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140 lines
6.9 KiB
HTML
140 lines
6.9 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>Define Subst (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Define Subst (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Define Subst (GNU Compiler Collection (GCC) Internals)">
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<meta name="resource-type" content="document">
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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="Define-Subst-Example.html#Define-Subst-Example" rel="next" title="Define Subst Example">
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<link href="Conditional-Execution.html#Conditional-Execution" rel="prev" title="Conditional Execution">
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</head>
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<body lang="en">
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<a name="Define-Subst"></a>
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<div class="header">
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<p>
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Next: <a href="Constant-Definitions.html#Constant-Definitions" accesskey="n" rel="next">Constant Definitions</a>, Previous: <a href="Conditional-Execution.html#Conditional-Execution" accesskey="p" rel="prev">Conditional Execution</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="RTL-Templates-Transformations"></a>
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<h3 class="section">17.21 RTL Templates Transformations</h3>
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<a name="index-define_005fsubst"></a>
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<p>For some hardware architectures there are common cases when the RTL
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templates for the instructions can be derived from the other RTL
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templates using simple transformations. E.g., <samp>i386.md</samp> contains
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an RTL template for the ordinary <code>sub</code> instruction—
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<code>*subsi_1</code>, and for the <code>sub</code> instruction with subsequent
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zero-extension—<code>*subsi_1_zext</code>. Such cases can be easily
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implemented by a single meta-template capable of generating a modified
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case based on the initial one:
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</p>
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<a name="index-define_005fsubst-4"></a>
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<div class="smallexample">
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<pre class="smallexample">(define_subst "<var>name</var>"
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[<var>input-template</var>]
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"<var>condition</var>"
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[<var>output-template</var>])
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</pre></div>
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<p><var>input-template</var> is a pattern describing the source RTL template,
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which will be transformed.
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</p>
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<p><var>condition</var> is a C expression that is conjunct with the condition
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from the input-template to generate a condition to be used in the
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output-template.
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</p>
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<p><var>output-template</var> is a pattern that will be used in the resulting
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template.
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</p>
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<p><code>define_subst</code> mechanism is tightly coupled with the notion of the
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subst attribute (see <a href="Subst-Iterators.html#Subst-Iterators">Subst Iterators</a>). The use of
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<code>define_subst</code> is triggered by a reference to a subst attribute in
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the transforming RTL template. This reference initiates duplication of
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the source RTL template and substitution of the attributes with their
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values. The source RTL template is left unchanged, while the copy is
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transformed by <code>define_subst</code>. This transformation can fail in the
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case when the source RTL template is not matched against the
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input-template of the <code>define_subst</code>. In such case the copy is
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deleted.
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</p>
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<p><code>define_subst</code> can be used only in <code>define_insn</code> and
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<code>define_expand</code>, it cannot be used in other expressions (e.g. in
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<code>define_insn_and_split</code>).
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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-Subst-Example.html#Define-Subst-Example" accesskey="1">Define Subst Example</a>:</td><td> </td><td align="left" valign="top">Example of <code>define_subst</code> work.
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Define-Subst-Pattern-Matching.html#Define-Subst-Pattern-Matching" accesskey="2">Define Subst Pattern Matching</a>:</td><td> </td><td align="left" valign="top">Process of template comparison.
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Define-Subst-Output-Template.html#Define-Subst-Output-Template" accesskey="3">Define Subst Output Template</a>:</td><td> </td><td align="left" valign="top">Generation of output template.
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</td></tr>
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</table>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Constant-Definitions.html#Constant-Definitions" accesskey="n" rel="next">Constant Definitions</a>, Previous: <a href="Conditional-Execution.html#Conditional-Execution" accesskey="p" rel="prev">Conditional Execution</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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