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128 lines
6.5 KiB
HTML
128 lines
6.5 KiB
HTML
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<head>
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<title>Control Flow (GNU Compiler Collection (GCC) Internals)</title>
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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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<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">15 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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