[Bug tree-optimization/77536] New: Vectorizer not maintaining relationship of relative block frequencies in absence of real profile data

["pthaugen at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77536

            Bug ID: 77536
           Summary: Vectorizer not maintaining relationship of relative
                    block frequencies in absence of real profile data
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pthaugen at gcc dot gnu.org
                CC: dje at gcc dot gnu.org, rguenth at gcc dot gnu.org,
                    wschmidt at gcc dot gnu.org
  Target Milestone: ---
              Host: powerpc64*-unknown-linux-gnu
            Target: powerpc64*-unknown-linux-gnu
             Build: powerpc64*-unknown-linux-gnu

// gcc -O3 -ffast-math -mcpu=power8 -mrecip=all -S test.c

void sub(float *restrict x, float *restrict y, float *restrict z, int n)
{
  int i;

  for (i = 0; i < n; i++)
    x[i] = z[i]/y[i];
}


In the dump before vectorizer (148t.ifcvt) the loop block has freq=8500 which
is almost 6X the freq of the entry block(1500). After vectoriztion the now
vectorized loop block has freq=662. While I agree the freq of the loop block
should drop some after vectorization, it should still maintain the relationship
of being the hottest block in the function.