1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3 <html> 4 <head> 5 <title>LLVM Branch Weight Metadata</title> 6 <link rel="stylesheet" href="llvm.css" type="text/css"> 7 </head> 8 <body> 9 10 <h1> 11 LLVM Branch Weight Metadata 12 </h1> 13 14 <ol> 15 <li><a href="#introduction">Introduction</a></li> 16 <li><a href="#supported_instructions">Supported Instructions</a></li> 17 <li><a href="#builtin_expect">Built-in "expect" Instruction </a></li> 18 <li><a href="#cfg_modifications">CFG Modifications</a></li> 19 </ol> 20 21 <div class="doc_author"> 22 <p>Written by <a href="mailto:jstaszak (a] apple.com">Jakub Staszak</a></p> 23 </div> 24 25 <h2> 26 <a name="introduction">Introduction</a> 27 </h2> 28 <div> 29 <p>Branch Weight Metadata represents branch weights as its likeliness to 30 be taken. Metadata is assigned to the <tt>TerminatorInst</tt> as a 31 <tt>MDNode</tt> of the <tt>MD_prof</tt> kind. The first operator is always a 32 <tt>MDString</tt> node with the string "branch_weights". Number of operators 33 depends on the terminator type.</p> 34 35 <p>Branch weights might be fetch from the profiling file, or generated based on 36 <a href="#builtin_expect"><tt>__builtin_expect</tt></a> instruction. 37 </p> 38 39 <p>All weights are represented as an unsigned 32-bit values, where higher value 40 indicates greater chance to be taken.</p> 41 </div> 42 43 <h2> 44 <a name="supported_instructions">Supported Instructions</a> 45 </h2> 46 47 <div> 48 <h4>BranchInst</h4> 49 <div> 50 <p>Metadata is only assign to the conditional branches. There are two extra 51 operarands, for the true and the false branch.</p> 52 </div> 53 <div class="doc_code"> 54 <pre> 55 !0 = metadata !{ 56 metadata !"branch_weights", 57 i32 <TRUE_BRANCH_WEIGHT>, 58 i32 <FALSE_BRANCH_WEIGHT> 59 } 60 </pre> 61 </div> 62 63 <h4>SwitchInst</h4> 64 <div> 65 <p>Branch weights are assign to every case (including <tt>default</tt> case 66 which is always case #0).</p> 67 </div> 68 <div class="doc_code"> 69 <pre> 70 !0 = metadata !{ 71 metadata !"branch_weights", 72 i32 <DEFAULT_BRANCH_WEIGHT> 73 [ , i32 <CASE_BRANCH_WEIGHT> ... ] 74 } 75 </pre> 76 </div> 77 78 <h4>IndirectBrInst</h4> 79 <div> 80 <p>Branch weights are assign to every destination.</p> 81 </div> 82 <div class="doc_code"> 83 <pre> 84 !0 = metadata !{ 85 metadata !"branch_weights", 86 i32 <LABEL_BRANCH_WEIGHT> 87 [ , i32 <LABEL_BRANCH_WEIGHT> ... ] 88 } 89 </pre> 90 </div> 91 92 <h4>Other</h4> 93 <div> 94 <p>Other terminator instructions are not allowed to contain Branch Weight 95 Metadata.</p> 96 </div> 97 </div> 98 99 <h2> 100 <a name="builtin_expect">Built-in "expect" Instructions</a> 101 </h2> 102 <div> 103 <p><tt>__builtin_expect(long exp, long c)</tt> instruction provides branch 104 prediction information. The return value is the value of <tt>exp</tt>.</p> 105 106 <p>It is especially useful in conditional statements. Currently Clang supports 107 two conditional statements: 108 </p> 109 <h4><tt>if</tt> statement</h4> 110 <div> 111 <p>The <tt>exp</tt> parameter is the condition. The <tt>c</tt> parameter is 112 the expected comparision value. If it is equal to 1 (true), the condition is 113 likely to be true, in other case condition is likely to be false. For example: 114 </p> 115 </div> 116 <div class="doc_code"> 117 <pre> 118 if (__builtin_expect(x > 0, 1)) { 119 // This block is likely to be taken. 120 } 121 </pre> 122 </div> 123 124 <h4><tt>switch</tt> statement</h4> 125 <div> 126 <p>The <tt>exp</tt> parameter is the value. The <tt>c</tt> parameter is the 127 expected value. If the expected value doesn't show on the cases list, the 128 <tt>default</tt> case is assumed to be likely taken.</p> 129 </div> 130 <div class="doc_code"> 131 <pre> 132 switch (__builtin_expect(x, 5)) { 133 default: break; 134 case 0: // ... 135 case 3: // ... 136 case 5: // This case is likely to be taken. 137 } 138 </pre> 139 </div> 140 </div> 141 142 <h2> 143 <a name="cfg_modifications">CFG Modifications</a> 144 </h2> 145 <div> 146 <p>Branch Weight Metatada is not proof against CFG changes. If terminator 147 operands' are changed some action should be taken. In other case some 148 misoptimizations may occur due to incorrent branch prediction information.</p> 149 </div> 150 151 <hr> 152 <address> 153 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 154 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 155 <a href="http://validator.w3.org/check/referer"><img 156 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 157 158 <a href="mailto:jstaszak (a] apple.com">Jakub Staszak</a><br> 159 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> 160 </address> 161 162 </body> 163 </html> 164
