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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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(see below). A copy of the license is included in the section entitled
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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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: Maintaining the CFG</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: Maintaining the CFG">
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<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Maintaining the CFG">
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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="Control-Flow.html#Control-Flow" rel="up" title="Control Flow">
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<link href="Liveness-information.html#Liveness-information" rel="next" title="Liveness information">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Maintaining-the-CFG"></a>
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<div class="header">
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<p>
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Next: <a href="Liveness-information.html#Liveness-information" accesskey="n" rel="next">Liveness information</a>, Previous: <a href="Profile-information.html#Profile-information" accesskey="p" rel="prev">Profile information</a>, Up: <a href="Control-Flow.html#Control-Flow" accesskey="u" rel="up">Control Flow</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="Maintaining-the-CFG-1"></a>
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<h3 class="section">14.4 Maintaining the CFG</h3>
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<a name="index-cfghooks_002eh"></a>
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<p>An important task of each compiler pass is to keep both the control
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flow graph and all profile information up-to-date. Reconstruction of
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the control flow graph after each pass is not an option, since it may be
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very expensive and lost profile information cannot be reconstructed at
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all.
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</p>
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<p>GCC has two major intermediate representations, and both use the
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<code>basic_block</code> and <code>edge</code> data types to represent control
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flow. Both representations share as much of the CFG maintenance code
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as possible. For each representation, a set of <em>hooks</em> is defined
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so that each representation can provide its own implementation of CFG
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manipulation routines when necessary. These hooks are defined in
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<samp>cfghooks.h</samp>. There are hooks for almost all common CFG
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manipulations, including block splitting and merging, edge redirection
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and creating and deleting basic blocks. These hooks should provide
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everything you need to maintain and manipulate the CFG in both the RTL
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and <code>GIMPLE</code> representation.
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</p>
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<p>At the moment, the basic block boundaries are maintained transparently
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when modifying instructions, so there rarely is a need to move them
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manually (such as in case someone wants to output instruction outside
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basic block explicitly).
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</p>
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<a name="index-BLOCK_005fFOR_005fINSN_002c-gimple_005fbb"></a>
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<p>In the RTL representation, each instruction has a
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<code>BLOCK_FOR_INSN</code> value that represents pointer to the basic block
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that contains the instruction. In the <code>GIMPLE</code> representation, the
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function <code>gimple_bb</code> returns a pointer to the basic block
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containing the queried statement.
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</p>
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<a name="index-GIMPLE-statement-iterators-1"></a>
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<p>When changes need to be applied to a function in its <code>GIMPLE</code>
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representation, <em>GIMPLE statement iterators</em> should be used. These
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iterators provide an integrated abstraction of the flow graph and the
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instruction stream. Block statement iterators are constructed using
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the <code>gimple_stmt_iterator</code> data structure and several modifiers are
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available, including the following:
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</p>
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<dl compact="compact">
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<dt><code>gsi_start</code>
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<a name="index-gsi_005fstart-1"></a>
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</dt>
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<dd><p>This function initializes a <code>gimple_stmt_iterator</code> that points to
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the first non-empty statement in a basic block.
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</p>
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</dd>
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<dt><code>gsi_last</code>
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<a name="index-gsi_005flast-1"></a>
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</dt>
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<dd><p>This function initializes a <code>gimple_stmt_iterator</code> that points to
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the last statement in a basic block.
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</p>
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</dd>
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<dt><code>gsi_end_p</code>
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<a name="index-gsi_005fend_005fp-1"></a>
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</dt>
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<dd><p>This predicate is <code>true</code> if a <code>gimple_stmt_iterator</code>
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represents the end of a basic block.
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</p>
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</dd>
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<dt><code>gsi_next</code>
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<a name="index-gsi_005fnext-1"></a>
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</dt>
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<dd><p>This function takes a <code>gimple_stmt_iterator</code> and makes it point to
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its successor.
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</p>
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</dd>
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<dt><code>gsi_prev</code>
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<a name="index-gsi_005fprev-1"></a>
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</dt>
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<dd><p>This function takes a <code>gimple_stmt_iterator</code> and makes it point to
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its predecessor.
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</p>
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</dd>
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<dt><code>gsi_insert_after</code>
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<a name="index-gsi_005finsert_005fafter-1"></a>
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</dt>
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<dd><p>This function inserts a statement after the <code>gimple_stmt_iterator</code>
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passed in. The final parameter determines whether the statement
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iterator is updated to point to the newly inserted statement, or left
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pointing to the original statement.
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</p>
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</dd>
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<dt><code>gsi_insert_before</code>
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<a name="index-gsi_005finsert_005fbefore-1"></a>
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</dt>
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<dd><p>This function inserts a statement before the <code>gimple_stmt_iterator</code>
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passed in. The final parameter determines whether the statement
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iterator is updated to point to the newly inserted statement, or left
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pointing to the original statement.
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</p>
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</dd>
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<dt><code>gsi_remove</code>
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<a name="index-gsi_005fremove-1"></a>
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</dt>
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<dd><p>This function removes the <code>gimple_stmt_iterator</code> passed in and
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rechains the remaining statements in a basic block, if any.
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</p></dd>
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</dl>
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<a name="index-BB_005fHEAD_002c-BB_005fEND"></a>
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<p>In the RTL representation, the macros <code>BB_HEAD</code> and <code>BB_END</code>
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may be used to get the head and end <code>rtx</code> of a basic block. No
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abstract iterators are defined for traversing the insn chain, but you
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can just use <code>NEXT_INSN</code> and <code>PREV_INSN</code> instead. See <a href="Insns.html#Insns">Insns</a>.
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</p>
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<a name="index-purge_005fdead_005fedges-1"></a>
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<p>Usually a code manipulating pass simplifies the instruction stream and
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the flow of control, possibly eliminating some edges. This may for
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example happen when a conditional jump is replaced with an
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unconditional jump, but also when simplifying possibly trapping
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instruction to non-trapping while compiling Java. Updating of edges
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is not transparent and each optimization pass is required to do so
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manually. However only few cases occur in practice. The pass may
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call <code>purge_dead_edges</code> on a given basic block to remove
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superfluous edges, if any.
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</p>
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<a name="index-redirect_005fedge_005fand_005fbranch_002c-redirect_005fjump"></a>
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<p>Another common scenario is redirection of branch instructions, but
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this is best modeled as redirection of edges in the control flow graph
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and thus use of <code>redirect_edge_and_branch</code> is preferred over more
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low level functions, such as <code>redirect_jump</code> that operate on RTL
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chain only. The CFG hooks defined in <samp>cfghooks.h</samp> should provide
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the complete API required for manipulating and maintaining the CFG.
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</p>
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<a name="index-split_005fblock"></a>
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<p>It is also possible that a pass has to insert control flow instruction
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into the middle of a basic block, thus creating an entry point in the
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middle of the basic block, which is impossible by definition: The
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block must be split to make sure it only has one entry point, i.e. the
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head of the basic block. The CFG hook <code>split_block</code> may be used
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when an instruction in the middle of a basic block has to become the
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target of a jump or branch instruction.
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</p>
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<a name="index-insert_005finsn_005fon_005fedge"></a>
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<a name="index-commit_005fedge_005finsertions"></a>
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<a name="index-gsi_005finsert_005fon_005fedge-1"></a>
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<a name="index-gsi_005fcommit_005fedge_005finserts-1"></a>
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<a name="index-edge-splitting"></a>
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<p>For a global optimizer, a common operation is to split edges in the
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flow graph and insert instructions on them. In the RTL
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representation, this can be easily done using the
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<code>insert_insn_on_edge</code> function that emits an instruction
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“on the edge”, caching it for a later <code>commit_edge_insertions</code>
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call that will take care of moving the inserted instructions off the
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edge into the instruction stream contained in a basic block. This
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includes the creation of new basic blocks where needed. In the
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<code>GIMPLE</code> representation, the equivalent functions are
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<code>gsi_insert_on_edge</code> which inserts a block statement
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iterator on an edge, and <code>gsi_commit_edge_inserts</code> which flushes
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the instruction to actual instruction stream.
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</p>
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<a name="index-verify_005fflow_005finfo"></a>
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<a name="index-CFG-verification"></a>
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<p>While debugging the optimization pass, the <code>verify_flow_info</code>
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function may be useful to find bugs in the control flow graph updating
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code.
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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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Next: <a href="Liveness-information.html#Liveness-information" accesskey="n" rel="next">Liveness information</a>, Previous: <a href="Profile-information.html#Profile-information" accesskey="p" rel="prev">Profile information</a>, Up: <a href="Control-Flow.html#Control-Flow" accesskey="u" rel="up">Control Flow</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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