[Bug tree-optimization/81549] New: PHI node should be eliminated if loop iterates enough times.

[amker at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81549

            Bug ID: 81549
           Summary: PHI node should be eliminated if loop iterates enough
                    times.
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amker at gcc dot gnu.org
  Target Milestone: ---

Given below test:

int a[10000], b[10000], c[10000];
int f(void)
{
  int i, n = 100;
  int t0 = a[0];
  int t1 = a[1];
     for (i = 0; i < n; i++)
       {
         a[i] = 1;
         int t2 = 2;
         t0 = t1;
         t1 = t2;
       }
     a[n] = t0;
     a[n+1] = t1;
  return 0;
}
Compile it at Ofast by default, the optimized dump is as:

  <bb 2> [1.00%] [count: INV]:
  t1_8 = a[1];
  ivtmp.9_17 = (unsigned long) &a;
  _16 = ivtmp.9_17 + 400;

  <bb 3> [99.00%] [count: INV]:
  # t1_20 = PHI <2(3), t1_8(2)>
  # ivtmp.9_2 = PHI <ivtmp.9_1(3), ivtmp.9_17(2)>
  _15 = (void *) ivtmp.9_2;
  MEM[base: _15, offset: 0B] = 1;
  ivtmp.9_1 = ivtmp.9_2 + 4;
  if (ivtmp.9_1 != _16)
    goto <bb 3>; [98.99%] [count: INV]
  else
    goto <bb 4>; [1.01%] [count: INV]

  <bb 4> [1.00%] [count: INV]:
  a[100] = t1_20;
  a[101] = 2;
  return 0;

We now eliminate one phi and leave another behind.  It is vrp1/dce2 when the
phi is eliminated.