[Bug tree-optimization/114056] New: ifcvt may introduce use of uninitialized variables

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114056

            Bug ID: 114056
           Summary: ifcvt may introduce use of uninitialized variables
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kristerw at gcc dot gnu.org
  Target Milestone: ---

The ifcvt pass may make the code more UB by doing operations on uninitialized
variables, which can be seen by compiling the following (from
gcc.c-torture/compile/pr80422.c) with -O2 for X86_64:


int a, c, f;
short b, d, e;

int fn1 (int h)
{ 
  return a > 2 || h > a ? h : h << a;
}

void fn2 ()
{ 
  int j, k;
  while (1)
    { 
      k = c && b;
      f &= e > (fn1 (k) && j);
      if (!d)
        break;
    }
}


What is happening here is that .LOOP_VECTORIZED (1, 2) != 0 branches to bb 16
with _17 uninitialized, which is then used in some calculations:

  _34 = .LOOP_VECTORIZED (2, 3);
  if (_34 != 0)
    goto <bb 45>; [100.00%]
  else
    goto <bb 46>; [100.00%]

  <bb 45> [local count: 77953654]:

  <bb 16> [local count: 708669600]:
  # _13 = PHI <_24(27), _17(D)(45)>
  _18 = _13 <= 0;
  _14 = _9 & _18;
  _27 = _13 > 0;
  _28 = _9 & _27;
  _29 = _13 < -29020049;
  _30 = ~_29;
  _31 = _14 & _30;
  _12 = _15 ? _3 : _13;
  _42 = (unsigned int) _12;
  _43 = _42 * 4294967222;
  _32 = _15 | _28;
  _33 = _31 | _32;
  _23 = _33 ? _43 : 4294967222;
  _24 = _33 ? _12 : _13;
  if (x_6(D) > _23)
    goto <bb 9>; [11.00%]
  else
    goto <bb 27>; [89.00%]

This does not affect the result, but the discussion about the semantics of
uninitialized variables on the mailing list a while back concluded that
operations on uninitialized data is UB (with a few exceptions related to moving
data...).