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  6  <title>LLVM Branch Weight Metadata</title>
  7  <link rel="stylesheet" href="llvm.css" type="text/css">
  8</head>
  9<body>
 10
 11<h1>
 12  LLVM Branch Weight Metadata
 13</h1>
 14
 15<ol>
 16  <li><a href="#introduction">Introduction</a></li>
 17  <li><a href="#supported_instructions">Supported Instructions</a></li>
 18  <li><a href="#builtin_expect">Built-in "expect" Instruction </a></li>
 19  <li><a href="#cfg_modifications">CFG Modifications</a></li>
 20</ol>
 21
 22<div class="doc_author">
 23  <p>Written by <a href="mailto:jstaszak@apple.com">Jakub Staszak</a></p>
 24</div>
 25
 26<h2>
 27  <a name="introduction">Introduction</a>
 28</h2>
 29<div>
 30<p>Branch Weight Metadata represents branch weights as its likeliness to
 31be taken. Metadata is assigned to the <tt>TerminatorInst</tt> as a
 32<tt>MDNode</tt> of the <tt>MD_prof</tt> kind. The first operator is always a
 33<tt>MDString</tt> node with the string "branch_weights". Number of operators
 34depends on the terminator type.</p>
 35
 36<p>Branch weights might be fetch from the profiling file, or generated based on
 37<a href="#builtin_expect"><tt>__builtin_expect</tt></a> instruction.
 38</p>
 39
 40<p>All weights are represented as an unsigned 32-bit values, where higher value
 41indicates greater chance to be taken.</p>
 42</div>
 43
 44<h2>
 45  <a name="supported_instructions">Supported Instructions</a>
 46</h2>
 47
 48<div>
 49  <h4>BranchInst</h4>
 50  <div>
 51    <p>Metadata is only assign to the conditional branches. There are two extra
 52    operarands, for the true and the false branch.</p>
 53  </div>
 54  <div class="doc_code">
 55  <pre>
 56!0 = metadata !{
 57  metadata !"branch_weights",
 58  i32 &lt;TRUE_BRANCH_WEIGHT&gt;,
 59  i32 &lt;FALSE_BRANCH_WEIGHT&gt;
 60}
 61  </pre>
 62  </div>
 63
 64  <h4>SwitchInst</h4>
 65  <div>
 66  <p>Branch weights are assign to every case (including <tt>default</tt> case
 67  which is always case #0).</p>
 68  </div>
 69  <div class="doc_code">
 70  <pre>
 71!0 = metadata !{
 72  metadata !"branch_weights",
 73  i32 &lt;DEFAULT_BRANCH_WEIGHT&gt;
 74  [ , i32 &lt;CASE_BRANCH_WEIGHT&gt; ... ]
 75}
 76  </pre>
 77  </div>
 78
 79  <h4>IndirectBrInst</h4>
 80  <div>
 81  <p>Branch weights are assign to every destination.</p>
 82  </div>
 83  <div class="doc_code">
 84  <pre>
 85!0 = metadata !{
 86  metadata !"branch_weights",
 87  i32 &lt;LABEL_BRANCH_WEIGHT&gt;
 88  [ , i32 &lt;LABEL_BRANCH_WEIGHT&gt; ... ]
 89}
 90  </pre>
 91  </div>
 92
 93  <h4>Other</h4>
 94  <div>
 95  <p>Other terminator instructions are not allowed to contain Branch Weight
 96  Metadata.</p>
 97  </div>
 98</div>
 99
100<h2>
101  <a name="builtin_expect">Built-in "expect" Instructions</a>
102</h2>
103<div>
104  <p><tt>__builtin_expect(long exp, long c)</tt> instruction provides branch
105  prediction information. The return value is the value of <tt>exp</tt>.</p>
106
107  <p>It is especially useful in conditional statements. Currently Clang supports
108  two conditional statements:
109  </p>
110  <h4><tt>if</tt> statement</h4>
111  <div>
112  <p>The <tt>exp</tt> parameter is the condition. The <tt>c</tt> parameter is
113  the expected comparision value. If it is equal to 1 (true), the condition is
114  likely to be true, in other case condition is likely to be false. For example:
115  </p>
116  </div>
117  <div class="doc_code">
118  <pre>
119  if (__builtin_expect(x &gt; 0, 1)) {
120    // This block is likely to be taken.
121  }
122  </pre>
123  </div>
124
125  <h4><tt>switch</tt> statement</h4>
126  <div>
127  <p>The <tt>exp</tt> parameter is the value. The <tt>c</tt> parameter is the
128  expected value. If the expected value doesn't show on the cases list, the
129  <tt>default</tt> case is assumed to be likely taken.</p>
130  </div>
131  <div class="doc_code">
132  <pre>
133  switch (__builtin_expect(x, 5)) {
134  default: break;
135  case 0:  // ...
136  case 3:  // ...
137  case 5:  // This case is likely to be taken.
138  }
139  </pre>
140  </div>
141</div>
142
143<h2>
144  <a name="cfg_modifications">CFG Modifications</a>
145</h2>
146<div>
147<p>Branch Weight Metatada is not proof against CFG changes. If terminator
148operands' are changed some action should be taken. In other case some
149misoptimizations may occur due to incorrent branch prediction information.</p>
150</div>
151
152<hr>
153<address>
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158
159  <a href="mailto:jstaszak@apple.com">Jakub Staszak</a><br>
160  <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
161</address>
162
163</body>
164</html>