[Bug tree-optimization/108360] [13 Regression] Dead Code Elimination Regression at -Os since r13-2048-g418b71c0d535bf

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108360

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
I now see the following IL into the last VRP pass

  <bb 2> [local count: 1073741824]:
  b.2_1 = b;
  _2 = b.2_1 <= 0;
  h.0_19 = (unsigned short) _2;
  _20 = h.0_19 + 65535;
  _21 = (short int) _20;
  _3 = _21 >= 0;
  _4 = (char) _3;
  f = _4;
  f.5_5 = (unsigned char) _3;
  _6 = f.5_5 << 4;
  e = _6;
  _23 = (short int) _3;
  _26 = _23 << 4;
  _8 = b.2_1 == _26;  [0,1]
  h_15 = (short int) _8; [0,1]
  _22 = h_15 + -1;  [-1,0]
  _18 = (unsigned int) _22;
  if (_18 <= 3) // we could substitute (b.2_1 == _26) here
    goto <bb 3>; [33.00%]
  else
    goto <bb 4>; [67.00%]

  <bb 3> [local count: 354334800]:
  foo ();

the last branch could be simplified by VRP, first we fail to replace
it by _18 == 0 which would then fold to _22 == 0 and then further to
the suggested compare.

Note I see -O2 and -O1 optimizing this just fine.  DOM2 threads a jump
here but some differences start earlier already.  It really looks some
phase-ordering bad luck triggers here, mainly triggered by early inlining
differences.