[Bug tree-optimization/105030] store motion if-change flag causes if-conversion optimization can't be taken.

[guihaoc at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105030

--- Comment #11 from HaoChen Gui <guihaoc at gcc dot gnu.org> ---
I tested C source code with Ofast. The Ofast enables data store race. It should
do store motion but it fails. The problem is on cselim pass. It does
conditional store replacement. The 'atemp' is converted to its alias set - 'MEM
<double> [(void *)&atemp]' in the if-else blocks and remains untouch out of the
if-else.

   atemp.0_5 = atemp;
  if (_4 < atemp.0_5)
    goto <bb 5>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 4> [local count: 477815113]:
  cstore_18 = MEM <double> [(void *)&atemp];

  <bb 5> [local count: 955630225]:
  # cstore_17 = PHI <cstore_18(4), _4(3)>
  MEM <double> [(void *)&atemp] = cstore_17;

Then in lim2 pass, it thinks 'atemp' and 'MEM <double> [(void *)&atemp]' are
independent. So the store can't be moved out.

Memory reference 1: *_3
Memory reference 2: atemp
Memory reference 3: MEM <double> [(void *)&atemp]
Querying dependency of refs 3 and 1: independent.
Querying dependency of refs 3 and 2: dependent.
Querying SM WAR dependencies of ref 3 in loop 1: dependent

I wonder if 'atemp' is equivalent to 'MEM <double> [(void *)&atemp]'. Why we
shall use alias-set zero in cselim pass.

 /* Make both store and load use alias-set zero as we have to
     deal with the case of the store being a conditional change
     of the dynamic type.  */

  while (handled_component_p (*basep))
    basep = &TREE_OPERAND (*basep, 0);
  if (TREE_CODE (*basep) == MEM_REF
      || TREE_CODE (*basep) == TARGET_MEM_REF)
    TREE_OPERAND (*basep, 1)
      = fold_convert (ptr_type_node, TREE_OPERAND (*basep, 1));
  else
    *basep = build2 (MEM_REF, TREE_TYPE (*basep),
                     build_fold_addr_expr (*basep),
                     build_zero_cst (ptr_type_node));