Re: [PATCH v5] c++: Move consteval folding to cp_fold_r

[Jason Merrill <jason@redhat.com>]

On 9/13/23 20:02, Marek Polacek wrote:
> On Wed, Sep 13, 2023 at 05:57:47PM -0400, Jason Merrill wrote:
>> On 9/13/23 16:56, Marek Polacek wrote:
>>> On Tue, Sep 12, 2023 at 05:26:25PM -0400, Jason Merrill wrote:
>>>> On 9/8/23 14:24, Marek Polacek wrote:
>>>>> +  switch (TREE_CODE (stmt))
>>>>> +    {
>>>>> +    /* Unfortunately we must handle code like
>>>>> +	 false ? bar () : 42
>>>>> +       where we have to check bar too.  */
>>>>> +    case COND_EXPR:
>>>>> +      if (cp_fold_immediate_r (&TREE_OPERAND (stmt, 1), walk_subtrees, data))
>>>>> +	return error_mark_node;
>>>>> +      if (TREE_OPERAND (stmt, 2)
>>>>> +	  && cp_fold_immediate_r (&TREE_OPERAND (stmt, 2), walk_subtrees, data))
>>>>> +	return error_mark_node;
>>>>
>>>> Is this necessary?  Doesn't walk_tree already walk into the arms of
>>>> COND_EXPR?
>>>
>>> Unfortunately yes.  The cp_fold call in cp_fold_r could fold the ?: into
>>> a constant before we see it here.  I've added a comment saying just that.
>>
>> Ah.  But in that case I guess we need to walk into the arms, not just check
>> the top-level expression in them.
>   
> Arg, of course.  I was fooled into thinking that it would recurse, but
> you're right.  Fixed by using cp_walk_tree as I intended.  Tested in
> consteval34.C.
> 
>> But maybe cp_fold_r should do that before the cp_fold, instead of this
>> function?
> 
> I...am not sure how that would be better than what I did.

Callers of cp_fold_immediate don't need this because cp_fold_r isn't 
involved, so it isn't folding anything.

cp_fold_r can walk the arms with cp_fold_r and then clear *walk_subtrees 
to avoid walking the arms again normally.

cp_fold_r uses data->pset to avoid walking the same tree twice; 
cp_fold_immediate_r currently doesn't do anything to avoid that.  If 
cp_fold_immediate_r doesn't itself call cp_walk_tree, cp_fold_immediate 
can use cp_walk_tree_without_duplicates.

>>>>> +      break;
>>>>> +
>>>>>         case PTRMEM_CST:
>>>>>           if (TREE_CODE (PTRMEM_CST_MEMBER (stmt)) == FUNCTION_DECL
>>>>>     	  && DECL_IMMEDIATE_FUNCTION_P (PTRMEM_CST_MEMBER (stmt)))
>>>>>     	{
>>>>> -	  if (!data->pset.add (stmt))
>>>>> +	  if (!data->pset.add (stmt) && (complain & tf_error))
>>>>>     	    error_at (PTRMEM_CST_LOCATION (stmt),
>>>>>     		      "taking address of an immediate function %qD",
>>>>>     		      PTRMEM_CST_MEMBER (stmt));
>>>>>     	  stmt = *stmt_p = build_zero_cst (TREE_TYPE (stmt));
>>>>
>>>> It looks like this will overwrite *stmt_p even if we didn't give an error.
>>>
>>> I suppose that could result in missing errors, adjusted.  And there's no
>>> point in setting stmt.
>>>>> -	  break;
>>>>> +	  return error_mark_node;
>>>>>     	}
>>>>>           break;
>>>>> +    /* Expand immediate invocations.  */
>>>>> +    case CALL_EXPR:
>>>>> +    case AGGR_INIT_EXPR:
>>>>> +      if (tree fn = cp_get_callee (stmt))
>>>>> +	if (TREE_CODE (fn) != ADDR_EXPR || ADDR_EXPR_DENOTES_CALL_P (fn))
>>>>> +	  if (tree fndecl = cp_get_fndecl_from_callee (fn, /*fold*/false))
>>>>> +	    if (DECL_IMMEDIATE_FUNCTION_P (fndecl))
>>>>> +	      {
>>>>> +		*stmt_p = stmt = cxx_constant_value (stmt, complain);
>>>>
>>>> Likewise.
>>>
>>> I think we have to keep setting *stmt_p to actually evaluate consteval
>>> functions.
>>
>> But only when it succeeds; we don't want to set it to error_mark_node if we
>> aren't complaining.
> 
> Hmm, probably not.  Fixed, thanks.
> 
> +	    if (DECL_IMMEDIATE_FUNCTION_P (fndecl))
> +	      {
> +		stmt = cxx_constant_value (stmt, complain);
> +		if (stmt == error_mark_node && (complain & tf_error))
> +		  return error_mark_node;
> +		*stmt_p = stmt;
> +	      }

This seems backwards; like with the ADDR_EXPR/PTRMEM_CST cases, I think 
we want to return error_mark_node regardless of complain, but only set 
*stmt_p when complaining.

Jason