http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50716
--- Comment #5 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-10-14
12:10:52 UTC ---
(In reply to comment #4)
> It's because we do
>
> align = MAX (TYPE_ALIGN (TREE_TYPE (exp)), get_object_alignment
> (exp));
>
> which discards the knowledge we have (exp is aligned to 4 bytes,
> get_object_alignment returns 4).
>
> Which can be fixed for example by
>
> align = get_object_alignment_1 (exp, &misalign);
> align = MAX (TYPE_ALIGN (TREE_TYPE (exp)), align);
> if (misalign != 0)
> align = (misalign & -misalign);
>
> so always honor an explicit knowledge about misalignment. Or less
> aggressively,
>
> align = get_object_alignment_1 (exp, &misalign);
> if (TYPE_ALIGN (TREE_TYPE (exp)) <= align)
> {
> if (misalign != 0)
> align = (misalign & -misalign);
> }
> else
> align = TYPE_ALIGN (TREE_TYPE (exp));
>
> thus only when the base alignment is at least that of the types alignment.
> Which means we'd treat a vector load from a misaligned int * pointer as
> aligned, but from a misaligned vector int * pointer not - maybe too
> surprising,
> I'd definitely go with the first variant. Any idea which reasonable
> case we'd miss here? Even *(vector int *)(int-ptr + 2) would be handled
> as aligned, get_object_alignment_1 would return 32 (int aligned), the
> misalign is truncated to the base alignment.
One odd result is that with -O the code would "work" while with -O0
it would segfault (we don't know anything about the alignment at -O0 as
CCP is not run).
