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85 lines
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85 lines
3.8 KiB
HTML
4 years ago
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<title>Peephole Definitions - GNU Compiler Collection (GCC) Internals</title>
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Machine-Desc.html#Machine-Desc" title="Machine Desc">
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<link rel="prev" href="Including-Patterns.html#Including-Patterns" title="Including Patterns">
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<link rel="next" href="Insn-Attributes.html#Insn-Attributes" title="Insn Attributes">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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<body>
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<div class="node">
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<a name="Peephole-Definitions"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>,
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Previous: <a rel="previous" accesskey="p" href="Including-Patterns.html#Including-Patterns">Including Patterns</a>,
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Up: <a rel="up" accesskey="u" href="Machine-Desc.html#Machine-Desc">Machine Desc</a>
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<hr>
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</div>
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<h3 class="section">16.18 Machine-Specific Peephole Optimizers</h3>
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<p><a name="index-peephole-optimizer-definitions-3711"></a><a name="index-defining-peephole-optimizers-3712"></a>
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In addition to instruction patterns the <samp><span class="file">md</span></samp> file may contain
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definitions of machine-specific peephole optimizations.
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<p>The combiner does not notice certain peephole optimizations when the data
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flow in the program does not suggest that it should try them. For example,
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sometimes two consecutive insns related in purpose can be combined even
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though the second one does not appear to use a register computed in the
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first one. A machine-specific peephole optimizer can detect such
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opportunities.
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<p>There are two forms of peephole definitions that may be used. The
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original <code>define_peephole</code> is run at assembly output time to
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match insns and substitute assembly text. Use of <code>define_peephole</code>
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is deprecated.
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<p>A newer <code>define_peephole2</code> matches insns and substitutes new
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insns. The <code>peephole2</code> pass is run after register allocation
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but before scheduling, which may result in much better code for
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targets that do scheduling.
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<ul class="menu">
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<li><a accesskey="1" href="define_005fpeephole.html#define_005fpeephole">define_peephole</a>: RTL to Text Peephole Optimizers
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<li><a accesskey="2" href="define_005fpeephole2.html#define_005fpeephole2">define_peephole2</a>: RTL to RTL Peephole Optimizers
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</ul>
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</body></html>
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