[Bug tree-optimization/105142] [12 Regression] Wrong code with -O2 since r12-2591

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105142

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
long long int c = 3623214276426624192LL;
unsigned short b;
char a = 42;
const long long &min(const long long &x, const long long &y) { return x < y ? x
: y; }
__attribute__((noipa)) void test() { b = min(a, min(a, c) +
5713568809962283044LL); }
int main() { test(); if (b != 42) __builtin_abort(); }

ifcombine "optimizes" the IMO correct:
  <bb 2> [local count: 1073741824]:
  a.1_1 = a;
  _2 = (long long int) a.1_1;
  _16 = c;
  if (_2 < _16)
    goto <bb 4>; [50.00%]
  else
    goto <bb 3>; [50.00%]

  <bb 3> [local count: 536870912]:
  _4 = _16 + 5713568809962283044;
  if (_4 > _2)
    goto <bb 4>; [20.00%]
  else
    goto <bb 5>; [80.00%]

  <bb 4> [local count: 536870912]:

  <bb 5> [local count: 1073741824]:
  # _5 = PHI <_2(4), _4(3)>
  _6 = (short unsigned int) _5;
  b = _6;
which for the testcase jumps from bb 2 to bb4 into:
  <bb 2> [local count: 1073741824]:
  a.1_1 = a;
  _2 = (long long int) a.1_1;
  _16 = c;
  _4 = _16 + 5713568809962283044;
  if (_2 < _4)
    goto <bb 3>; [60.00%]
  else
    goto <bb 4>; [40.00%]

  <bb 3> [local count: 536870912]:

  <bb 4> [local count: 1073741824]:
  # _5 = PHI <_2(3), _4(2)>
  _6 = (short unsigned int) _5;
  b = _6;
which invokes UB on the _16 + 5713568809962283044 addition (signed integer
overflow).  So, either it needs to make sure the no longer conditional addition
is performed in unsigned type, or it needs to give up.