[Bug tree-optimization/119399] New: Overlap check in vectorized code may invoke UB

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119399

            Bug ID: 119399
           Summary: Overlap check in vectorized code may invoke UB
           Product: gcc
           Version: 15.0
            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: ---

Compile the function below for X86_64 with -O3:

void foo(int *p, int *q, int n)
{
  for (int i = 0; i < n; i++)
    p[i] = q[i] + 1;
}

The vectorizer adds code performing an overlap check:

  <bb 14>:
  _7 = q_11(D) + 4;
  _25 = p_12(D) - _7;
  _26 = (sizetype) _25;
  _27 = _26 > 8;
  _28 = _27;
  if (_28 != 0)
    goto <bb 11>;
  else
    goto <bb 12>;

This takes the difference between two pointers that may point to different
objects, which invokes UB if the distance between the objects are too large to
fit in the result type.