[Bug tree-optimization/93538] New: equality of address of first member to address to enclosing object not folded

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93538

            Bug ID: 93538
           Summary: equality of address of first member to address to
                    enclosing object not folded
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The inequality below is trivially false but since GCC 4.6 the middle-end fails
to fold it into a constant (it is ultimately folded in RTL).  In GCC 4.5 it was
folded by the ealias pass.

$ cat a.c && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout a.c
struct A { int a[1]; };

void f (struct A *p)
{
  void *q = p->a;
  if (p != q)
    __builtin_abort ();
}

;; Function f (f, funcdef_no=0, decl_uid=1932, cgraph_uid=1, symbol_order=0)

f (struct A * p)
{
  void * q;

  <bb 2> [local count: 1073741824]:
  q_2 = &p_1(D)->a;
  if (p_1(D) != q_2)
    goto <bb 3>; [0.00%]
  else
    goto <bb 4>; [100.00%]

  <bb 3> [count: 0]:
  __builtin_abort ();

  <bb 4> [local count: 1073741824]:
  return;

}