[Bug tree-optimization/64601] Missed PRE on std::vector move assignment

["glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64601

--- Comment #7 from Marc Glisse <glisse at gcc dot gnu.org> ---
I am testing the following hack, I hope there will be at least 1 failure in the
testsuite that will help me understand why it is wrong. It bootstraps and gives
accesses like MEM[(struct _Vector_impl *)v_4(D)]._M_start.

> struct B { int i; int j; };
> struct B b;
> int *p = &((struct A *)&b)->i;
> *p = 0;

Hmmm, yeah, that's ugly... I wish it weren't legal... I'll have to play with it
to see exactly how bad it is... Thanks for the example.

--- tree-ssa-forwprop.c    (revision 219694)
+++ tree-ssa-forwprop.c    (working copy)
@@ -777,20 +777,41 @@ forward_propagate_addr_expr_1 (tree name
                     new_def_rhs);
       else if (is_gimple_min_invariant (new_def_rhs))
     gimple_assign_set_rhs_with_ops (use_stmt_gsi, NOP_EXPR, new_def_rhs);
       else
     return false;
       gcc_assert (gsi_stmt (*use_stmt_gsi) == use_stmt);
       update_stmt (use_stmt);
       return true;
     }

+  /* *&X -> X.  */
+  if (TREE_CODE (lhs) == MEM_REF
+      && TREE_OPERAND (lhs, 0) == name
+      && integer_zerop (TREE_OPERAND (lhs, 1))
+      && useless_type_conversion_p (TREE_TYPE (lhs),
+                    TREE_TYPE (TREE_OPERAND (def_rhs, 0))))
+    {
+      gimple_assign_set_lhs (use_stmt, unshare_expr (TREE_OPERAND (def_rhs,
0)));
+      tidy_after_forward_propagate_addr (use_stmt);
+      return true;
+    }
+  else if (TREE_CODE (rhs) == MEM_REF
+      && TREE_OPERAND (rhs, 0) == name
+      && integer_zerop (TREE_OPERAND (rhs, 1))
+      && useless_type_conversion_p (TREE_TYPE (rhs),
+                    TREE_TYPE (TREE_OPERAND (def_rhs, 0))))
+    {
+      gimple_assign_set_rhs1 (use_stmt, unshare_expr (TREE_OPERAND (def_rhs,
0)));
+      tidy_after_forward_propagate_addr (use_stmt);
+      return true;
+    }
   /* Now strip away any outer COMPONENT_REF/ARRAY_REF nodes from the LHS.
      ADDR_EXPR will not appear on the LHS.  */
   tree *lhsp = gimple_assign_lhs_ptr (use_stmt);
   while (handled_component_p (*lhsp))
     lhsp = &TREE_OPERAND (*lhsp, 0);
   lhs = *lhsp;

   /* Now see if the LHS node is a MEM_REF using NAME.  If so,
      propagate the ADDR_EXPR into the use of NAME and fold the result.  */
   if (TREE_CODE (lhs) == MEM_REF