[Bug tree-optimization/19637] Missed constant propagation with placement new

[rguenth at gcc dot gnu dot org]

------- Additional Comments From rguenth at gcc dot gnu dot org  2005-02-10 09:49 -------
The difference between foo_void and foo_void_offset is that for foo_void PRE
cannot see that (struct Foo *) &i[0] is equivalent to (struct Foo *) &i.  As
such, for foo_void we end up with

<bb 0>:
  __p_2 = &i[0];
  this_6 = (struct Foo *) &i[0];
  this_6->i[0] = 1;

<L3>:;
  i.3_7 = (struct Foo *) &i;
  D.1777_8 = i.3_7->i[0];
  return D.1777_8;

while for foo_void_offset the two uses of i are redundant and one is
removed:

<bb 0>:
  __p_2 = &i[0];
  this_6 = (struct Foo *) &i[0];
  this_6->i[0] = 1;

<L3>:;
  D.1786_7 = this_6;
  D.1785_8 = D.1786_7->i[0];
  return D.1785_8;


So it seems we either need to teach PRE the equivalency between
(struct Foo *) &i[0] and (struct Foo *) &i, or fold should canonicalize
them to one form (which one? I guess &i[0]).

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19637