[llvm-dev] Vectorization width not correct using #pragma clang loop vectorize_width

[Friedman, Eli via llvm-dev]

On 9/20/2018 2:15 PM, hameeza ahmed wrote:
> Hello,
> I m trying to set vector width using #pragma clang loop 
> vectorize_width(32) but i m getting width 8 for the following kernel;
>
> *i m getting following output when i compiled;*
> *
> *
> *clang -O3  correlation.c   -Rpass=loop-vectorize -emit-llvm 
> -march=knl    -S  -o 1.ll
> correlation.c:38:9: remark: vectorized loop (vectorization width: 8, 
> interleaved count: 4) [-Rpass=loop-vectorize]
>         for (j = 0; j < M; j++)
>         ^
> *

With AVX-512, an instruction can operate on at most 8 double-precision 
lanes.  The vectorizer recognizes that, and interleaves the loop so you 
get 8*4==32 scalar iterations per iteration of the vectorized loop.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project

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