[patch] for PR23509

[Zdenek Dvorak]

Hello,

> >we lose the VUSE, which could in worst case lead to misscompilations.
> >In this particular case it only confuses ivopts that expect 
> >name_mem_tag
> >or type_mem_tag to be set for every dereferenced pointer, and leads to
> >ICE.
> 
> Then how does copy prop works?  This is exactly what copy prop does
> and I don't see we losing VUSE there.

copy propagation merges the alias information (see
tree-ssa-copy.c:merge_alias_info).  Which of course is an
alternative solution for the problem; I may prepare a patch to
use it if preferable.

> This looks more like a bug in
> IV-OPTs not recording the aliasing info right when it transforms the
> loop.

The loss of VUSEs occurs before ivopts, so they have nothing to do with
it.

Zdenek