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Re: [gomp4 simd, RFC] Simple fix to override vectorization cost estimation.


I will get some tests.
As for cost analysis - simply consider the pragma as a request to
vectorize. How can I - as a developer - enforce it beyond the pragma?

On Wed, Nov 13, 2013 at 12:55 PM, Richard Biener <rguenther@suse.de> wrote:
> On Tue, 12 Nov 2013, Sergey Ostanevich wrote:
>
>> The reason patch was in its original state is because we want
>> to notify user that his assumption of profitability may be wrong.
>> This is not a part of any spec and as far as I know ICC does not
>> notify user about the case. Still it can be a good hint for those
>> users who tries to get as much as possible performance.
>>
>> Richard's comment on the vectorization problems is about the same -
>> to inform user that his attempt to force vectorization is failed.
>>
>> As for profitable or not - sometimes I believe it's impossible to be
>> precise. For OMP we have case of a vector version of a function
>> and we have no chance to figure out whether it is profitable to use
>> it or to loose it. If we can't map the loop for any vector length
>> other than 1 - I believe in this case we have to bail out and report.
>> Is it about 'never profitable'?
>
> For example.  I think we should report non-vectorized loops
> that are marked with force_vect anyway, with -Wdisabled-optimization.
> Another case is that a loop may be profitable to vectorize if
> the ISA supports a gather instruction but otherwise not.  Or if the
> ISA supports efficient vector construction from N not loop
> invariant scalars (for vectorization of strided loads).
>
> Simply disregarding all of the cost analysis sounds completely
> bogus to me.
>
> I'd simply go for the diagnostic for now, not changing anything else.
> We want to have a good understanding about why the cost model is
> so bad that we have to force to ignore it for #pragma simd - thus we
> want testcases.
>
> Richard.
>
>>
>> On Tue, Nov 12, 2013 at 6:35 PM, Richard Biener <rguenther@suse.de> wrote:
>> > On 11/12/13 3:16 PM, Jakub Jelinek wrote:
>> >> On Tue, Nov 12, 2013 at 05:46:14PM +0400, Sergey Ostanevich wrote:
>> >>> ivdep just substitutes all cross-iteration data analysis,
>> >>> nothing related to cost model. ICC does not cancel its
>> >>> cost model in case of #pragma ivdep
>> >>>
>> >>> as for the safelen - OMP standart treats it as a limitation
>> >>> for the vector length. this means if no safelen is present
>> >>> an arbitrary vector length can be used.
>> >>
>> >> I was talking about GCC loop->safelen, which is INT_MAX for #pragma omp simd
>> >> without safelen clause or #pragma simd without vectorlength clause.
>> >>
>> >>> so I believe loop->force_vect is the only trigger to disregard
>> >>> the cost model
>> >>
>> >> Anyway, in that case I think the originally posted patch is wrong,
>> >> if we want to treat force_vect as disregard all the cost model and
>> >> force vectorization (well, the name of the field already kind of suggest
>> >> that), then IMHO we should treat it the same as -fvect-cost-model=unlimited
>> >> for those loops.
>> >
>> > Err - the user may have a specific sub-architecture in mind when using
>> > #pragma simd, if you say we should completely ignore the cost model
>> > then should we also sorry () if we cannot vectorize the loop (either
>> > because of GCC deficiencies or lack of sub-target support)?
>> >
>> > That said, at least in the cases that the cost model says the loop
>> > is never profitable to vectorize we should follow its advice.
>> >
>> > Richard.
>> >
>> >> Thus (untested):
>> >>
>> >> 2013-11-12  Jakub Jelinek  <jakub@redhat.com>
>> >>
>> >>       * tree-vect-loop.c (vect_estimate_min_profitable_iters): Use
>> >>       unlimited cost model also for force_vect loops.
>> >>
>> >> --- gcc/tree-vect-loop.c.jj   2013-11-12 12:09:40.000000000 +0100
>> >> +++ gcc/tree-vect-loop.c      2013-11-12 15:11:43.821404330 +0100
>> >> @@ -2702,7 +2702,7 @@ vect_estimate_min_profitable_iters (loop
>> >>    void *target_cost_data = LOOP_VINFO_TARGET_COST_DATA (loop_vinfo);
>> >>
>> >>    /* Cost model disabled.  */
>> >> -  if (unlimited_cost_model ())
>> >> +  if (unlimited_cost_model () || LOOP_VINFO_LOOP (loop_vinfo)->force_vect)
>> >>      {
>> >>        dump_printf_loc (MSG_NOTE, vect_location, "cost model disabled.\n");
>> >>        *ret_min_profitable_niters = 0;
>> >>
>> >>       Jakub
>> >>
>> >
>>
>>
>
> --
> Richard Biener <rguenther@suse.de>
> SUSE / SUSE Labs
> SUSE LINUX Products GmbH - Nuernberg - AG Nuernberg - HRB 16746
> GF: Jeff Hawn, Jennifer Guild, Felix Imend


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