p">(</span><span class="n">__builtin_expect</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="mi">5</span><span class="p">))</span> <span class="p">{</span>
<span class="k">default</span><span class="o">:</span> <span class="k">break</span><span class="p">;</span>
<span class="k">case</span> <span class="mi">0</span><span class="o">:</span> <span class="c1">// ...</span>
<span class="k">case</span> <span class="mi">3</span><span class="o">:</span> <span class="c1">// ...</span>
<span class="k">case</span> <span class="mi">5</span><span class="o">:</span> <span class="c1">// This case is likely to be taken.</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="cfg-modifications">
<h2><a class="toc-backref" href="#id10">CFG Modifications</a><a class="headerlink" href="#cfg-modifications" title="Permalink to this headline">¶</a></h2>
<p>Branch Weight Metatada is not proof against CFG changes. If terminator operands’
are changed some action should be taken. In other case some misoptimizations may
occur due to incorrent branch prediction information.</p>
</div>
<div class="section" id="function-entry-counts">
<h2><a class="toc-backref" href="#id11">Function Entry Counts</a><a class="headerlink" href="#function-entry-counts" title="Permalink to this headline">¶</a></h2>
<p>To allow comparing different functions durint inter-procedural analysis and
optimization, <code class="docutils literal"><span class="pre">MD_prof</span></code> nodes can also be assigned to a function definition.
The first operand is a string indicating the name of the associated counter.</p>
<p>Currently, one counter is supported: “function_entry_count”. This is a 64-bit
counter that indicates the number of times that this function was invoked (in
the case of instrumentation-based profiles). In the case of sampling-based
profiles, this counter is an approximation of how many times the function was
invoked.</p>
<p>For example, in the code below, the instrumentation for function foo()
indicates that it was called 2,590 times at runtime.</p>
<div class="highlight-llvm"><div class="highlight"><pre><span></span><span class="k">define</span> <span class="k">i32</span> <span class="vg">@foo</span><span class="p">()</span> <span class="nv">!prof</span> <span class="nv nv-Anonymous">!1</span> <span class="p">{</span>
<span class="k">ret</span> <span class="k">i32</span> <span class="m">0</span>
<span class="p">}</span>
<span class="nv nv-Anonymous">!1</span> <span class="p">=</span> <span class="p">!{</span><span class="nv">!"function_entry_count"</span><span class="p">,</span> <span class="k">i64</span> <span class="m">2590</span><span class="p">}</span>
</pre></div>
</div>
</div>
</div>
</div>
</div>
<div class="clearer"></div>
</div>
<div class="related" role="navigation" aria-label="related navigation">
<h3>Navigation</h3>
<ul>
<li class="right" style="margin-right: 10px">
<a href="genindex.html" title="General Index"
>index</a></li>
<li class="right" >
<a href="Bugpoint.html" title="LLVM bugpoint tool: design and usage"
>next</a> |</li>
<li class="right" >
<a href="BlockFrequencyTerminology.html" title="LLVM Block Frequency Terminology"
>previous</a> |</li>
<li><a href="http://llvm.org/">LLVM Home</a> | </li>
<li><a href="index.html">Documentation</a>»</li>
</ul>
</div>
<div class="footer" role="contentinfo">
© Copyright 2003-2017, LLVM Project.
Last updated on 2017-09-14.
Created using <a href="http://sphinx-doc.org/">Sphinx</a> 1.5.6.
</div>
</body>
</html>
|