/3rd_party/llvm/docs/BranchWeightMetadata.html

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  3. <html>
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  4. <head>
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  5.   <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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  6.   <title>LLVM Branch Weight Metadata</title>
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  7.   <link rel="stylesheet" href="llvm.css" type="text/css">
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  8. </head>
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  9. <body>
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  10. 

  11. <h1>
    12. 

  12.   LLVM Branch Weight Metadata
    13. 

  13. </h1>
    14. 

  14. 

  15. <ol>
    16. 

  16.   <li><a href="#introduction">Introduction</a></li>
    17. 

  17.   <li><a href="#supported_instructions">Supported Instructions</a></li>
    18. 

  18.   <li><a href="#builtin_expect">Built-in "expect" Instruction </a></li>
    19. 

  19.   <li><a href="#cfg_modifications">CFG Modifications</a></li>
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  20. </ol>
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  21. 

  22. <div class="doc_author">
    23. 

  23.   <p>Written by <a href="mailto:jstaszak@apple.com">Jakub Staszak</a></p>
    24. 

  24. </div>
    25. 

  25. 

  26. <h2>
    27. 

  27.   <a name="introduction">Introduction</a>
    28. 

  28. </h2>
    29. 

  29. <div>
    30. 

  30. <p>Branch Weight Metadata represents branch weights as its likeliness to
    31. 

  31. be taken. Metadata is assigned to the <tt>TerminatorInst</tt> as a
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  32. <tt>MDNode</tt> of the <tt>MD_prof</tt> kind. The first operator is always a
    33. 

  33. <tt>MDString</tt> node with the string "branch_weights". Number of operators
    34. 

  34. depends on the terminator type.</p>
    35. 

  35. 

  36. <p>Branch weights might be fetch from the profiling file, or generated based on
    37. 

  37. <a href="#builtin_expect"><tt>__builtin_expect</tt></a> instruction.
    38. 

  38. </p>
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  39. 

  40. <p>All weights are represented as an unsigned 32-bit values, where higher value
    41. 

  41. indicates greater chance to be taken.</p>
    42. 

  42. </div>
    43. 

  43. 

  44. <h2>
    45. 

  45.   <a name="supported_instructions">Supported Instructions</a>
    46. 

  46. </h2>
    47. 

  47. 

  48. <div>
    49. 

  49.   <h4>BranchInst</h4>
    50. 

  50.   <div>
    51. 

  51.     <p>Metadata is only assign to the conditional branches. There are two extra
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  52.     operarands, for the true and the false branch.</p>
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  53.   </div>
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  54.   <div class="doc_code">
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  55.   <pre>
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  56. !0 = metadata !{
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  57.   metadata !"branch_weights",
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  58.   i32 &lt;TRUE_BRANCH_WEIGHT&gt;,
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  59.   i32 &lt;FALSE_BRANCH_WEIGHT&gt;
    60. 

  60. }
    61. 

  61.   </pre>
    62. 

  62.   </div>
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  63. 

  64.   <h4>SwitchInst</h4>
    65. 

  65.   <div>
    66. 

  66.   <p>Branch weights are assign to every case (including <tt>default</tt> case
    67. 

  67.   which is always case #0).</p>
    68. 

  68.   </div>
    69. 

  69.   <div class="doc_code">
    70. 

  70.   <pre>
    71. 

  71. !0 = metadata !{
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  72.   metadata !"branch_weights",
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  73.   i32 &lt;DEFAULT_BRANCH_WEIGHT&gt;
    74. 

  74.   [ , i32 &lt;CASE_BRANCH_WEIGHT&gt; ... ]
    75. 

  75. }
    76. 

  76.   </pre>
    77. 

  77.   </div>
    78. 

  78. 

  79.   <h4>IndirectBrInst</h4>
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  80.   <div>
    81. 

  81.   <p>Branch weights are assign to every destination.</p>
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  82.   </div>
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  83.   <div class="doc_code">
    84. 

  84.   <pre>
    85. 

  85. !0 = metadata !{
    86. 

  86.   metadata !"branch_weights",
    87. 

  87.   i32 &lt;LABEL_BRANCH_WEIGHT&gt;
    88. 

  88.   [ , i32 &lt;LABEL_BRANCH_WEIGHT&gt; ... ]
    89. 

  89. }
    90. 

  90.   </pre>
    91. 

  91.   </div>
    92. 

  92. 

  93.   <h4>Other</h4>
    94. 

  94.   <div>
    95. 

  95.   <p>Other terminator instructions are not allowed to contain Branch Weight
    96. 

  96.   Metadata.</p>
    97. 

  97.   </div>
    98. 

  98. </div>
    99. 

  99. 

  100. <h2>
    101. 

  101.   <a name="builtin_expect">Built-in "expect" Instructions</a>
    102. 

  102. </h2>
    103. 

  103. <div>
    104. 

  104.   <p><tt>__builtin_expect(long exp, long c)</tt> instruction provides branch
    105. 

  105.   prediction information. The return value is the value of <tt>exp</tt>.</p>
    106. 

  106. 

  107.   <p>It is especially useful in conditional statements. Currently Clang supports
    108. 

  108.   two conditional statements:
    109. 

  109.   </p>
    110. 

  110.   <h4><tt>if</tt> statement</h4>
    111. 

  111.   <div>
    112. 

  112.   <p>The <tt>exp</tt> parameter is the condition. The <tt>c</tt> parameter is
    113. 

  113.   the expected comparision value. If it is equal to 1 (true), the condition is
    114. 

  114.   likely to be true, in other case condition is likely to be false. For example:
    115. 

  115.   </p>
    116. 

  116.   </div>
    117. 

  117.   <div class="doc_code">
    118. 

  118.   <pre>
    119. 

  119.   if (__builtin_expect(x &gt; 0, 1)) {
    120. 

  120.     // This block is likely to be taken.
    121. 

  121.   }
    122. 

  122.   </pre>
    123. 

  123.   </div>
    124. 

  124. 

  125.   <h4><tt>switch</tt> statement</h4>
    126. 

  126.   <div>
    127. 

  127.   <p>The <tt>exp</tt> parameter is the value. The <tt>c</tt> parameter is the
    128. 

  128.   expected value. If the expected value doesn't show on the cases list, the
    129. 

  129.   <tt>default</tt> case is assumed to be likely taken.</p>
    130. 

  130.   </div>
    131. 

  131.   <div class="doc_code">
    132. 

  132.   <pre>
    133. 

  133.   switch (__builtin_expect(x, 5)) {
    134. 

  134.   default: break;
    135. 

  135.   case 0:  // ...
    136. 

  136.   case 3:  // ...
    137. 

  137.   case 5:  // This case is likely to be taken.
    138. 

  138.   }
    139. 

  139.   </pre>
    140. 

  140.   </div>
    141. 

  141. </div>
    142. 

  142. 

  143. <h2>
    144. 

  144.   <a name="cfg_modifications">CFG Modifications</a>
    145. 

  145. </h2>
    146. 

  146. <div>
    147. 

  147. <p>Branch Weight Metatada is not proof against CFG changes. If terminator
    148. 

  148. operands' are changed some action should be taken. In other case some
    149. 

  149. misoptimizations may occur due to incorrent branch prediction information.</p>
    150. 

  150. </div>
    151. 

  151. 

  152. <hr>
    153. 

  153. <address>
    154. 

  154.   <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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    158. 

  158. 

  159.   <a href="mailto:jstaszak@apple.com">Jakub Staszak</a><br>
    160. 

  160.   <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
    161. 

  161. </address>
    162. 

  162. 

  163. </body>
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  164. </html>
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