[Bug tree-optimization/104639] [12 Regression] Useless loop not fully optimized anymore

[jakub at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104639

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The ranger itself can't, i_2(D) in the PHI arg is ~[4, 4], and _3 is still [0,
1] aka VARYING.
Yes, phiopt could handle this by seeing a PHI result is only used in an
equality comparison and try to figure out something for it using range info or
so, or vrp2 could.
In GCC 11, it was indeed PRE that optimized that:
  if (i_2(D) == 4)
    goto <bb 4>; [97.00%]
  else
    goto <bb 3>; [3.00%]

  <bb 3> [local count: 3540129]:

  <bb 4> [local count: 118111600]:
  # i_6 = PHI <i_2(D)(3), 6(2)>
  _3 = i_6 != 0;
which is what we have on the trunk until optimized with:
  _Bool _1;
  _Bool prephitmp_7;

  <bb 2> [local count: 118111600]:
  if (i_2(D) == 4)
    goto <bb 4>; [97.00%]
  else
    goto <bb 3>; [3.00%]

  <bb 3> [local count: 3540129]:
  _1 = i_2(D) != 0;

  <bb 4> [local count: 118111600]:
  # i_6 = PHI <i_2(D)(3), 6(2)>
  # prephitmp_7 = PHI <_1(3), 1(2)>
and later reassoc2 optimizes that to just
  _6 = i_2(D) != 0;
  return _6;