alexey-bataev wrote:
> > > FWIW, I don't think there's a reasonable, safe way to collapse loops like
> > > this and maintain parallel semantics, but ICBW.
> >
> >
> > I think, there is a proper way. Just annotate these instructions with
> > something like:
> > ```
> > if (first inner loop iteration) {
> > arr[i][i] = ...;
> > }
> > ```
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > and it should be correct
>
> I think you're right for this particular test case, but I don't think that
> solution works in general. Consider something like:
>
> ```
> int arr1[N][N], arr2[N];
>
> #pragma omp parallel for collapse(2)
> {
> for (int i = 0; i < N; i++) {
> arr2[i] = i;
> for (int j = 0; j < N; j++) {
> arr1[i][j] += arr2[i];
> }
> }
> }
> ```
>
> Now, if collapsed loop iterations take place in parallel or in an arbitrary
> order, there could be a read from arr2[i] before the write (in the "first"
> inner loop iteration). That is, I think we'd have something like:
>
> ```
> for (int ij = 0; ij < N*N; ij++) {
> int i = ij / N, j = ij % N;
> if (j == 0) {
> arr2[i] = i;
> }
> arr1[i][j] += arr2[i];
> }
> ```
>
> and that's still not safe.
something like
```
bool Init[N] = {false};
for (int ij = 0; ij < N*N; ij++) {
int i = ij / N, j = ij % N;
#pragma omp critical
if (!Init[i]) {
arr2[i] = i;
Init[i] = true;
}
arr1[i][j] += arr2[i];
}
```
remarks or anything else still is not correct in this case.
https://github.com/llvm/llvm-project/pull/96087
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