[Bug tree-optimization/33136] [4.1/4.2/4.3 Regression] wrong code due to alias with allocation in loop

["dberlin at dberlin dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #23 from dberlin at gcc dot gnu dot org  2007-08-24 16:21 -------
Subject: Re:  [4.1/4.2/4.3 Regression] wrong code due to alias with allocation
in loop

On 24 Aug 2007 16:16:44 -0000, jakub at gcc dot gnu dot org
<gcc-bugzilla@gcc.gnu.org> wrote:
>
>
> ------- Comment #22 from jakub at gcc dot gnu dot org  2007-08-24 16:16 -------
> Not sure how, could I pass the buck to you then?
>
> My thought was just that may_alias_p could for STRUCT_FIELD_TAG call
> ipa_type_escape_field_does_not_clobber (TREE_TYPE (SFT_PARENT_VAR (var)),
> TREE_TYPE (var)) (i.e. don't care about PTRs type, just say if that field's
> address wasn't ever taken that it can't alias.  That would be similar to how
> alias.c uses this function (again, doesn't use the pointer type at all).
>
This would work too.
I will take care of it in about a week if you don't get their first.

> Anyway, I found that current GCC 4.2 miscompiles following modified testcase
> (works with 4.1 and the trunk, though neither 4.1 nor 4.3 actually optimize bar
> function as they could).  4.2 optimizes bar and misoptimizes baz, by assuming
> *x = 4; will not clobber s.b.
For 4.2, the safest thing to do is just remove the call from may_alias_p.
I serously doubt this will have any real effect on performance.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33136