[clang] Unable to vectorize loop with connectivity table on result index

[bemichel]

Hi,

I try to find if there is already an issue on the subject, but I did not find any.

I work on loop like this (https://godbolt.org/z/fchd4e54e), where the final index on the result array y is given by the integer table face_cell :

void compute_local_extrema(
  const int                nface, 
  const int                ncell, 
  const int*    __restrict face_cell,
  const double* __restrict x, 
        double* __restrict y) {
  int il, ir;

#pragma clang loop distribute(enable) vectorize(enable) interleave(enable)
  for (int iface=0; iface<nface; ++iface) {
    il = face_cell[iface];
    ir = face_cell[iface+nface];
    y[il] = std::max(y[il], x[ir]);
  }
}

It seems that the auto-vectorize does not work on the kind of loop, despite the fact that it seems working on GCC (https://godbolt.org/z/944q8M6T1 from 12.2 to 13.3).

Is there a way to finally vectorize this kind of loop ?
If it is possible, can you give me some advices on how to do this?

Thanks a lot for your help.

Bertrand M.

[fhahn]

I think it should vectorize with #pragma clang loop vectorize(assume_safety). Currently it doesn't work as expected, as some metadata gets dropped, but that should be fixed with #115605

[bemichel]

Thanks a lot,
It works fine (but only with specialization function std::fmax) : https://godbolt.org/z/EGq44c8vv
Bertrand M.

[fhahn]

@bemichel I think with fhahn@9e0ea8c, std::max() should now work

[bemichel]

Great thanks !!!