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Re: combine permutations in gimple


On Wed, Aug 15, 2012 at 2:53 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> On Wed, Aug 15, 2012 at 02:36:54PM +0200, Richard Guenther wrote:
>> On Wed, Aug 15, 2012 at 2:29 PM, Jakub Jelinek <jakub@redhat.com> wrote:
>> > On Wed, Aug 15, 2012 at 02:15:03PM +0200, Richard Guenther wrote:
>> >> Ok.  That would still leave us with the issue Ramana brought up - the
>> >> target hook returning true unconditionally if a generic permute is implemented.
>> >> We just avoid generic expansion by tree-vect-generic.c that way.
>> >
>> > Yeah, if there is a generic permute, can_vec_perm_p will return true always
>> > for the modes for which it is available.  Which is why I've been suggesting
>> > also the target hook which should return false if only generic permute is
>> > going to be used.
>>
>> Well.  What about returning a cost instead?  We don't want to transform
>> two insns to four either.
>
> That could work, it would just be a lot of work to change it (and more
> costly).  E.g. on i386 for 16-byte vectors with certain ISAs we return true
> right away, when returning cost we'd need to go the slow path always
> even when the caller is just interested in a boolean flag whether it is
> possible or not.
>
> CCing rth as the author of the hooks.

We'd want more accurate costs for the vectorizer cost model anyways.

Richard.

>         Jakub


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