[Bug tree-optimization/87355] missed comparison optimizations (grep DFA, x86-64)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87355

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-12-25

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.
for f, if GCC would swap around the conditional, we would get decent code.

That is like this:

int f1 (int n, int i)
{
  if (n == THRESHOLD)
    return i;
  if (n >= THRESHOLD)
    return 0;
  return -1;
}

If we have:
  <bb 2> [local count: 1073741823]:
  if (n_3(D) > 99)
    goto <bb 3>; [96.19%]
  else
    goto <bb 5>; [3.81%]

  <bb 3> [local count: 1032832263]:
  if (n_3(D) == 100)
    goto <bb 5>; [34.00%]
  else
    goto <bb 4>; [66.00%]

  <bb 4> [local count: 681669290]:

  <bb 5> [local count: 1073741824]:
  # _1 = PHI <0(4), -1(2), i_4(D)(3)>

We should notice that the conditional at bb 3 is independent and that swapping
would get us a PHI which has 0, -1 which can then be phiopt'ed.