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 <a name="Edges"></a>
 <div class="header">
 <p>
 Next: <a href="Profile-information.html#Profile-information" accesskey="n" rel="next">Profile information</a>, Previous: <a href="Basic-Blocks.html#Basic-Blocks" accesskey="p" rel="prev">Basic Blocks</a>, Up: <a href="Control-Flow.html#Control-Flow" accesskey="u" rel="up">Control Flow</a> &nbsp; [<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>
 </div>
 <hr>
 <a name="Edges-1"></a>
 <h3 class="section">14.2 Edges</h3>

 <a name="index-edge-in-the-flow-graph"></a>
 <a name="index-edge"></a>
 <p>Edges represent possible control flow transfers from the end of some
 basic block A to the head of another basic block B.  We say that A is
 a predecessor of B, and B is a successor of A.  Edges are represented
 in GCC with the <code>edge</code> data type.  Each <code>edge</code> acts as a
 link between two basic blocks: The <code>src</code> member of an edge
 points to the predecessor basic block of the <code>dest</code> basic block.
 The members <code>preds</code> and <code>succs</code> of the <code>basic_block</code> data
 type point to type-safe vectors of edges to the predecessors and
 successors of the block.
 </p>
 <a name="index-edge-iterators"></a>
 <p>When walking the edges in an edge vector, <em>edge iterators</em> should
 be used.  Edge iterators are constructed using the
 <code>edge_iterator</code> data structure and several methods are available
 to operate on them:
 </p>
 <dl compact="compact">
 <dt><code>ei_start</code>
 <a name="index-ei_005fstart"></a>
 </dt>
 <dd><p>This function initializes an <code>edge_iterator</code> that points to the
 first edge in a vector of edges.
 </p>
 </dd>
 <dt><code>ei_last</code>
 <a name="index-ei_005flast"></a>
 </dt>
 <dd><p>This function initializes an <code>edge_iterator</code> that points to the
 last edge in a vector of edges.
 </p>
 </dd>
 <dt><code>ei_end_p</code>
 <a name="index-ei_005fend_005fp"></a>
 </dt>
 <dd><p>This predicate is <code>true</code> if an <code>edge_iterator</code> represents
 the last edge in an edge vector.
 </p>
 </dd>
 <dt><code>ei_one_before_end_p</code>
 <a name="index-ei_005fone_005fbefore_005fend_005fp"></a>
 </dt>
 <dd><p>This predicate is <code>true</code> if an <code>edge_iterator</code> represents
 the second last edge in an edge vector.
 </p>
 </dd>
 <dt><code>ei_next</code>
 <a name="index-ei_005fnext"></a>
 </dt>
 <dd><p>This function takes a pointer to an <code>edge_iterator</code> and makes it
 point to the next edge in the sequence.
 </p>
 </dd>
 <dt><code>ei_prev</code>
 <a name="index-ei_005fprev"></a>
 </dt>
 <dd><p>This function takes a pointer to an <code>edge_iterator</code> and makes it
 point to the previous edge in the sequence.
 </p>
 </dd>
 <dt><code>ei_edge</code>
 <a name="index-ei_005fedge"></a>
 </dt>
 <dd><p>This function returns the <code>edge</code> currently pointed to by an
 <code>edge_iterator</code>.
 </p>
 </dd>
 <dt><code>ei_safe_safe</code>
 <a name="index-ei_005fsafe_005fsafe"></a>
 </dt>
 <dd><p>This function returns the <code>edge</code> currently pointed to by an
 <code>edge_iterator</code>, but returns <code>NULL</code> if the iterator is
 pointing at the end of the sequence.  This function has been provided
 for existing code makes the assumption that a <code>NULL</code> edge
 indicates the end of the sequence.
 </p>
 </dd>
 </dl>

 <p>The convenience macro <code>FOR_EACH_EDGE</code> can be used to visit all of
 the edges in a sequence of predecessor or successor edges.  It must
 not be used when an element might be removed during the traversal,
 otherwise elements will be missed.  Here is an example of how to use
 the macro:
 </p>
 <div class="smallexample">
 <pre class="smallexample">edge e;
 edge_iterator ei;

 FOR_EACH_EDGE (e, ei, bb-&gt;succs)
   {
      if (e-&gt;flags &amp; EDGE_FALLTHRU)
        break;
   }
 </pre></div>

 <a name="index-fall_002dthru"></a>
 <p>There are various reasons why control flow may transfer from one block
 to another.  One possibility is that some instruction, for example a
 <code>CODE_LABEL</code>, in a linearized instruction stream just always
 starts a new basic block.  In this case a <em>fall-thru</em> edge links
 the basic block to the first following basic block.  But there are
 several other reasons why edges may be created.  The <code>flags</code>
 field of the <code>edge</code> data type is used to store information
 about the type of edge we are dealing with.  Each edge is of one of
 the following types:
 </p>
 <dl compact="compact">
 <dt><em>jump</em></dt>
 <dd><p>No type flags are set for edges corresponding to jump instructions.
 These edges are used for unconditional or conditional jumps and in
 RTL also for table jumps.  They are the easiest to manipulate as they
 may be freely redirected when the flow graph is not in SSA form.
 </p>
 </dd>
 <dt><em>fall-thru</em></dt>
 <dd><a name="index-EDGE_005fFALLTHRU_002c-force_005fnonfallthru"></a>
 <p>Fall-thru edges are present in case where the basic block may continue
 execution to the following one without branching.  These edges have
 the <code>EDGE_FALLTHRU</code> flag set.  Unlike other types of edges, these
 edges must come into the basic block immediately following in the
 instruction stream.  The function <code>force_nonfallthru</code> is
 available to insert an unconditional jump in the case that redirection
 is needed.  Note that this may require creation of a new basic block.
 </p>
 </dd>
 <dt><em>exception handling</em></dt>
 <dd><a name="index-exception-handling"></a>
 <a name="index-EDGE_005fABNORMAL_002c-EDGE_005fEH"></a>
 <p>Exception handling edges represent possible control transfers from a
 trapping instruction to an exception handler.  The definition of
 &ldquo;trapping&rdquo; varies.  In C++, only function calls can throw, but for
 Java and Ada, exceptions like division by zero or segmentation fault are
 defined and thus each instruction possibly throwing this kind of
 exception needs to be handled as control flow instruction.  Exception
 edges have the <code>EDGE_ABNORMAL</code> and <code>EDGE_EH</code> flags set.
 </p>
 <a name="index-purge_005fdead_005fedges"></a>
 <p>When updating the instruction stream it is easy to change possibly
 trapping instruction to non-trapping, by simply removing the exception
 edge.  The opposite conversion is difficult, but should not happen
 anyway.  The edges can be eliminated via <code>purge_dead_edges</code> call.
 </p>
 <a name="index-REG_005fEH_005fREGION_002c-EDGE_005fABNORMAL_005fCALL"></a>
 <p>In the RTL representation, the destination of an exception edge is
 specified by <code>REG_EH_REGION</code> note attached to the insn.
 In case of a trapping call the <code>EDGE_ABNORMAL_CALL</code> flag is set
 too.  In the <code>GIMPLE</code> representation, this extra flag is not set.
 </p>
 <a name="index-may_005ftrap_005fp_002c-tree_005fcould_005ftrap_005fp"></a>
 <p>In the RTL representation, the predicate <code>may_trap_p</code> may be used
 to check whether instruction still may trap or not.  For the tree
 representation, the <code>tree_could_trap_p</code> predicate is available,
 but this predicate only checks for possible memory traps, as in
 dereferencing an invalid pointer location.
 </p>

 </dd>
 <dt><em>sibling calls</em></dt>
 <dd><a name="index-sibling-call"></a>
 <a name="index-EDGE_005fABNORMAL_002c-EDGE_005fSIBCALL"></a>
 <p>Sibling calls or tail calls terminate the function in a non-standard
 way and thus an edge to the exit must be present.
 <code>EDGE_SIBCALL</code> and <code>EDGE_ABNORMAL</code> are set in such case.
 These edges only exist in the RTL representation.
 </p>
 </dd>
 <dt><em>computed jumps</em></dt>
 <dd><a name="index-computed-jump"></a>
 <a name="index-EDGE_005fABNORMAL"></a>
 <p>Computed jumps contain edges to all labels in the function referenced
 from the code.  All those edges have <code>EDGE_ABNORMAL</code> flag set.
 The edges used to represent computed jumps often cause compile time
 performance problems, since functions consisting of many taken labels
 and many computed jumps may have <em>very</em> dense flow graphs, so
 these edges need to be handled with special care.  During the earlier
 stages of the compilation process, GCC tries to avoid such dense flow
 graphs by factoring computed jumps.  For example, given the following
 series of jumps,
 </p>
 <div class="smallexample">
 <pre class="smallexample">  goto *x;
   [ &hellip; ]

   goto *x;
   [ &hellip; ]

   goto *x;
   [ &hellip; ]
 </pre></div>

 <p>factoring the computed jumps results in the following code sequence
 which has a much simpler flow graph:
 </p>
 <div class="smallexample">
 <pre class="smallexample">  goto y;
   [ &hellip; ]

   goto y;
   [ &hellip; ]

   goto y;
   [ &hellip; ]

 y:
   goto *x;
 </pre></div>

 <a name="index-pass_005fduplicate_005fcomputed_005fgotos"></a>
 <p>However, the classic problem with this transformation is that it has a
 runtime cost in there resulting code: An extra jump.  Therefore, the
 computed jumps are un-factored in the later passes of the compiler
 (in the pass called <code>pass_duplicate_computed_gotos</code>).
 Be aware of that when you work on passes in that area.  There have
 been numerous examples already where the compile time for code with
 unfactored computed jumps caused some serious headaches.
 </p>
 </dd>
 <dt><em>nonlocal goto handlers</em></dt>
 <dd><a name="index-nonlocal-goto-handler"></a>
 <a name="index-EDGE_005fABNORMAL_002c-EDGE_005fABNORMAL_005fCALL"></a>
 <p>GCC allows nested functions to return into caller using a <code>goto</code>
 to a label passed to as an argument to the callee.  The labels passed
 to nested functions contain special code to cleanup after function
 call.  Such sections of code are referred to as &ldquo;nonlocal goto
 receivers&rdquo;.  If a function contains such nonlocal goto receivers, an
 edge from the call to the label is created with the
 <code>EDGE_ABNORMAL</code> and <code>EDGE_ABNORMAL_CALL</code> flags set.
 </p>
 </dd>
 <dt><em>function entry points</em></dt>
 <dd><a name="index-function-entry-point_002c-alternate-function-entry-point"></a>
 <a name="index-LABEL_005fALTERNATE_005fNAME"></a>
 <p>By definition, execution of function starts at basic block 0, so there
 is always an edge from the <code>ENTRY_BLOCK_PTR</code> to basic block 0.
 There is no <code>GIMPLE</code> representation for alternate entry points at
 this moment.  In RTL, alternate entry points are specified by
 <code>CODE_LABEL</code> with <code>LABEL_ALTERNATE_NAME</code> defined.  This
 feature is currently used for multiple entry point prologues and is
 limited to post-reload passes only.  This can be used by back-ends to
 emit alternate prologues for functions called from different contexts.
 In future full support for multiple entry functions defined by Fortran
 90 needs to be implemented.
 </p>
 </dd>
 <dt><em>function exits</em></dt>
 <dd><p>In the pre-reload representation a function terminates after the last
 instruction in the insn chain and no explicit return instructions are
 used.  This corresponds to the fall-thru edge into exit block.  After
 reload, optimal RTL epilogues are used that use explicit (conditional)
 return instructions that are represented by edges with no flags set.
 </p>
 </dd>
 </dl>


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	<a name="Edges"></a>
	<div class="header">
	<p>
	Next: <a href="Profile-information.html#Profile-information" accesskey="n" rel="next">Profile information</a>, Previous: <a href="Basic-Blocks.html#Basic-Blocks" accesskey="p" rel="prev">Basic Blocks</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>
	</div>
	<hr>
	<a name="Edges-1"></a>
	<h3 class="section">14.2 Edges</h3>

	<a name="index-edge-in-the-flow-graph"></a>
	<a name="index-edge"></a>
	<p>Edges represent possible control flow transfers from the end of some
	basic block A to the head of another basic block B. We say that A is
	a predecessor of B, and B is a successor of A. Edges are represented
	in GCC with the <code>edge</code> data type. Each <code>edge</code> acts as a
	link between two basic blocks: The <code>src</code> member of an edge
	points to the predecessor basic block of the <code>dest</code> basic block.
	The members <code>preds</code> and <code>succs</code> of the <code>basic_block</code> data
	type point to type-safe vectors of edges to the predecessors and
	successors of the block.
	</p>
	<a name="index-edge-iterators"></a>
	<p>When walking the edges in an edge vector, <em>edge iterators</em> should
	be used. Edge iterators are constructed using the
	<code>edge_iterator</code> data structure and several methods are available
	to operate on them:
	</p>
	<dl compact="compact">
	<dt><code>ei_start</code>
	<a name="index-ei_005fstart"></a>
	</dt>
	<dd><p>This function initializes an <code>edge_iterator</code> that points to the
	first edge in a vector of edges.
	</p>
	</dd>
	<dt><code>ei_last</code>
	<a name="index-ei_005flast"></a>
	</dt>
	<dd><p>This function initializes an <code>edge_iterator</code> that points to the
	last edge in a vector of edges.
	</p>
	</dd>
	<dt><code>ei_end_p</code>
	<a name="index-ei_005fend_005fp"></a>
	</dt>
	<dd><p>This predicate is <code>true</code> if an <code>edge_iterator</code> represents
	the last edge in an edge vector.
	</p>
	</dd>
	<dt><code>ei_one_before_end_p</code>
	<a name="index-ei_005fone_005fbefore_005fend_005fp"></a>
	</dt>
	<dd><p>This predicate is <code>true</code> if an <code>edge_iterator</code> represents
	the second last edge in an edge vector.
	</p>
	</dd>
	<dt><code>ei_next</code>
	<a name="index-ei_005fnext"></a>
	</dt>
	<dd><p>This function takes a pointer to an <code>edge_iterator</code> and makes it
	point to the next edge in the sequence.
	</p>
	</dd>
	<dt><code>ei_prev</code>
	<a name="index-ei_005fprev"></a>
	</dt>
	<dd><p>This function takes a pointer to an <code>edge_iterator</code> and makes it
	point to the previous edge in the sequence.
	</p>
	</dd>
	<dt><code>ei_edge</code>
	<a name="index-ei_005fedge"></a>
	</dt>
	<dd><p>This function returns the <code>edge</code> currently pointed to by an
	<code>edge_iterator</code>.
	</p>
	</dd>
	<dt><code>ei_safe_safe</code>
	<a name="index-ei_005fsafe_005fsafe"></a>
	</dt>
	<dd><p>This function returns the <code>edge</code> currently pointed to by an
	<code>edge_iterator</code>, but returns <code>NULL</code> if the iterator is
	pointing at the end of the sequence. This function has been provided
	for existing code makes the assumption that a <code>NULL</code> edge
	indicates the end of the sequence.
	</p>
	</dd>
	</dl>

	<p>The convenience macro <code>FOR_EACH_EDGE</code> can be used to visit all of
	the edges in a sequence of predecessor or successor edges. It must
	not be used when an element might be removed during the traversal,
	otherwise elements will be missed. Here is an example of how to use
	the macro:
	</p>
	<div class="smallexample">
	<pre class="smallexample">edge e;
	edge_iterator ei;

	FOR_EACH_EDGE (e, ei, bb->succs)
	{
	if (e->flags & EDGE_FALLTHRU)
	break;
	}
	</pre></div>

	<a name="index-fall_002dthru"></a>
	<p>There are various reasons why control flow may transfer from one block
	to another. One possibility is that some instruction, for example a
	<code>CODE_LABEL</code>, in a linearized instruction stream just always
	starts a new basic block. In this case a <em>fall-thru</em> edge links
	the basic block to the first following basic block. But there are
	several other reasons why edges may be created. The <code>flags</code>
	field of the <code>edge</code> data type is used to store information
	about the type of edge we are dealing with. Each edge is of one of
	the following types:
	</p>
	<dl compact="compact">
	<dt><em>jump</em></dt>
	<dd><p>No type flags are set for edges corresponding to jump instructions.
	These edges are used for unconditional or conditional jumps and in
	RTL also for table jumps. They are the easiest to manipulate as they
	may be freely redirected when the flow graph is not in SSA form.
	</p>
	</dd>
	<dt><em>fall-thru</em></dt>
	<dd><a name="index-EDGE_005fFALLTHRU_002c-force_005fnonfallthru"></a>
	<p>Fall-thru edges are present in case where the basic block may continue
	execution to the following one without branching. These edges have
	the <code>EDGE_FALLTHRU</code> flag set. Unlike other types of edges, these
	edges must come into the basic block immediately following in the
	instruction stream. The function <code>force_nonfallthru</code> is
	available to insert an unconditional jump in the case that redirection
	is needed. Note that this may require creation of a new basic block.
	</p>
	</dd>
	<dt><em>exception handling</em></dt>
	<dd><a name="index-exception-handling"></a>
	<a name="index-EDGE_005fABNORMAL_002c-EDGE_005fEH"></a>
	<p>Exception handling edges represent possible control transfers from a
	trapping instruction to an exception handler. The definition of
	“trapping” varies. In C++, only function calls can throw, but for
	Java and Ada, exceptions like division by zero or segmentation fault are
	defined and thus each instruction possibly throwing this kind of
	exception needs to be handled as control flow instruction. Exception
	edges have the <code>EDGE_ABNORMAL</code> and <code>EDGE_EH</code> flags set.
	</p>
	<a name="index-purge_005fdead_005fedges"></a>
	<p>When updating the instruction stream it is easy to change possibly
	trapping instruction to non-trapping, by simply removing the exception
	edge. The opposite conversion is difficult, but should not happen
	anyway. The edges can be eliminated via <code>purge_dead_edges</code> call.
	</p>
	<a name="index-REG_005fEH_005fREGION_002c-EDGE_005fABNORMAL_005fCALL"></a>
	<p>In the RTL representation, the destination of an exception edge is
	specified by <code>REG_EH_REGION</code> note attached to the insn.
	In case of a trapping call the <code>EDGE_ABNORMAL_CALL</code> flag is set
	too. In the <code>GIMPLE</code> representation, this extra flag is not set.
	</p>
	<a name="index-may_005ftrap_005fp_002c-tree_005fcould_005ftrap_005fp"></a>
	<p>In the RTL representation, the predicate <code>may_trap_p</code> may be used
	to check whether instruction still may trap or not. For the tree
	representation, the <code>tree_could_trap_p</code> predicate is available,
	but this predicate only checks for possible memory traps, as in
	dereferencing an invalid pointer location.
	</p>

	</dd>
	<dt><em>sibling calls</em></dt>
	<dd><a name="index-sibling-call"></a>
	<a name="index-EDGE_005fABNORMAL_002c-EDGE_005fSIBCALL"></a>
	<p>Sibling calls or tail calls terminate the function in a non-standard
	way and thus an edge to the exit must be present.
	<code>EDGE_SIBCALL</code> and <code>EDGE_ABNORMAL</code> are set in such case.
	These edges only exist in the RTL representation.
	</p>
	</dd>
	<dt><em>computed jumps</em></dt>
	<dd><a name="index-computed-jump"></a>
	<a name="index-EDGE_005fABNORMAL"></a>
	<p>Computed jumps contain edges to all labels in the function referenced
	from the code. All those edges have <code>EDGE_ABNORMAL</code> flag set.
	The edges used to represent computed jumps often cause compile time
	performance problems, since functions consisting of many taken labels
	and many computed jumps may have <em>very</em> dense flow graphs, so
	these edges need to be handled with special care. During the earlier
	stages of the compilation process, GCC tries to avoid such dense flow
	graphs by factoring computed jumps. For example, given the following
	series of jumps,
	</p>
	<div class="smallexample">
	<pre class="smallexample"> goto *x;
	[ … ]

	goto *x;
	[ … ]

	goto *x;
	[ … ]
	</pre></div>

	<p>factoring the computed jumps results in the following code sequence
	which has a much simpler flow graph:
	</p>
	<div class="smallexample">
	<pre class="smallexample"> goto y;
	[ … ]

	goto y;
	[ … ]

	goto y;
	[ … ]

	y:
	goto *x;
	</pre></div>

	<a name="index-pass_005fduplicate_005fcomputed_005fgotos"></a>
	<p>However, the classic problem with this transformation is that it has a
	runtime cost in there resulting code: An extra jump. Therefore, the
	computed jumps are un-factored in the later passes of the compiler
	(in the pass called <code>pass_duplicate_computed_gotos</code>).
	Be aware of that when you work on passes in that area. There have
	been numerous examples already where the compile time for code with
	unfactored computed jumps caused some serious headaches.
	</p>
	</dd>
	<dt><em>nonlocal goto handlers</em></dt>
	<dd><a name="index-nonlocal-goto-handler"></a>
	<a name="index-EDGE_005fABNORMAL_002c-EDGE_005fABNORMAL_005fCALL"></a>
	<p>GCC allows nested functions to return into caller using a <code>goto</code>
	to a label passed to as an argument to the callee. The labels passed
	to nested functions contain special code to cleanup after function
	call. Such sections of code are referred to as “nonlocal goto
	receivers”. If a function contains such nonlocal goto receivers, an
	edge from the call to the label is created with the
	<code>EDGE_ABNORMAL</code> and <code>EDGE_ABNORMAL_CALL</code> flags set.
	</p>
	</dd>
	<dt><em>function entry points</em></dt>
	<dd><a name="index-function-entry-point_002c-alternate-function-entry-point"></a>
	<a name="index-LABEL_005fALTERNATE_005fNAME"></a>
	<p>By definition, execution of function starts at basic block 0, so there
	is always an edge from the <code>ENTRY_BLOCK_PTR</code> to basic block 0.
	There is no <code>GIMPLE</code> representation for alternate entry points at
	this moment. In RTL, alternate entry points are specified by
	<code>CODE_LABEL</code> with <code>LABEL_ALTERNATE_NAME</code> defined. This
	feature is currently used for multiple entry point prologues and is
	limited to post-reload passes only. This can be used by back-ends to
	emit alternate prologues for functions called from different contexts.
	In future full support for multiple entry functions defined by Fortran
	90 needs to be implemented.
	</p>
	</dd>
	<dt><em>function exits</em></dt>
	<dd><p>In the pre-reload representation a function terminates after the last
	instruction in the insn chain and no explicit return instructions are
	used. This corresponds to the fall-thru edge into exit block. After
	reload, optimal RTL epilogues are used that use explicit (conditional)
	return instructions that are represented by edges with no flags set.
	</p>
	</dd>
	</dl>


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