[Bug middle-end/20638] gcc doesn't take advantage of attribute malloc on alloca

[ghazi at gcc dot gnu dot org]

------- Additional Comments From ghazi at gcc dot gnu dot org  2005-04-28 13:11 -------
Roger Sayle provided this feedback to me in private email and agreed to have it 
copied here:

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I've glanced through the code and I suspect I know why this isn't
working.  The MALLOC attribute is traditionally handled in calls.c
where it attaches a REG_NOALIAS note to the relevant call_insn.
In the case of alloca, however, we don't go through calls.c and
instead inline calls to alloca via expand_builtin_alloca in
builtins.c instead.  My guess is that we need to add a REG_NOALIAS
note, or mark/assign the alias set of this pointer in builtins.c.

Interestingly, this doesn't explain why the builtins-13.c tests
aren't now being optimized by tree-ssa, where the "malloc" attribute
should now be getting used during the early alias analysis passes.

Unfortunately, I'm not an expert of how to present aliasing information
at the RTL-level.  But the above should help explain why alloca behaves
differently to malloc/calloc/strdup etc.

Roger


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20638