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131 lines
6.4 KiB
HTML
131 lines
6.4 KiB
HTML
4 years ago
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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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<html>
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<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
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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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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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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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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Jump Patterns (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Jump Patterns (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Jump Patterns (GNU Compiler Collection (GCC) Internals)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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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="Looping-Patterns.html#Looping-Patterns" rel="next" title="Looping Patterns">
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<link href="Dependent-Patterns.html#Dependent-Patterns" rel="prev" title="Dependent Patterns">
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Jump-Patterns"></a>
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<div class="header">
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<p>
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Next: <a href="Looping-Patterns.html#Looping-Patterns" accesskey="n" rel="next">Looping Patterns</a>, Previous: <a href="Dependent-Patterns.html#Dependent-Patterns" accesskey="p" rel="prev">Dependent 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="Defining-Jump-Instruction-Patterns"></a>
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<h3 class="section">17.12 Defining Jump Instruction Patterns</h3>
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<a name="index-jump-instruction-patterns"></a>
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<a name="index-defining-jump-instruction-patterns"></a>
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<p>GCC does not assume anything about how the machine realizes jumps.
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The machine description should define a single pattern, usually
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a <code>define_expand</code>, which expands to all the required insns.
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</p>
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<p>Usually, this would be a comparison insn to set the condition code
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and a separate branch insn testing the condition code and branching
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or not according to its value. For many machines, however,
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separating compares and branches is limiting, which is why the
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more flexible approach with one <code>define_expand</code> is used in GCC.
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The machine description becomes clearer for architectures that
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have compare-and-branch instructions but no condition code. It also
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works better when different sets of comparison operators are supported
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by different kinds of conditional branches (e.g. integer vs. floating-point),
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or by conditional branches with respect to conditional stores.
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</p>
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<p>Two separate insns are always used if the machine description represents
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a condition code register using the legacy RTL expression <code>(cc0)</code>,
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and on most machines that use a separate condition code register
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(see <a href="Condition-Code.html#Condition-Code">Condition Code</a>). For machines that use <code>(cc0)</code>, in
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fact, the set and use of the condition code must be separate and
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adjacent<a name="DOCF4" href="#FOOT4"><sup>4</sup></a>, thus
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allowing flags in <code>cc_status</code> to be used (see <a href="Condition-Code.html#Condition-Code">Condition Code</a>) and
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so that the comparison and branch insns could be located from each other
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by using the functions <code>prev_cc0_setter</code> and <code>next_cc0_user</code>.
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</p>
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<p>Even in this case having a single entry point for conditional branches
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is advantageous, because it handles equally well the case where a single
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comparison instruction records the results of both signed and unsigned
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comparison of the given operands (with the branch insns coming in distinct
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signed and unsigned flavors) as in the x86 or SPARC, and the case where
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there are distinct signed and unsigned compare instructions and only
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one set of conditional branch instructions as in the PowerPC.
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</p>
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<div class="footnote">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h3><a name="FOOT4" href="#DOCF4">(4)</a></h3>
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<p><code>note</code> insns can separate them, though.</p>
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</div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Looping-Patterns.html#Looping-Patterns" accesskey="n" rel="next">Looping Patterns</a>, Previous: <a href="Dependent-Patterns.html#Dependent-Patterns" accesskey="p" rel="prev">Dependent 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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</body>
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</html>
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