/3rd_party/llvm/docs/BranchWeightMetadata.html
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1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3<html> 4<head> 5 <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 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 <TRUE_BRANCH_WEIGHT>, 59 i32 <FALSE_BRANCH_WEIGHT> 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 <DEFAULT_BRANCH_WEIGHT> 74 [ , i32 <CASE_BRANCH_WEIGHT> ... ] 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 <LABEL_BRANCH_WEIGHT> 88 [ , i32 <LABEL_BRANCH_WEIGHT> ... ] 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 > 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> 154 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 155 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 156 <a href="http://validator.w3.org/check/referer"><img 157 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 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>