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164 lines
4.4 KiB
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164 lines
4.4 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<title>LLVM Branch Weight Metadata</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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</head>
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<body>
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<h1>
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LLVM Branch Weight Metadata
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</h1>
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<ol>
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<li><a href="#introduction">Introduction</a></li>
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<li><a href="#supported_instructions">Supported Instructions</a></li>
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<li><a href="#builtin_expect">Built-in "expect" Instruction </a></li>
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<li><a href="#cfg_modifications">CFG Modifications</a></li>
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</ol>
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<div class="doc_author">
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<p>Written by <a href="mailto:jstaszak@apple.com">Jakub Staszak</a></p>
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</div>
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<h2>
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<a name="introduction">Introduction</a>
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</h2>
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<div>
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<p>Branch Weight Metadata represents branch weights as its likeliness to
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be taken. Metadata is assigned to the <tt>TerminatorInst</tt> as a
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<tt>MDNode</tt> of the <tt>MD_prof</tt> kind. The first operator is always a
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<tt>MDString</tt> node with the string "branch_weights". Number of operators
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depends on the terminator type.</p>
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<p>Branch weights might be fetch from the profiling file, or generated based on
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<a href="#builtin_expect"><tt>__builtin_expect</tt></a> instruction.
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</p>
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<p>All weights are represented as an unsigned 32-bit values, where higher value
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indicates greater chance to be taken.</p>
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</div>
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<h2>
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<a name="supported_instructions">Supported Instructions</a>
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</h2>
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<div>
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<h4>BranchInst</h4>
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<div>
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<p>Metadata is only assign to the conditional branches. There are two extra
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operarands, for the true and the false branch.</p>
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</div>
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<div class="doc_code">
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<pre>
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!0 = metadata !{
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metadata !"branch_weights",
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i32 <TRUE_BRANCH_WEIGHT>,
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i32 <FALSE_BRANCH_WEIGHT>
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}
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</pre>
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</div>
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<h4>SwitchInst</h4>
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<div>
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<p>Branch weights are assign to every case (including <tt>default</tt> case
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which is always case #0).</p>
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</div>
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<div class="doc_code">
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<pre>
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!0 = metadata !{
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metadata !"branch_weights",
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i32 <DEFAULT_BRANCH_WEIGHT>
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[ , i32 <CASE_BRANCH_WEIGHT> ... ]
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}
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</pre>
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</div>
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<h4>IndirectBrInst</h4>
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<div>
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<p>Branch weights are assign to every destination.</p>
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</div>
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<div class="doc_code">
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<pre>
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!0 = metadata !{
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metadata !"branch_weights",
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i32 <LABEL_BRANCH_WEIGHT>
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[ , i32 <LABEL_BRANCH_WEIGHT> ... ]
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}
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</pre>
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</div>
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<h4>Other</h4>
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<div>
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<p>Other terminator instructions are not allowed to contain Branch Weight
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Metadata.</p>
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</div>
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</div>
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<h2>
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<a name="builtin_expect">Built-in "expect" Instructions</a>
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</h2>
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<div>
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<p><tt>__builtin_expect(long exp, long c)</tt> instruction provides branch
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prediction information. The return value is the value of <tt>exp</tt>.</p>
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<p>It is especially useful in conditional statements. Currently Clang supports
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two conditional statements:
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</p>
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<h4><tt>if</tt> statement</h4>
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<div>
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<p>The <tt>exp</tt> parameter is the condition. The <tt>c</tt> parameter is
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the expected comparision value. If it is equal to 1 (true), the condition is
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likely to be true, in other case condition is likely to be false. For example:
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</p>
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</div>
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<div class="doc_code">
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<pre>
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if (__builtin_expect(x > 0, 1)) {
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// This block is likely to be taken.
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}
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</pre>
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</div>
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<h4><tt>switch</tt> statement</h4>
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<div>
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<p>The <tt>exp</tt> parameter is the value. The <tt>c</tt> parameter is the
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expected value. If the expected value doesn't show on the cases list, the
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<tt>default</tt> case is assumed to be likely taken.</p>
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</div>
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<div class="doc_code">
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<pre>
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switch (__builtin_expect(x, 5)) {
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default: break;
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case 0: // ...
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case 3: // ...
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case 5: // This case is likely to be taken.
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}
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</pre>
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</div>
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</div>
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<h2>
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<a name="cfg_modifications">CFG Modifications</a>
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</h2>
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<div>
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<p>Branch Weight Metatada is not proof against CFG changes. If terminator
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operands' are changed some action should be taken. In other case some
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misoptimizations may occur due to incorrent branch prediction information.</p>
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</div>
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<hr>
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<address>
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<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
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<a href="mailto:jstaszak@apple.com">Jakub Staszak</a><br>
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<a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
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</address>
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</body>
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</html>
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