[Bug tree-optimization/52558] write introduction incorrect wrt the C++11 memory model

[aldyh at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52558

--- Comment #14 from Aldy Hernandez <aldyh at gcc dot gnu.org> 2012-04-10 
17:07:22 UTC ---
Richard G., or perhaps another aliasing expert.  I am working on a patch for
this problem.  Could you pontificate as to why no optimization pass has been
able to figure out that g_2_lsm.6_12 == g_2 below?

  # VUSE <.MEM_9(D)>
  g_2_lsm.6_12 = g_2;            <-- g_2_lsm set to g_2
  if (pretmp.4_1 != 0)
    goto <bb 3>;
  else
    goto <bb 5>;

<bb 3>:
  # VUSE <.MEM_9(D)>
  D.1883_17 = g_2;
  if (g_2_lsm.6_12 != D.1883_17)    <-- g_2_lsm compared with g_2
    goto <bb 4>;

Why can't anyone figure out that g_2_lsm is g_2?  Am I building the conditions
and stores incorrectly, is there a missing annotation, or is something else
amok here?

Thanks.