[llvm-dev] Loop unrolling pragma and OpenMP

[Michael Kruse via llvm-dev]

Unfortunately, this does not work, by design. In the compiler pipeline
"#pragma omp parallel for" is lowered in the front-end, while "#pragma
clang loop unroll_count(N)" is in the LoopUnroll mid-end-pass.
"#pragma omp parallel for" cannot be applied on a loop that has not
yet been unrolled in the front-end. In practice, the #pragma clang
loop will just be ignored.

Michael

Am Mo., 24. Mai 2021 um 03:02 Uhr schrieb ardi via llvm-dev
<llvm-dev at lists.llvm.org>:
>
> Hi,
>
> Newer OpenMP versions include a construct for loop unrolling, but,
> however, I was wondering if the regular clang pragma for loop
> unrolling can be safely mixed with an OpenMP loop,without using the
> new OpenMP feature.
>
> I mean, is this guaranteed to work?
>
> #pragma omp parallel for
> #pragma clang loop unroll_count(N)
> for(i=0;i<n;i++) {
>    whatever();
> }
>
> Thanks a lot!
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