[Bug tree-optimization/33136] [4.1/4.2/4.3 Regression] wrong code due to alias with allocation in loop

["jakub at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #22 from jakub at gcc dot gnu dot org  2007-08-24 16:16 -------
Not sure how, could I pass the buck to you then?

My thought was just that may_alias_p could for STRUCT_FIELD_TAG call
ipa_type_escape_field_does_not_clobber (TREE_TYPE (SFT_PARENT_VAR (var)),
TREE_TYPE (var)) (i.e. don't care about PTRs type, just say if that field's
address wasn't ever taken that it can't alias.  That would be similar to how
alias.c uses this function (again, doesn't use the pointer type at all).

Anyway, I found that current GCC 4.2 miscompiles following modified testcase
(works with 4.1 and the trunk, though neither 4.1 nor 4.3 actually optimize bar
function as they could).  4.2 optimizes bar and misoptimizes baz, by assuming
*x = 4; will not clobber s.b.

/* { dg-do run } */
/* { dg-options "-O2" } */

extern void abort (void);

struct S
{
  struct S *a;
  int b;
  float f;
};

static struct S s;

static int *
__attribute__((noinline, const))
foo (void)
{
  return &s.b;
}

float
__attribute__((noinline))
bar (float *f)
{
  s.f = 1.0;
  *f = 4.0;
  return s.f;
}

int
__attribute__((noinline))
baz (int *x)
{
  s.b = 1;
  *x = 4;
  return s.b;
}

int
t (void)
{
  float f = 8.0;
  return bar (&f) + baz (foo ());
}

int
main (void)
{
  if (t () != 5)
    abort ();
  return 0;
}


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33136