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<h3 class="section">12.4 Alias analysis</h3>
<p><a name="index-alias-2530"></a><a name="index-flow_002dsensitive-alias-analysis-2531"></a><a name="index-flow_002dinsensitive-alias-analysis-2532"></a>
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.
<ol type=1 start=1>
<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>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.
<pre class="smallexample"> int i;
int foo (void)
{
# .MEM_3 = VDEF <.MEM_2(D)>
i = 1;
# VUSE <.MEM_3>
return i;
}
</pre>
<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>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.
<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>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:
<ul>
<li><code>pt_solution_includes</code>
<li><code>pt_solutions_intersect</code>
</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.
<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>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>.
<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:
<ul>
<li><code>refs_may_alias_p</code>
<li><code>ref_maybe_used_by_stmt_p</code>
<li><code>stmt_may_clobber_ref_p</code>
</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.
</ol>
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