[Bug middle-end/50716] Segmentation fault caused by misaligned vector access

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

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).