[Bug tree-optimization/51877] New: [4.7 Regression] XEmacs miscompilation due to tail merging

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51877

             Bug #: 51877
           Summary: [4.7 Regression] XEmacs miscompilation due to tail
                    merging
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: ja...@gcc.gnu.org
        ReportedBy: ja...@gcc.gnu.org
                CC: vr...@gcc.gnu.org


extern void abort (void);
struct A { int a; char b[32]; } a, b;

__attribute__((noinline, noclone))
struct A
bar (int x)
{
  struct A r;
  static int n;
  r.a = ++n;
  __builtin_memset (r.b, 0, sizeof (r.b));
  r.b[0] = x;
  return r;
}

__attribute__((noinline, noclone))
void
baz (void)
{
  asm volatile ("" : : : "memory");
}

__attribute__((noinline, noclone))
void
foo (struct A *x, int y)
{
  if (y == 6)
    a = bar (7);
  else
    *x = bar (7);
  baz ();
}

int
main ()
{
  a = bar (3);
  b = bar (4);
  if (a.a != 1 || a.b[0] != 3 || b.a != 2 || b.b[0] != 4)
    abort ();
  foo (&b, 0);
  if (a.a != 1 || a.b[0] != 3 || b.a != 3 || b.b[0] != 7)
    abort ();
  foo (&b, 6);
  if (a.a != 4 || a.b[0] != 7 || b.a != 3 || b.b[0] != 7)
    abort ();
  return 0;
}

is miscompiled at -O2 (distilled from XEmacs).  Tail merging surprisingly
decides to tail merge the two calls, eventhough they have different lhs.