[PATCH] D103907: [LV] Parallel annotated loop does not imply all loads can be hoisted.

[Michael Kruse via Phabricator via llvm-commits]

Meinersbur added a comment.

I am in favor of this. Would be great if we had another person had a look. @pjaaskel? @fhahn?

To make another case: I think this kind speculation causes undefined behaviour/implementation-dependent semantics. For example:

  #pragma omp simd // adds llvm.loop.parallel_accesses
  for (int i = 0; i < N; i+=1) {
    double *C = i ? A : B;
    V = C[i].;
  }

which an optimization might transform to

  #pragma omp simd
  for (int i = 0; i < N; i+=1) {
    if (i)
      V = A[i];
    if (!i)
      V = *B;
  }

This moved from an unconditional memory access (no if-conversion) to two speculable accesses. In cases such as `N == 1`, A was never accessed (and might not have been allocated), but it would after an if-conversion. IMHO it is not predictable what memory is going to accessed.


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  https://reviews.llvm.org/D103907/new/

https://reviews.llvm.org/D103907