Re: [PATCH] Fix PR tree-optimization/51315

[Richard Sandiford <rdsandiford at googlemail dot com>]

Eric Botcazou <ebotcazou@adacore.com> writes:
>> 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).
>
> Adjusted version attached, regression-tested on SPARC/Solaris.  I'll do a full 
> testing cycle if it is OK for mainline.
>
> get_object_or_type_alignment doesn't exist on the 4.6 branch, the expanders for 
> MEM_REF and TARGET_MEM_REF do:
>
>   MAX (TYPE_ALIGN (TREE_TYPE (exp)),
>        get_object_alignment (exp, BIGGEST_ALIGNMENT)))
>
> instead, so I presume I should do the same for them in tree_non_aligned_mem_p?
>
>
> 2011-12-07  Eric Botcazou  <ebotcazou@adacore.com>
>
>         PR tree-optimization/51315
> 	* tree.h (get_object_or_type_alignment): Declare.
> 	* expr.c (get_object_or_type_alignment): Move to...
> 	* builtins.c (get_object_or_type_alignment): ...here.  Add assertion.
>         * 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 use get_object_or_type_alignment for them.
>         (build_accesses_from_assign): Adjust for above change.
>         (access_precludes_ipa_sra_p): Likewise.

Sorry for the late notice, but this regressed memcpy-1.c for n32 and n64
on mips64-linux-gnu.  I realise memcpy-1.c isn't viewed as being a proper
SRA test, but in this case I think it really is showing a genuine problem.
We have:

    struct a {int a,b,c;} a;
    int test(struct a a)
    {
    struct a nasty_local;
    __builtin_memcpy (&nasty_local,&a, sizeof(a));
    return nasty_local.a;
    }

We apply LOCAL_ALIGNMENT to nasty_local during estimated_stack_frame_size,
so we have a VAR_DECL (nasty_local) with 64-bit alignment and a PARM_DECL
(a) with 32-bit alignment.  This fails the condition:

      if (STRICT_ALIGNMENT
	  && tree_non_aligned_mem_p (rhs, get_object_alignment (lhs)))
        lacc->grp_unscalarizable_region = 1;

because LHS has 64-bit alignment but RHS has 32-bit alignment.

Richard