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| <a name="Alias-analysis-1"></a> |
| <h3 class="section">12.4 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 <.MEM_2(D)> |
| i = 1; |
| # VUSE <.MEM_3> |
| 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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