llvm dev - May 2021 - Loop unrolling pragma and OpenMP

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!

Michael implements the new features and knows :)

On 5/24/21 3:01 AM, ardi via llvm-dev wrote:
> 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!
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/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!
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev