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125 lines
6.1 KiB
HTML
125 lines
6.1 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 Output Template (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Define Subst Output Template (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Define Subst Output Template (GNU Compiler Collection (GCC) Internals)">
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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="Define-Subst.html#Define-Subst" rel="up" title="Define Subst">
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<link href="Constant-Definitions.html#Constant-Definitions" rel="next" title="Constant Definitions">
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<link href="Define-Subst-Pattern-Matching.html#Define-Subst-Pattern-Matching" rel="prev" title="Define Subst Pattern Matching">
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</style>
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</head>
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<body lang="en">
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<a name="Define-Subst-Output-Template"></a>
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<div class="header">
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<p>
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Previous: <a href="Define-Subst-Pattern-Matching.html#Define-Subst-Pattern-Matching" accesskey="p" rel="prev">Define Subst Pattern Matching</a>, Up: <a href="Define-Subst.html#Define-Subst" accesskey="u" rel="up">Define Subst</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="Generation-of-output-template-in-define_005fsubst"></a>
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<h4 class="subsection">17.21.3 Generation of output template in <code>define_subst</code></h4>
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<a name="index-define_005fsubst-3"></a>
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<p>If all necessary checks for <code>define_subst</code> application pass, a new
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RTL-pattern, based on the output-template, is created to replace the old
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template. Like in input-patterns, meanings of some RTL expressions are
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changed when they are used in output-patterns of a <code>define_subst</code>.
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Thus, <code>match_dup</code> is used for copying the whole expression from the
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original pattern, which matched corresponding <code>match_operand</code> from
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the input pattern.
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</p>
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<p><code>match_dup N</code> is used in the output template to be replaced with
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the expression from the original pattern, which matched
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<code>match_operand N</code> from the input pattern. As a consequence,
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<code>match_dup</code> cannot be used to point to <code>match_operand</code>s from
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the output pattern, it should always refer to a <code>match_operand</code>
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from the input pattern.
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</p>
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<p>In the output template one can refer to the expressions from the
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original pattern and create new ones. For instance, some operands could
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be added by means of standard <code>match_operand</code>.
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</p>
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<p>After replacing <code>match_dup</code> with some RTL-subtree from the original
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pattern, it could happen that several <code>match_operand</code>s in the
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output pattern have the same indexes. It is unknown, how many and what
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indexes would be used in the expression which would replace
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<code>match_dup</code>, so such conflicts in indexes are inevitable. To
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overcome this issue, <code>match_operands</code> and <code>match_operators</code>,
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which were introduced into the output pattern, are renumerated when all
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<code>match_dup</code>s are replaced.
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</p>
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<p>Number of alternatives in <code>match_operand</code>s introduced into the
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output template <code>M</code> could differ from the number of alternatives in
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the original pattern <code>N</code>, so in the resultant pattern there would
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be <code>N*M</code> alternatives. Thus, constraints from the original pattern
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would be duplicated <code>N</code> times, constraints from the output pattern
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would be duplicated <code>M</code> times, producing all possible combinations.
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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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Previous: <a href="Define-Subst-Pattern-Matching.html#Define-Subst-Pattern-Matching" accesskey="p" rel="prev">Define Subst Pattern Matching</a>, Up: <a href="Define-Subst.html#Define-Subst" accesskey="u" rel="up">Define Subst</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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