On Tue, Dec 6, 2011 at 11:24 PM, Eric Botcazou <ebotca...@adacore.com> wrote:
> Hi,
>
> this is a regression present on mainline and 4.6 branch at -O for the SPARC.
> The compiler generates an unaligned access for the memcpy call in:
>
> char *s
>
> union
> {
> int32_t i;
> char a[sizeof (int32_t)];
> }
> v;
>
> memcpy (v.a, s, sizeof (int32_t));
>
> The memcpy call is folded to an assignment between a couple of MEM_REFs:
>
> MEM[(char * {ref-all})&v] = MEM[(char * {ref-all})s_1];
>
> with type 'char a[4]'. The problem is that SRA turns this into:
>
> v$i_27 = MEM[(char * {ref-all})s_1].i;
>
> but s isn't aligned enough for an int32_t access.
>
>
> I don't think we can scalarize in this case on strict-alignment targets. The
> SRA pass already contains an alignment check, but it is purely internal to the
> RHS and this is clearly not sufficient anymore with MEM_REFs.
>
> The proposed fix is to enhance this alignment check to take into account both
> the RHS and the LHS (much like the memcpy folder). I think it subsumes the
> previous check, which could be viewed as a check involving the RHS and the
> type of the LHS. But there is a hitch: get_object_alignment is conservative
> for MEM_REF (unlike for INDIRECT_REF) so a trick is needed in order not to
> pessimize in some cases.
>
> Tested on SPARC/Solaris. OK for mainline and 4.6 branch?
You are basically (but non-optimally) locally re-implementing
what expr.c:get_object_or_type_alignment does, for the
bare MEM_REF case (you don't consider offsets that
do not change the alignment, nor alignment information
present on the SSA name).
Note that in expr.c gete_object_or_type_alignment is only
called on [TARGET_]MEM_REFs (I didn't export that function
exactly because of its restrictions, if we want to export it
we should assert it is only called for [TARGET_]MEM_REFs).
Thus, apart from the exp_align computation the patch looks ok
(it's only non-optimal, not incorrect).
Thanks,
Richard.
>
> 2011-12-06 Eric Botcazou <ebotca...@adacore.com>
>
> PR tree-optimization/51315
> * tree-sra.c (tree_non_mode_aligned_mem_p): Rename to...
> (tree_non_aligned_mem_p): ...this. Add ALIGN parameter. Look into
> MEM_REFs and deal with simple dereferences specially.
> (build_accesses_from_assign): Adjust for above change.
> (access_precludes_ipa_sra_p): Likewise.
>
>
> 2011-12-06 Eric Botcazou <ebotca...@adacore.com>
>
> * gcc.c-torture/execute/20111206-1.c: New test.
>
>
> --
> Eric Botcazou
