[PATCH PR78114]Refine gfortran.dg/vect/fast-math-mgrid-resid.f

[Richard Biener]

On Fri, Nov 18, 2016 at 5:57 PM, Bin.Cheng <amker.cheng@gmail.com> wrote:
> On Fri, Nov 18, 2016 at 4:52 PM, Michael Matz <matz@suse.de> wrote:
>> Hi,
>>
>> On Thu, 17 Nov 2016, Bin.Cheng wrote:
>>
>>> B) Depending on ilp, I think below test strings fail for long time with haswell:
>>> ! { dg-final { scan-tree-dump-times "Executing predictive commoning
>>> without unrolling" 1 "pcom" { target lp64 } } }
>>> ! { dg-final { scan-tree-dump-times "Executing predictive commoning
>>> without unrolling" 2 "pcom" { target ia32 } } }
>>> Because vectorizer choose vf==4 in this case, and there is no
>>> predictive commoning opportunities at all.
>>> Also the newly added test string fails in this case too because the
>>> prolog peeled iterates more than 1 times.
>>
>> Btw, this probably means that on haswell (or other archs with vf==4) mgrid
>> is slower than necessary.  On mgrid you really really want predictive
>> commoning to happen.  Vectorization isn't that interesting there.
> Interesting, I will check if there is difference between 2/4 vf.  we
> do have cases that smaller vf is better and should be chosen, though
> for different reasons.

At some time in the past we had predictive commoning done before
vectorization (GCC 4.3 at least).

Patch is ok meanwhile.

Richard.

> Thanks,
> bin
>>
>>
>> Ciao,
>> Michael.