RFC: [ARM] Disable peeling

[Richard Biener]

On Wed, Dec 12, 2012 at 6:50 PM, Andi Kleen <andi@firstfloor.org> wrote:
> "H.J. Lu" <hjl.tools@gmail.com> writes:
>>
>> i386.c has
>>
>>    {
>>       /* When not optimize for size, enable vzeroupper optimization for
>>          TARGET_AVX with -fexpensive-optimizations and split 32-byte
>>          AVX unaligned load/store.  */
>
> This is only for the load, not for deciding whether peeling is
> worthwhile or not.
>
> I believe it's unimplemented for x86 at this point. There isn't even a
> hook for it. Any hook that is added should ideally work for both ARM64
> and x86. This would imply it would need to handle different vector
> sizes.

There is

/* Implement targetm.vectorize.builtin_vectorization_cost.  */
static int
ix86_builtin_vectorization_cost (enum vect_cost_for_stmt type_of_cost,
                                 tree vectype,
                                 int misalign ATTRIBUTE_UNUSED)
{
...
      case unaligned_load:
      case unaligned_store:
        return ix86_cost->vec_unalign_load_cost;

which indeed doesn't distinguish between unaligned load/store cost.  Still
it does distinguish between aligned and unaligned load/store cost.

Now look at the cost tables and see different unaligned vs. aligned costs
dependent on the target CPU.

generic32 and generic64 have:

  1,                                    /* vec_align_load_cost.  */
  2,                                    /* vec_unalign_load_cost.  */
  1,                                    /* vec_store_cost.  */

The missed piece in the vectorizer is that peeling for alignment should have the
option to turn itself off based on that costs (and analysis).

Richard.