[Bug tree-optimization/109539] [13/14 Regression] Compile-time hog in gimple-ssa-warn-access.cc since r13-6706-gadb70c2d1060b3

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109539

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
So what we try to do is, when check_pointer_uses follows the use chain
of 'ptr' through say

 ptr2 = ptr + 4;

then it wants to visit ptr2 uses as well.  For PHIs we get to

 ptr2 = PHI <..., ptr, ...>;

and we want to consider uses of ptr2 only if all other PHI arguments
are "related" to 'ptr'.  To prove that we perform complicated gymnastics,
in particular the only real check done is

  if (!ptr_derefs_may_alias_p (p, q))
    return false;

and that's of course incredibly weak.  To disprove relatedness
the pointers_related_p recurses, but only when either of both
pointers are defined by a PHI.  It uses pointer-query to skip
chains of pointer adjustments up to such definition.

So we basically do forward relatedness in check_pointer_uses and
then backward relatedness in pointers_related_p.

I think we should eschew that completely and only try to handle "forward"
PHIs by the iteration in check_pointer_uses itself.