[llvm-dev] Difference between clang -Oz and opt -Oz

[Fernando Magno Quintao Pereira via llvm-dev]

Dear LLVMers,

Why does opt -Oz and clang -Oz produce different codes? And could
someone point me to some documentation/discussion that explains the
differences between them?

Regards,

Fernando

------------------

Example
=======

This program below ends up with 74 instructions with opt -Oz, and 22
instructions with clang -Oz (LLVM 10.0). Opt unrolls the loop 2x,
whereas clang doesn't:

void prefix_sum(int *src, int *dst, int N) {
  if (0 < N) {
    int i = 0;
    do {
      int tmp = 0;
      int j = 0;
      if (j < i) {
        do  {
          tmp += src[j];
          j++;
        } while (j < i);
        dst[i] = tmp;
      }
      i++;
    } while (i < N);
  }
}

I can see some optimizations with clang, that might not run with opt when I do:

echo 'int;' | clang -xc -Oz - -o /dev/null -\#\#\#

Would that be the reason for the difference?