jtb20 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.
https://github.com/llvm/llvm-project/pull/96087
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