GNU Compiler Collection (GCC) Internals: Alias analysis

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### 13.4 Alias analysis

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.

1. Memory SSA form.

   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.

   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.

       int i;
       int foo (void)
       {
         # .MEM_3 = VDEF <.MEM_2(D)>
         i = 1;
         # VUSE <.MEM_3>
         return i;
       }

   The virtual SSA names in this case are `.MEM_2(D)` and `.MEM_3`. The store to the global variable `i` defines `.MEM_3` invalidating `.MEM_2(D)`. The load from `i` uses that new state `.MEM_3`.

   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.

2. Points-to and escape analysis.

   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.

   This points-to solution for a given SSA name pointer is stored in the `pt_solution` sub-structure of the `SSA_NAME_PTR_INFO` record. The following accessor functions are available:

   - `pt_solution_includes`
   - `pt_solutions_intersect`

   Points-to analysis also computes the solution for two special set of pointers, `ESCAPED` and `CALLUSED`. Those represent all memory that has escaped the scope of analysis or that is used by pure or nested const calls.

3. Type-based alias analysis

   Type-based alias analysis is frontend dependent though generic support is provided by the middle-end in `alias.cc`. TBAA code is used by both tree optimizers and RTL optimizers.

   Every language that wishes to perform language-specific alias analysis should define a function that computes, given a `tree` 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 `c_get_alias_set`.

4. Tree alias-oracle

   The tree alias-oracle provides means to disambiguate two memory references and memory references against statements. The following queries are available:

   - `refs_may_alias_p`
   - `ref_maybe_used_by_stmt_p`
   - `stmt_may_clobber_ref_p`

   In addition to those two kind of statement walkers are available walking statements related to a reference ref. `walk_non_aliased_vuses` 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. `walk_aliased_vdefs` walks over dominating memory defining statements and calls back on each statement clobbering ref providing its aliasing VDEF. The walk stops if asked to.

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