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178 lines
9.7 KiB
HTML
178 lines
9.7 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>GIMPLE (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="GIMPLE (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="GIMPLE (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="index.html#Top" rel="up" title="Top">
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<link href="Tuple-representation.html#Tuple-representation" rel="next" title="Tuple representation">
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<link href="Java-Trees.html#Java-Trees" rel="prev" title="Java Trees">
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<style type="text/css">
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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="GIMPLE"></a>
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<div class="header">
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<p>
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Next: <a href="Tree-SSA.html#Tree-SSA" accesskey="n" rel="next">Tree SSA</a>, Previous: <a href="GENERIC.html#GENERIC" accesskey="p" rel="prev">GENERIC</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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="GIMPLE-1"></a>
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<h2 class="chapter">12 GIMPLE</h2>
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<a name="index-GIMPLE-2"></a>
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<p>GIMPLE is a three-address representation derived from GENERIC by
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breaking down GENERIC expressions into tuples of no more than 3
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operands (with some exceptions like function calls). GIMPLE was
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heavily influenced by the SIMPLE IL used by the McCAT compiler
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project at McGill University, though we have made some different
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choices. For one thing, SIMPLE doesn’t support <code>goto</code>.
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</p>
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<p>Temporaries are introduced to hold intermediate values needed to
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compute complex expressions. Additionally, all the control
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structures used in GENERIC are lowered into conditional jumps,
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lexical scopes are removed and exception regions are converted
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into an on the side exception region tree.
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</p>
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<p>The compiler pass which converts GENERIC into GIMPLE is referred to as
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the ‘<samp>gimplifier</samp>’. The gimplifier works recursively, generating
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GIMPLE tuples out of the original GENERIC expressions.
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</p>
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<p>One of the early implementation strategies used for the GIMPLE
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representation was to use the same internal data structures used
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by front ends to represent parse trees. This simplified
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implementation because we could leverage existing functionality
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and interfaces. However, GIMPLE is a much more restrictive
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representation than abstract syntax trees (AST), therefore it
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does not require the full structural complexity provided by the
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main tree data structure.
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</p>
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<p>The GENERIC representation of a function is stored in the
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<code>DECL_SAVED_TREE</code> field of the associated <code>FUNCTION_DECL</code>
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tree node. It is converted to GIMPLE by a call to
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<code>gimplify_function_tree</code>.
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</p>
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<p>If a front end wants to include language-specific tree codes in the tree
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representation which it provides to the back end, it must provide a
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definition of <code>LANG_HOOKS_GIMPLIFY_EXPR</code> which knows how to
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convert the front end trees to GIMPLE. Usually such a hook will involve
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much of the same code for expanding front end trees to RTL. This function
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can return fully lowered GIMPLE, or it can return GENERIC trees and let the
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main gimplifier lower them the rest of the way; this is often simpler.
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GIMPLE that is not fully lowered is known as “High GIMPLE” and
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consists of the IL before the pass <code>pass_lower_cf</code>. High GIMPLE
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contains some container statements like lexical scopes
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(represented by <code>GIMPLE_BIND</code>) and nested expressions (e.g.,
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<code>GIMPLE_TRY</code>), while “Low GIMPLE” exposes all of the
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implicit jumps for control and exception expressions directly in
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the IL and EH region trees.
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</p>
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<p>The C and C++ front ends currently convert directly from front end
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trees to GIMPLE, and hand that off to the back end rather than first
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converting to GENERIC. Their gimplifier hooks know about all the
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<code>_STMT</code> nodes and how to convert them to GENERIC forms. There
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was some work done on a genericization pass which would run first, but
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the existence of <code>STMT_EXPR</code> meant that in order to convert all
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of the C statements into GENERIC equivalents would involve walking the
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entire tree anyway, so it was simpler to lower all the way. This
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might change in the future if someone writes an optimization pass
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which would work better with higher-level trees, but currently the
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optimizers all expect GIMPLE.
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</p>
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<p>You can request to dump a C-like representation of the GIMPLE form
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with the flag <samp>-fdump-tree-gimple</samp>.
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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="Tuple-representation.html#Tuple-representation" accesskey="1">Tuple representation</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Class-hierarchy-of-GIMPLE-statements.html#Class-hierarchy-of-GIMPLE-statements" accesskey="2">Class hierarchy of GIMPLE statements</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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<tr><td align="left" valign="top">• <a href="GIMPLE-instruction-set.html#GIMPLE-instruction-set" accesskey="3">GIMPLE instruction set</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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<tr><td align="left" valign="top">• <a href="GIMPLE-Exception-Handling.html#GIMPLE-Exception-Handling" accesskey="4">GIMPLE Exception Handling</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Temporaries.html#Temporaries" accesskey="5">Temporaries</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Operands.html#Operands" accesskey="6">Operands</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Manipulating-GIMPLE-statements.html#Manipulating-GIMPLE-statements" accesskey="7">Manipulating GIMPLE statements</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Tuple-specific-accessors.html#Tuple-specific-accessors" accesskey="8">Tuple specific accessors</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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<tr><td align="left" valign="top">• <a href="GIMPLE-sequences.html#GIMPLE-sequences" accesskey="9">GIMPLE sequences</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Sequence-iterators.html#Sequence-iterators">Sequence iterators</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Adding-a-new-GIMPLE-statement-code.html#Adding-a-new-GIMPLE-statement-code">Adding a new GIMPLE statement code</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Statement-and-operand-traversals.html#Statement-and-operand-traversals">Statement and operand traversals</a>:</td><td> </td><td align="left" valign="top">
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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="Tree-SSA.html#Tree-SSA" accesskey="n" rel="next">Tree SSA</a>, Previous: <a href="GENERIC.html#GENERIC" accesskey="p" rel="prev">GENERIC</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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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