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<a name="Alias-analysis"></a>
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<p>
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<hr>
<a name="Alias-analysis-1"></a>
<h3 class="section">13.4 Alias analysis</h3>
<a name="index-alias"></a>
<a name="index-flow_002dsensitive-alias-analysis"></a>
<a name="index-flow_002dinsensitive-alias-analysis"></a>

<p>Alias analysis in GIMPLE SSA form consists of two pieces.  First
the virtual SSA web ties conflicting memory accesses and provides
a SSA use-def chain and SSA immediate-use chains for walking
possibly dependent memory accesses.  Second an alias-oracle can
be queried to disambiguate explicit and implicit memory references.
</p>
<ol>
<li> Memory SSA form.

<p>All statements that may use memory have exactly one accompanied use of
a virtual SSA name that represents the state of memory at the
given point in the IL.
</p>
<p>All statements that may define memory have exactly one accompanied
definition of a virtual SSA name using the previous state of memory
and defining the new state of memory after the given point in the IL.
</p>
<div class="smallexample">
<pre class="smallexample">int i;
int foo (void)
{
  # .MEM_3 = VDEF &lt;.MEM_2(D)&gt;
  i = 1;
  # VUSE &lt;.MEM_3&gt;
  return i;
}
</pre></div>

<p>The virtual SSA names in this case are <code>.MEM_2(D)</code> and
<code>.MEM_3</code>.  The store to the global variable <code>i</code>
defines <code>.MEM_3</code> invalidating <code>.MEM_2(D)</code>.  The
load from <code>i</code> uses that new state <code>.MEM_3</code>.
</p>
<p>The virtual SSA web serves as constraints to SSA optimizers
preventing illegitimate code-motion and optimization.  It
also provides a way to walk related memory statements.
</p>
</li><li> Points-to and escape analysis.

<p>Points-to analysis builds a set of constraints from the GIMPLE
SSA IL representing all pointer operations and facts we do
or do not know about pointers.  Solving this set of constraints
yields a conservatively correct solution for each pointer
variable in the program (though we are only interested in
SSA name pointers) as to what it may possibly point to.
</p>
<p>This points-to solution for a given SSA name pointer is stored
in the <code>pt_solution</code> sub-structure of the
<code>SSA_NAME_PTR_INFO</code> record.  The following accessor
functions are available:
</p>
<ul>
<li> <code>pt_solution_includes</code>
</li><li> <code>pt_solutions_intersect</code>
</li></ul>

<p>Points-to analysis also computes the solution for two special
set of pointers, <code>ESCAPED</code> and <code>CALLUSED</code>.  Those
represent all memory that has escaped the scope of analysis
or that is used by pure or nested const calls.
</p>
</li><li> Type-based alias analysis

<p>Type-based alias analysis is frontend dependent though generic
support is provided by the middle-end in <code>alias.c</code>.  TBAA
code is used by both tree optimizers and RTL optimizers.
</p>
<p>Every language that wishes to perform language-specific alias analysis
should define a function that computes, given a <code>tree</code>
node, an alias set for the node.  Nodes in different alias sets are not
allowed to alias.  For an example, see the C front-end function
<code>c_get_alias_set</code>.
</p>
</li><li> Tree alias-oracle

<p>The tree alias-oracle provides means to disambiguate two memory
references and memory references against statements.  The following
queries are available:
</p>
<ul>
<li> <code>refs_may_alias_p</code>
</li><li> <code>ref_maybe_used_by_stmt_p</code>
</li><li> <code>stmt_may_clobber_ref_p</code>
</li></ul>

<p>In addition to those two kind of statement walkers are available
walking statements related to a reference ref.
<code>walk_non_aliased_vuses</code> walks over dominating memory defining
statements and calls back if the statement does not clobber ref
providing the non-aliased VUSE.  The walk stops at
the first clobbering statement or if asked to.
<code>walk_aliased_vdefs</code> walks over dominating memory defining
statements and calls back on each statement clobbering ref
providing its aliasing VDEF.  The walk stops if asked to.
</p>
</li></ol>


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