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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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| <a name="Control-Flow-Graph"></a> |
| <h2 class="chapter">14 Control Flow Graph</h2> |
| <a name="index-CFG_002c-Control-Flow-Graph"></a> |
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| <p>A control flow graph (CFG) is a data structure built on top of the |
| intermediate code representation (the RTL or <code>GIMPLE</code> instruction |
| stream) abstracting the control flow behavior of a function that is |
| being compiled. The CFG is a directed graph where the vertices |
| represent basic blocks and edges represent possible transfer of |
| control flow from one basic block to another. The data structures |
| used to represent the control flow graph are defined in |
| <samp>basic-block.h</samp>. |
| </p> |
| <p>In GCC, the representation of control flow is maintained throughout |
| the compilation process, from constructing the CFG early in |
| <code>pass_build_cfg</code> to <code>pass_free_cfg</code> (see <samp>passes.def</samp>). |
| The CFG takes various different modes and may undergo extensive |
| manipulations, but the graph is always valid between its construction |
| and its release. This way, transfer of information such as data flow, |
| a measured profile, or the loop tree, can be propagated through the |
| passes pipeline, and even from <code>GIMPLE</code> to <code>RTL</code>. |
| </p> |
| <p>Often the CFG may be better viewed as integral part of instruction |
| chain, than structure built on the top of it. Updating the compiler’s |
| intermediate representation for instructions can not be easily done |
| without proper maintenance of the CFG simultaneously. |
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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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| <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. |
| </td></tr> |
| <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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| <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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| <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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