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Re: Type-based alias analysis and alias sets
- From: Richard Guenther <rguenther at suse dot de>
- To: Eric Botcazou <ebotcazou at adacore dot com>
- Cc: gcc at gcc dot gnu dot org
- Date: Fri, 23 Oct 2009 13:52:36 +0200 (CEST)
- Subject: Re: Type-based alias analysis and alias sets
- References: <200910231331.41698.ebotcazou@adacore.com>
On Fri, 23 Oct 2009, Eric Botcazou wrote:
> Hi Richard,
>
> I just (re-)discovered that the new TBAA machinery is quite aggressive and
> breaks cases that used to work in Ada (-O2 testcase for SPARC64 attached).
>
> The problem boils down to this:
>
> D.1416_1 = (struct p__rec &) &r.F;
> r.F = ...
> ... = D.1416_1->d;
>
> DSE computes that the store to r.F is dead and eliminates it at -O2 because
> ultimately nonaliasing_component_refs_p returns false:
>
> /* If we have two type access paths B1.path1 and B2.path2 they may
> only alias if either B1 is in B2.path2 or B2 is in B1.path1. */
> return false;
>
> [Shouldn't nonaliasing_component_refs_p be named aliasing_component_refs_p or
> component_refs_may_alias_p instead]?
Err, yes ;) I named it after the RTL variant in alias.c.
> Yes, it's a blatant type-punning case but all the structure types are given
> the same alias set (struct p__rec, type of r, type of F) and 'd' is not
> addressable so all the memory accesses are done with the same alias set.
>
> The root of the problem is that same_type_for_tbaa never returns true since
> the types don't have the same TYPE_CANONICAL (rightfully so, they are not
> equivalent) so we fall back to the final return of nonaliasing_c_r_p.
>
> Shouldn't this final return be 'true' instead of 'false', like the final
> return in indirect_ref_may_alias_decl_p, so that the ultimate fallback is the
> comparison of alias sets like it used to be?
I changed this default to false somewhen in the past. There was a
big fat comment there on the alias-improvements branch (where I
wondered if returning false would be a safe thing to do).
I didn't find (or could construct) a single C or C++ testcase that
wasn't fine with the new default, so I switched it (IIRC the default
is disambiguating the most cases).
I'm fine with switching it back though, this time with a comment
explaining why it is not safe (instead of just speculating) and
a testcase (I guess you now indeed have one). Care to prepare
a patch?
Thanks,
Richard.