Question on cfg_remove_useless_stmts_bb

[Jeffrey A Law]

On Tue, 2004-08-17 at 04:57, Richard Kenner wrote:
>       if (TREE_CODE (stmt) == ASM_EXPR
>           || (TREE_CODE (stmt) == MODIFY_EXPR
>               && (TREE_OPERAND (stmt, 0) == var
>                   || TREE_CODE (stmt, 0) == val)))
> 
>     Ought to do the trick.
> 
> In this case, yes, but I'm not convinced that it's complete.  Couldn't
> VAL be a more complex expression here and we have to verify that none of
> its operands were modified?  That issue is basically the other motivation
> for my query (the primary one being, of course, "how did this ever work?"!).
VAL can only be a _DECL node or a constant.  

  if (TREE_CODE (cond) == VAR_DECL || TREE_CODE (cond) == PARM_DECL)
    {
      var = cond;
      val = (bb->pred->flags & EDGE_FALSE_VALUE
             ? boolean_false_node : boolean_true_node);
    }
  else if (TREE_CODE (cond) == TRUTH_NOT_EXPR
           && (TREE_CODE (TREE_OPERAND (cond, 0)) == VAR_DECL
               || TREE_CODE (TREE_OPERAND (cond, 0)) == PARM_DECL))
    {
      var = TREE_OPERAND (cond, 0);
      val = (bb->pred->flags & EDGE_FALSE_VALUE
             ? boolean_true_node : boolean_false_node);
    }
  else
    {
      if (bb->pred->flags & EDGE_FALSE_VALUE)
        cond = invert_truthvalue (cond);
      if (TREE_CODE (cond) == EQ_EXPR
          && (TREE_CODE (TREE_OPERAND (cond, 0)) == VAR_DECL
              || TREE_CODE (TREE_OPERAND (cond, 0)) == PARM_DECL)
          && (TREE_CODE (TREE_OPERAND (cond, 1)) == VAR_DECL
              || TREE_CODE (TREE_OPERAND (cond, 1)) == PARM_DECL
              || TREE_CONSTANT (TREE_OPERAND (cond, 1))))
        {
          var = TREE_OPERAND (cond, 0);
          val = TREE_OPERAND (cond, 1);
        }
      else
        return;