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130 lines
5.8 KiB
HTML
130 lines
5.8 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>Overview (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Overview (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Overview (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 name="Generator" content="makeinfo">
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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="Patterns.html#Patterns" rel="next" title="Patterns">
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<link href="Machine-Desc.html#Machine-Desc" rel="prev" title="Machine Desc">
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<style type="text/css">
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ul.no-bullet {list-style: none}
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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="Overview"></a>
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<div class="header">
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<p>
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Next: <a href="Patterns.html#Patterns" accesskey="n" rel="next">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="Overview-of-How-the-Machine-Description-is-Used"></a>
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<h3 class="section">17.1 Overview of How the Machine Description is Used</h3>
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<p>There are three main conversions that happen in the compiler:
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</p>
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<ol>
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<li> The front end reads the source code and builds a parse tree.
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</li><li> The parse tree is used to generate an RTL insn list based on named
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instruction patterns.
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</li><li> The insn list is matched against the RTL templates to produce assembler
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code.
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</li></ol>
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<p>For the generate pass, only the names of the insns matter, from either a
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named <code>define_insn</code> or a <code>define_expand</code>. The compiler will
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choose the pattern with the right name and apply the operands according
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to the documentation later in this chapter, without regard for the RTL
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template or operand constraints. Note that the names the compiler looks
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for are hard-coded in the compiler—it will ignore unnamed patterns and
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patterns with names it doesn’t know about, but if you don’t provide a
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named pattern it needs, it will abort.
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</p>
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<p>If a <code>define_insn</code> is used, the template given is inserted into the
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insn list. If a <code>define_expand</code> is used, one of three things
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happens, based on the condition logic. The condition logic may manually
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create new insns for the insn list, say via <code>emit_insn()</code>, and
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invoke <code>DONE</code>. For certain named patterns, it may invoke <code>FAIL</code> to tell the
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compiler to use an alternate way of performing that task. If it invokes
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neither <code>DONE</code> nor <code>FAIL</code>, the template given in the pattern
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is inserted, as if the <code>define_expand</code> were a <code>define_insn</code>.
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</p>
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<p>Once the insn list is generated, various optimization passes convert,
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replace, and rearrange the insns in the insn list. This is where the
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<code>define_split</code> and <code>define_peephole</code> patterns get used, for
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example.
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</p>
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<p>Finally, the insn list’s RTL is matched up with the RTL templates in the
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<code>define_insn</code> patterns, and those patterns are used to emit the
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final assembly code. For this purpose, each named <code>define_insn</code>
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acts like it’s unnamed, since the names are ignored.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Patterns.html#Patterns" accesskey="n" rel="next">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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