[Bug target/111591] ppc64be: miscompilation with -mstrict-align / -O3

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111591

Michael Matz <matz at gcc dot gnu.org> changed:

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                 CC|                            |matz at gcc dot gnu.org

--- Comment #28 from Michael Matz <matz at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #27)
> (In reply to Kewen Lin from comment #26)
> > Thanks for the clarification! Is it possible to update the alias set for the
> > indirect accesses as well? since we know the address is originally taken
> > from one coalesced decl (also update its propagated ones).

That's not generally possible, the address-taking and the actual access might
be
separated by arbitrary obfuscating code:

   char *p = &x;
   char *p2 = get_some_pointer(p);
   *p2 = ...

Here p2 may, or may not, point to x.  So we'd need to be fairly conservative
here ...

> I suppose we could record a bitmap of all decls participating in any
> coalescing, check whether a MEM could possibly refer to any of them
> via the points-to API

... which the points-to API of course will be.

> and then force alias-set zero for those.

So that will work.  But I wonder if the result then won't be that essentially
all of the mem accesses will get alias set zero, at least if there was any
coalescing.  At that point we may also bite the bullet and just do away
with any TBAA alias sets in RTL at all.

> We
> could also try to do sophisticated analysis to make assigning a new
> alias-set for each coalesce group work, merging groups when there's
> indirect accesses that could alias a member of more than a single
> group.

Question is if the sophistication is worth it.