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

[glisse at gcc dot gnu.org]

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

--- Comment #8 from Marc Glisse <glisse at gcc dot gnu.org> ---
With the modified hack below, the testsuite passes, except:

gcc.dg/tree-ssa/scev-[34].c those go from xfail to pass, that sounds good
g++.dg/tree-ssa/pr31146.C seems that the scan-tree-dump is too strict
c-c++-common/ubsan/object-size-9.c for some reason it prints <memory cannot be
printed> instead of a bunch of 0s but I can't reproduce outside the dg
framework, doesn't seem very important.

I had to add a couple checks compared to the first version. Giving up on
volatile operands, sure, there is probably a better way but whatever. Forwprop
tends to feed artificial &MEM_REF[(void*)...] to this function in many cases
(POINTER_PLUS_EXPR in particular), which causes a number of issues (including,
surprisingly, assuming too strong alignment). Testing for ptr_type_node is a
hack, but the necessity for it is just an artifact of placing the
transformation there, it shouldn't be necessary if I put it elsewhere.

The transformation is triggered by dereferences, and compares the types of
objects, not pointers, so it isn't obvious that the issues you were describing
apply to it.

--- tree-ssa-forwprop.c    (revision 219694)
+++ tree-ssa-forwprop.c    (working copy)
@@ -777,20 +777,49 @@ 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
+      && !gimple_has_volatile_ops (use_stmt)
+      && TREE_OPERAND (lhs, 0) == name
+      && integer_zerop (TREE_OPERAND (lhs, 1))
+      && (TREE_CODE (TREE_OPERAND (def_rhs, 0)) != MEM_REF
+      || TREE_TYPE (TREE_OPERAND (TREE_OPERAND (def_rhs, 0), 1))
+         != ptr_type_node)
+      && 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
+      && !gimple_has_volatile_ops (use_stmt)
+      && TREE_OPERAND (rhs, 0) == name
+      && integer_zerop (TREE_OPERAND (rhs, 1))
+      && (TREE_CODE (TREE_OPERAND (def_rhs, 0)) != MEM_REF
+      || TREE_TYPE (TREE_OPERAND (TREE_OPERAND (def_rhs, 0), 1))
+         != ptr_type_node)
+      && 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