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99 lines
4.2 KiB
HTML
99 lines
4.2 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Overview - GNU Compiler Collection (GCC) Internals</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<meta name="generator" content="makeinfo 4.13">
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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="next" href="Patterns.html#Patterns" title="Patterns">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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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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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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<style type="text/css"><!--
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Overview"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Patterns.html#Patterns">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">16.1 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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<ol type=1 start=1>
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<li>The front end reads the source code and builds a parse tree.
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<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>The insn list is matched against the RTL templates to produce assembler
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code.
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</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>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>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>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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</body></html>
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