[Bug tree-optimization/91108] New: [8/9/10 Regression] Fails to pun through unions

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91108

            Bug ID: 91108
           Summary: [8/9/10 Regression] Fails to pun through unions
           Product: gcc
           Version: 9.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

The following testcase fails to support our promise for punning through
union members if the access happens through the union.

/* { dg-do run } */
/* { dg-options "-O3 -fstrict-aliasing" } */

union U {
  struct A { int : 2; int x : 8; } a;
  struct B { int : 6; int x : 8; } b;
};

int __attribute__((noipa))
foo (union U *p, union U *q)
{
  p->a.x = 1;
  q->b.x = 1;
  return p->a.x;
}

int
main()
{
  union U x;
  if (foo (&x, &x) != x.a.x)
    __builtin_abort ();
  return 0;
}