[Bug tree-optimization/105337] [12 Regression] wrong code at -Os and above on x86_64-linux-gnu

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105337

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu.org

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I'd say that ivopts is at fault here.  Before that we have:
  <bb 2> [local count: 312727306]:

  <bb 3> [local count: 805306369]:
  # b_14 = PHI <b_9(6), 0(2)>
  # c_15 = PHI <c_7(6), c_5(D)(2)>
  # ivtmp_3 = PHI <ivtmp_2(6), 3(2)>
  a.0_1 = a;
  if (a.0_1 == 0)
    goto <bb 7>; [5.50%]
  else
    goto <bb 4>; [94.50%]
...
  <bb 7> [local count: 44291850]:
  # b_4 = PHI <b_14(3)>

  <bb 5> [local count: 312727306]:
  # b_10 = PHI <b_4(7), 3(8)>
  return b_10;
as the path actually taken at runtime, yes, we use uninitialized c in the PHI,
but that doesn't correspond to UB at runtime.
But ivopts changes that to:
  <bb 2> [local count: 312727306]:
  _19 = (unsigned int) c_5(D);
  _20 = _19 + 4294967293;
  _21 = (int) _20;

  <bb 3> [local count: 805306369]:
  # c_15 = PHI <c_7(6), c_5(D)(2)>
  _13 = (unsigned int) c_5(D);
  _12 = (unsigned int) c_15;
  _11 = -_12;
  _18 = _11 + _13;
  b_14 = (int) _18;
  a.0_1 = a;
  if (a.0_1 == 0)
    goto <bb 7>; [5.50%]
  else
    goto <bb 4>; [94.50%]
...
  <bb 7> [local count: 44291850]:
  # b_4 = PHI <b_14(3)>

  <bb 5> [local count: 312727306]:
  # b_10 = PHI <b_4(7), 3(8)>
  return b_10;
and now it uses the uninitialized value in arithmetics in the actually executed
code.  And, even the return value which gets a copy of b_14 now depends on that
uninitialized value.  I think we had some other PR about this...
Then comes ccp4 and figures out that many values are UNDEFINED and optimizes it
into return 3;