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<title>GNU Compiler Collection (GCC) Internals: Alias analysis</title>
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<a name="Alias-analysis"></a>
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<div class="header">
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<p>
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Next: <a href="Memory-model.html#Memory-model" accesskey="n" rel="next">Memory model</a>, Previous: <a href="SSA.html#SSA" accesskey="p" rel="prev">SSA</a>, Up: <a href="Tree-SSA.html#Tree-SSA" accesskey="u" rel="up">Tree SSA</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="Alias-analysis-1"></a>
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<h3 class="section">12.4 Alias analysis</h3>
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<a name="index-alias"></a>
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<a name="index-flow_002dsensitive-alias-analysis"></a>
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<a name="index-flow_002dinsensitive-alias-analysis"></a>
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<p>Alias analysis in GIMPLE SSA form consists of two pieces. First
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the virtual SSA web ties conflicting memory accesses and provides
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a SSA use-def chain and SSA immediate-use chains for walking
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possibly dependent memory accesses. Second an alias-oracle can
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be queried to disambiguate explicit and implicit memory references.
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</p>
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<ol>
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<li> Memory SSA form.
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<p>All statements that may use memory have exactly one accompanied use of
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a virtual SSA name that represents the state of memory at the
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given point in the IL.
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</p>
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<p>All statements that may define memory have exactly one accompanied
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definition of a virtual SSA name using the previous state of memory
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and defining the new state of memory after the given point in the IL.
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</p>
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<div class="smallexample">
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<pre class="smallexample">int i;
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int foo (void)
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{
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# .MEM_3 = VDEF <.MEM_2(D)>
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i = 1;
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# VUSE <.MEM_3>
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return i;
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}
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</pre></div>
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<p>The virtual SSA names in this case are <code>.MEM_2(D)</code> and
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<code>.MEM_3</code>. The store to the global variable <code>i</code>
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defines <code>.MEM_3</code> invalidating <code>.MEM_2(D)</code>. The
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load from <code>i</code> uses that new state <code>.MEM_3</code>.
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</p>
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<p>The virtual SSA web serves as constraints to SSA optimizers
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preventing illegitimate code-motion and optimization. It
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also provides a way to walk related memory statements.
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</p>
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</li><li> Points-to and escape analysis.
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<p>Points-to analysis builds a set of constraints from the GIMPLE
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SSA IL representing all pointer operations and facts we do
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or do not know about pointers. Solving this set of constraints
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yields a conservatively correct solution for each pointer
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variable in the program (though we are only interested in
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SSA name pointers) as to what it may possibly point to.
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</p>
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<p>This points-to solution for a given SSA name pointer is stored
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in the <code>pt_solution</code> sub-structure of the
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<code>SSA_NAME_PTR_INFO</code> record. The following accessor
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functions are available:
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</p>
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<ul>
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<li> <code>pt_solution_includes</code>
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</li><li> <code>pt_solutions_intersect</code>
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</li></ul>
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<p>Points-to analysis also computes the solution for two special
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set of pointers, <code>ESCAPED</code> and <code>CALLUSED</code>. Those
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represent all memory that has escaped the scope of analysis
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or that is used by pure or nested const calls.
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</p>
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</li><li> Type-based alias analysis
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<p>Type-based alias analysis is frontend dependent though generic
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support is provided by the middle-end in <code>alias.c</code>. TBAA
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code is used by both tree optimizers and RTL optimizers.
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</p>
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<p>Every language that wishes to perform language-specific alias analysis
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should define a function that computes, given a <code>tree</code>
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node, an alias set for the node. Nodes in different alias sets are not
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allowed to alias. For an example, see the C front-end function
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<code>c_get_alias_set</code>.
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</p>
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</li><li> Tree alias-oracle
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<p>The tree alias-oracle provides means to disambiguate two memory
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references and memory references against statements. The following
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queries are available:
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</p>
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<ul>
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<li> <code>refs_may_alias_p</code>
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</li><li> <code>ref_maybe_used_by_stmt_p</code>
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</li><li> <code>stmt_may_clobber_ref_p</code>
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</li></ul>
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<p>In addition to those two kind of statement walkers are available
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walking statements related to a reference ref.
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<code>walk_non_aliased_vuses</code> walks over dominating memory defining
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statements and calls back if the statement does not clobber ref
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providing the non-aliased VUSE. The walk stops at
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the first clobbering statement or if asked to.
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<code>walk_aliased_vdefs</code> walks over dominating memory defining
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statements and calls back on each statement clobbering ref
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providing its aliasing VDEF. The walk stops if asked to.
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</p>
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</li></ol>
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<div class="header">
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<p>
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Next: <a href="Memory-model.html#Memory-model" accesskey="n" rel="next">Memory model</a>, Previous: <a href="SSA.html#SSA" accesskey="p" rel="prev">SSA</a>, Up: <a href="Tree-SSA.html#Tree-SSA" accesskey="u" rel="up">Tree SSA</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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