129 lines
6.6 KiB
HTML
129 lines
6.6 KiB
HTML
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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) 1988-2016 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 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>GNU Compiler Collection (GCC) Internals: Control Flow</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: Control Flow">
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<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Control Flow">
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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="Basic-Blocks.html#Basic-Blocks" rel="next" title="Basic Blocks">
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<link href="Reading-RTL.html#Reading-RTL" rel="prev" title="Reading RTL">
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Control-Flow"></a>
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<div class="header">
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<p>
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Next: <a href="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" accesskey="n" rel="next">Loop Analysis and Representation</a>, Previous: <a href="RTL.html#RTL" accesskey="p" rel="prev">RTL</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="Control-Flow-Graph"></a>
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<h2 class="chapter">14 Control Flow Graph</h2>
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<a name="index-CFG_002c-Control-Flow-Graph"></a>
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<a name="index-basic_002dblock_002eh"></a>
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<p>A control flow graph (CFG) is a data structure built on top of the
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intermediate code representation (the RTL or <code>GIMPLE</code> instruction
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stream) abstracting the control flow behavior of a function that is
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being compiled. The CFG is a directed graph where the vertices
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represent basic blocks and edges represent possible transfer of
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control flow from one basic block to another. The data structures
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used to represent the control flow graph are defined in
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<samp>basic-block.h</samp>.
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</p>
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<p>In GCC, the representation of control flow is maintained throughout
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the compilation process, from constructing the CFG early in
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<code>pass_build_cfg</code> to <code>pass_free_cfg</code> (see <samp>passes.def</samp>).
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The CFG takes various different modes and may undergo extensive
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manipulations, but the graph is always valid between its construction
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and its release. This way, transfer of information such as data flow,
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a measured profile, or the loop tree, can be propagated through the
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passes pipeline, and even from <code>GIMPLE</code> to <code>RTL</code>.
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</p>
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<p>Often the CFG may be better viewed as integral part of instruction
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chain, than structure built on the top of it. Updating the compiler’s
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intermediate representation for instructions can not be easily done
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without proper maintenance of the CFG simultaneously.
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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="Basic-Blocks.html#Basic-Blocks" accesskey="1">Basic Blocks</a>:</td><td> </td><td align="left" valign="top">The definition and representation of basic blocks.
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Edges.html#Edges" accesskey="2">Edges</a>:</td><td> </td><td align="left" valign="top">Types of edges and their representation.
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Profile-information.html#Profile-information" accesskey="3">Profile information</a>:</td><td> </td><td align="left" valign="top">Representation of frequencies and probabilities.
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Maintaining-the-CFG.html#Maintaining-the-CFG" accesskey="4">Maintaining the CFG</a>:</td><td> </td><td align="left" valign="top">Keeping the control flow graph and up to date.
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</td></tr>
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<tr><td align="left" valign="top">• <a href="Liveness-information.html#Liveness-information" accesskey="5">Liveness information</a>:</td><td> </td><td align="left" valign="top">Using and maintaining liveness information.
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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="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" accesskey="n" rel="next">Loop Analysis and Representation</a>, Previous: <a href="RTL.html#RTL" accesskey="p" rel="prev">RTL</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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