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

Tian, Xinmin via llvm-dev llvm-dev at lists.llvm.org
Fri Jan 19 09:54:05 PST 2018

Tom, your understanding is correct per OpenMP SIMD model. Our implementation behaves as you stated. Which is not part of LLVM main trunk yet.


From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Tom Sun via llvm-dev
Sent: Friday, January 19, 2018 12:39 AM
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] Does OpenMP hints bypass the vectorisation legality check in llvm

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,

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