114 lines
5.2 KiB
HTML
114 lines
5.2 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>GNU Compiler Collection (GCC) Internals: Peephole Definitions</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: Peephole Definitions">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Machine-Desc.html#Machine-Desc" rel="up" title="Machine Desc">
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<link href="define_005fpeephole.html#define_005fpeephole" rel="next" title="define_peephole">
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<link href="Including-Patterns.html#Including-Patterns" rel="prev" title="Including Patterns">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Peephole-Definitions"></a>
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<div class="header">
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<p>
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Next: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="n" rel="next">Insn Attributes</a>, Previous: <a href="Including-Patterns.html#Including-Patterns" accesskey="p" rel="prev">Including Patterns</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Machine_002dSpecific-Peephole-Optimizers"></a>
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<h3 class="section">16.18 Machine-Specific Peephole Optimizers</h3>
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<a name="index-peephole-optimizer-definitions"></a>
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<a name="index-defining-peephole-optimizers"></a>
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<p>In addition to instruction patterns the <samp>md</samp> file may contain
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definitions of machine-specific peephole optimizations.
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</p>
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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>
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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>
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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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</p>
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<table class="menu" border="0" cellspacing="0">
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<tr><td align="left" valign="top">• <a href="define_005fpeephole.html#define_005fpeephole" accesskey="1">define_peephole</a>:</td><td> </td><td align="left" valign="top">RTL to Text Peephole Optimizers
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</td></tr>
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<tr><td align="left" valign="top">• <a href="define_005fpeephole2.html#define_005fpeephole2" accesskey="2">define_peephole2</a>:</td><td> </td><td align="left" valign="top">RTL to RTL Peephole Optimizers
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</td></tr>
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</table>
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</body>
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</html>
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