[Bug tree-optimization/98598] Missed opportunity to optimize dependent loads in loops

["jiangning.liu at amperecomputing dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98598

--- Comment #2 from Jiangning Liu <jiangning.liu at amperecomputing dot com> ---
Loop distribution can only handle very simple case. If the inner loop has
complicated control flow and other memory accesses with loop-carried
dependence, it would be hard to handle it. For example,

int foo (int n, int m, A *pa) {
  int sum;

  for (int i = 0; i < n; i++) {
    for (int j = 0; j < m; j++) {
      sum += pa[j].pb->pc->val;  // each value is repeatedly loaded "n" times
      sum = sum % 7;
    }
    sum = sum % 13;
  }

  return sum;
}

Alternatively, we can detect "invariant" dependent memory loads for the nested
loops with alias conflict checked. If the outer loop is hot enough, we could
have a chance to "hoist" them to create cache.

As for temp storage, is it a gcc's rule of thumb not to introduce temp storage
on heap, or it is just gcc doesn't have it yet and we want to have it?