[RFC] S/390: Alignment peeling prolog generation

[Richard Biener]

On Wed, Apr 12, 2017 at 9:50 AM, Robin Dapp <rdapp@linux.vnet.ibm.com> wrote:
>> Note I was very conservative here to allow store bandwidth starved
>> CPUs to benefit from aligning a store.
>>
>> I think it would be reasonable to apply the same heuristic to the
>> store case that we only peel for same cost if peeling would at least
>> align two refs.
>
> Do you mean checking if peeling aligns >= 2 refs for sure? (i.e. with a
> known misalignment) Or the same as currently via
> STMT_VINFO_SAME_ALIGN_REFS just for stores and .length() >= 2?

The latter.

> Is checking via vect_peeling_hash_choose_best_peeling () too costly or
> simply unnecessary if we already know the costs for aligned and
> unaligned are the same?

This one only works for known misalignment, otherwise it's overkill.

OTOH if with some refactoring we can end up using a single cost model
that would be great.  That is for the SAME_ALIGN_REFS we want to
choose the unknown misalignment with the maximum number of
SAME_ALIGN_REFS.  And if we know the misalignment of a single
ref then we still may want to align a unknown misalign ref if that has
more SAME_ALIGN_REFS (I think we always choose the known-misalign
one currently).

Richard.

> Regards
>  Robin
>