[llvm-dev] Does OpenMP hints bypass the vectorisation legality check in llvm

[Hal Finkel via llvm-dev]

Hi, Tom,

Yes, OpenMP SIMD hints are currently translated into metadata that is
used by the vectorizer. If you'd like to know how that's done, see:

http://llvm.org/docs/LangRef.html#llvm-mem-parallel-loop-access-metadata

(and should also enable the vectorizer:
http://llvm.org/docs/LangRef.html#llvm-loop-vectorize-enable-metadata)

The relevant Clang code is in clang/lib/CodeGen/CGLoopInfo.cpp and
lib/CodeGen/CGStmtOpenMP.cpp (see, in particular, the comments around
emitSimdlenSafelenClause and EmitOMPSimdInit regarding safelen and
semantics).

The relevant check for the vectorizer is in LoopAccessAnalysis.cpp:

  const bool IsAnnotatedParallel = TheLoop->isAnnotatedParallel();

where isAnnotatedParallel checks for the parallel-loop-access metadata
(that indicates that the loop is data parallel) and is part of LoopInfo.

 -Hal


On 01/19/2018 02:38 AM, Tom Sun via llvm-dev wrote:
>
> Hi all,
>
> I am currently looking into how "#pragma omp for simd" is actually
> recognized in llvm. To my knowledge, clang will parse it and set
> metadata in IR to indicate this force-vectorization hint and later
> optimization passes would read it and vectorize the marked loop.
> Therefore, the loop should be vectorized even the compiler think it
> might not be safe to do so?
>
> So my assumption is that such force-vectorization hints should bypass
> both the vectorization legality and cost model check. However, in
> LoopVectorize.cpp, I can't see how this is done. All loops will be
> sent to a legality check of LVL.canVectorize() and, if this condition
> does not fit, it return to false directly without actually reaching
> the vectorization stage. 
>
> Is there anything wrong with my assumption made on the use of
> force-vectorization hints?
>
> Thanks in advance,
>
> T
>
>
>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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