[PATCH] #pragma vectorize

[Hal Finkel]

----- Original Message -----
> From: "Nadav Rotem" <nrotem at apple.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Tyler Nowicki" <tnowicki at apple.com>, cfe-commits at cs.uiuc.edu, "Alexey Bataev" <alexey.bataev at intel.com>,
> "Alexander Musman" <alexander.musman at gmail.com>, "Chandler Carruth" <chandlerc at google.com>
> Sent: Tuesday, April 22, 2014 11:34:48 AM
> Subject: Re: [PATCH] #pragma vectorize
> 
> Hi Hal,
> 
> 
> Thanks for the review and for your support of this feature.
> 
> 
> 
> 
> 
> I feel strongly that we should separate the unrolling pragma from the
> vectorization pragma. The fact that modulo unrolling is implemented
> by our loop vectorizer is an implementation detail that I do not
> want to expose to our users directly.
> 
> 
> I think that the name ‘unroll’ is misleading because what the
> vectorizer does is not the usual loop unrolling. The vectorizer uses
> two or more SIMD registers to perform the widened scalar operations.
> I would like to allow users to control this special kind of
> unrolling within the vectorization pragma. Maybe we should give it a
> different name, like ‘widen’ ?

Okay, this is reasonable, but I'm still not sure it should be in a pragma called 'vectorize'. This particular transformation can be applied (and more often than not is applied) to scalar code. Strictly speaking it is an optimization that exposes ILP, and has little to do with vectorization.

That having been said, the fact that the correctness model for this 'widening' operation is similar (although not identical) to that for vectorization, and thus implemented by the vectorizer, is not facet of the implementation we should expose to the user. However, pragma widen is fine with me too.

 -Hal

> 
> 
> 
> 
> Also, we have a concatenation unroller which performs unrolling
> separate from the vectorizer. I think we should do something like
> this:
> 
> 1. For the purpose of this patch, please split off the unrolling into
> a separate pragma:
> #pragma unroll(_value_ | enable | disable)
> 
> 2. In the future, this syntax will be enhanced to something like
> this:
> #pragma unroll(unroll-spec-list)
> 
> unroll-spec-list:
> kind_prefix_opt unroll-spec
> 
> unroll-spec:
> _value_
> enable
> disable
> 
> kind_prefix:
> kind :
> 
> kind:
> sequenced :
> unsequenced :
> any :
> 
> [this sequenced vs unsequenced terminology is what we decided we
> liked for the parallel algorithms library being considered in WG21,
> and I think it applies just as well here]
> 
> In our implementation, 'unsequenced' unrolling means the modulo
> unrolling performed by the loop vectorizer. 'sequenced' unrolling
> means the concatenation unrolling performed by the generic unroller.
> 
> Adding Chandler, he might have some opinion on my use of the
> sequenced vs. unsequenced suggestion.
> 
> Also, adding Alexey and Alexander who have done some similar work in
> clang-omp.
> 
> -Hal
> 
> ----- Original Message -----
> 
> 
> From: "Tyler Nowicki" < tnowicki at apple.com >
> To: cfe-commits at cs.uiuc.edu
> Cc: "Nadav Rotem" < nrotem at apple.com >
> Sent: Monday, April 21, 2014 6:23:02 PM
> Subject: [PATCH] #pragma vectorize
> 
> 
> 
> Hi,
> 
> Please review the attached patch for adding pragma vectorize syntax /
> vectorization hints to clang.
> 
> pragma vectorize
> * supports the options enable, disable, unroll(_value_), and
> width(_value_)
> * options are turned into vectorization hints that are used during
> codegen to add metadata to the conditional branch of the for, while,
> and do-while loops.
> * enable forces the vectorizer to consider the loop, for example when
> compiling with Os
> * disable prevents vectorization of the loop
> * The _value_ specified by unroll(_value_) and width(_value_) must be
> a positive integer. It will be used to set the
> llvm.vectorizer.unroll or llvm.vectorizer.width metadata values.
> 
> Thank you,
> 
> Tyler Nowicki
> Apple
> 
> 
> 
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> 
> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory