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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/53774] Reassociator generates non-canonical addition
Date: Tue, 26 Jun 2012 14:24:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53774

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|NEW                         |ASSIGNED
         AssignedTo|unassigned at gcc dot       |rguenth at gcc dot gnu.org
                   |gnu.org                     |

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-06-26 14:24:13 UTC ---
I am testing instead

@@ -2299,8 +2299,16 @@ rewrite_expr_tree (gimple stmt, unsigned
              print_gimple_stmt (dump_file, stmt, 0, 0);
            }

-         gimple_assign_set_rhs1 (stmt, oe1->op);
-         gimple_assign_set_rhs2 (stmt, oe2->op);
+         if (tree_swap_operands_p (oe1->op, oe2->op, true))
+           {
+             gimple_assign_set_rhs1 (stmt, oe2->op);
+             gimple_assign_set_rhs2 (stmt, oe1->op);
+           }
+         else
+           {
+             gimple_assign_set_rhs1 (stmt, oe1->op);
+             gimple_assign_set_rhs2 (stmt, oe2->op);
+           }
          update_stmt (stmt);
          if (rhs1 != oe1->op && rhs1 != oe2->op)
            remove_visited_stmt_chain (rhs1);

which fixes it.  OTOH there are many other places reassoc adjusts
stmt operands (but it eventually relies on operand order).  Adding
a canonicalize_operand_order_and_update_stmt () wherever we
call update_stmt afer adjusting operands might be better ... or
simply calling fold_stmt on all stmts we touch which does the re-ordering, too.

Well, I'm not really sure where we should enforce canonical ordering ...