Re: [PATCH] c, c++: attribute format on a ctor with a vbase [PR101833, PR47634]

[Jason Merrill <jason@redhat.com>]

On 4/8/22 15:21, Marek Polacek wrote:
> On Wed, Apr 06, 2022 at 04:55:54PM -0400, Jason Merrill wrote:
>> On 4/1/22 15:14, Marek Polacek wrote:
>>> Attribute format takes three arguments: archetype, string-index, and
>>> first-to-check.  The last two specify the position in the function
>>> parameter list.  r63030 clarified that "Since non-static C++ methods have
>>> an implicit this argument, the arguments of such methods should be counted
>>> from two, not one, when giving values for string-index and first-to-check."
>>> Therefore one has to write
>>>
>>>     struct D {
>>>       D(const char *, ...) __attribute__((format(printf, 2, 3)));
>>>     };
>>>
>>> However -- and this is the problem in this PR -- ctors with virtual
>>> bases also get two additional parameters: the in-charge parameter and
>>> the VTT parameter (added in maybe_retrofit_in_chrg).  In fact we'll end up
>>> with two clones of the ctor: an in-charge and a not-in-charge version (see
>>> build_cdtor_clones).  That means that the argument position the user
>>> specified in the attribute argument will refer to different arguments,
>>> depending on which constructor we're currently dealing with.  This can
>>> cause a range of problems: wrong errors, confusing warnings, or crashes.
>>>
>>> This patch corrects that; for C we don't have to do anything, and in C++
>>> we can use num_artificial_parms_for.  It would be wrong to rewrite the
>>> attributes the user supplied, so I've added an extra parameter called
>>> adjust_pos.
>>>
>>> Attribute format_arg is not affected, because it requires that the
>>> function returns "const char *" which will never be the case for cdtors.
>>>
>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>>
>>> 	PR c++/101833
>>> 	PR c++/47634
>>>
>>> gcc/c-family/ChangeLog:
>>>
>>> 	* c-attribs.cc (positional_argument): Add new argument adjust_pos,
>>> 	use it.
>>> 	* c-common.cc (check_function_arguments): Pass fndecl to
>>> 	check_function_format.
>>> 	* c-common.h (check_function_format): Adjust declaration.
>>> 	(maybe_adjust_arg_pos_for_attribute): Add.
>>> 	(positional_argument): Adjust declaration.
>>> 	* c-format.cc (decode_format_attr): Add fndecl argument.  Pass it to
>>> 	maybe_adjust_arg_pos_for_attribute.  Adjust calls to get_constant.
>>
>> I wonder about, instead of adding another parameter, allowing the current
>> fntype parameter to be the fndecl when we have one.
>>
>> And then that gets passed down into positional_argument, so we can call
>> maybe_adjust_arg_pos_for_attribute there, and adjust the return value
>> appropriately so we don't need the extra adjustment in get_constant?
> 
> Unfortunately I can't do that.  positional_argument can't return the
> adjusted position, because get_constant returns it and in decode_format_attr
> it's used to rewrite the arguments in the attribute list:
> 
>    tree *format_num_expr = &TREE_VALUE (TREE_CHAIN (args));
>    tree *first_arg_num_expr = &TREE_VALUE (TREE_CHAIN (TREE_CHAIN (args)));
>    ...
>      if (tree val = get_constant (fntype, atname, *format_num_expr,
>                                 2, &info->format_num, 0, validated_p,
>                                 adjust_pos))
>      *format_num_expr = val;

Could we not do that?  Currently isn't it just overwriting the value 
with the same value after default_conversion?  Maybe do that conversion 
directly in decode_format_attr instead?

> Replacing the arguments in the attribute list would lead to problems, because
> when we're processing the constructor clone without the additional parameters,
> the adjusted argument position would be out of whack at this point.
> 
> I've attempted to reduce the number of parameters, but it hardly seemed like
> a win, here's the patch I came up with:
> 
> diff --git a/gcc/c-family/c-attribs.cc b/gcc/c-family/c-attribs.cc
> index 6e17847ec9e..972476fbdf4 100644
> --- a/gcc/c-family/c-attribs.cc
> +++ b/gcc/c-family/c-attribs.cc
> @@ -594,7 +594,7 @@ attribute_takes_identifier_p (const_tree attr_id)
>   }
>   
>   /* Verify that argument value POS at position ARGNO to attribute NAME
> -   applied to function TYPE refers to a function parameter at position
> +   applied to function FNTYPE refers to a function parameter at position
>      POS and the expected type CODE.  Treat CODE == INTEGER_TYPE as
>      matching all C integral types except bool.  If successful, return
>      POS after default conversions, if any.  Otherwise, issue appropriate
> diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
> index 6f08b55d4a7..ffa36673ec0 100644
> --- a/gcc/c-family/c-common.cc
> +++ b/gcc/c-family/c-common.cc
> @@ -6069,7 +6069,7 @@ check_function_arguments (location_t loc, const_tree fndecl, const_tree fntype,
>     /* Check for errors in format strings.  */
>   
>     if (warn_format || warn_suggest_attribute_format)
> -    check_function_format (fntype, fndecl, TYPE_ATTRIBUTES (fntype), nargs,
> +    check_function_format (fndecl, TYPE_ATTRIBUTES (fntype), nargs,
>   			   argarray, arglocs);
>   
>     if (warn_format)
> diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
> index db6ff07db37..b68dc8f7d69 100644
> --- a/gcc/c-family/c-common.h
> +++ b/gcc/c-family/c-common.h
> @@ -857,7 +857,7 @@ extern void check_function_arguments_recurse (void (*)
>   					      opt_code);
>   extern bool check_builtin_function_arguments (location_t, vec<location_t>,
>   					      tree, tree, int, tree *);
> -extern void check_function_format (const_tree, const_tree, tree, int, tree *,
> +extern void check_function_format (const_tree, tree, int, tree *,
>   				   vec<location_t> *);
>   extern bool attribute_fallthrough_p (tree);
>   extern tree handle_format_attribute (tree *, tree, tree, int, bool *);
> diff --git a/gcc/c-family/c-format.cc b/gcc/c-family/c-format.cc
> index 87ae7bb73b0..6f0199dfcff 100644
> --- a/gcc/c-family/c-format.cc
> +++ b/gcc/c-family/c-format.cc
> @@ -70,7 +70,7 @@ static GTY(()) tree local_gimple_ptr_node;
>   static GTY(()) tree local_cgraph_node_ptr_node;
>   static GTY(()) tree locus;
>   
> -static bool decode_format_attr (const_tree, const_tree, tree, tree,
> +static bool decode_format_attr (const_tree, tree, tree,
>   				function_format_info *, bool);
>   static format_type decode_format_type (const char *, bool * = NULL);
>   
> @@ -330,17 +330,19 @@ get_constant (const_tree fntype, const_tree atname, tree expr, int argno,
>     return NULL_TREE;
>   }
>   
> -/* Decode the arguments to a "format" attribute into a
> -   function_format_info structure.  It is already known that the list
> -   is of the right length.  If VALIDATED_P is true, then these
> -   attributes have already been validated and must not be erroneous;
> -   if false, it will give an error message.  Returns true if the
> -   attributes are successfully decoded, false otherwise.  */
> +/* Decode the arguments to a "format" attribute into a function_format_info
> +   structure.  It is already known that the list is of the right length.  If
> +   VALIDATED_P is true, then these attributes have already been validated and
> +   must not be erroneous; if false, it will give an error message.  FN is
> +   either a FUNCTION_TYPE or a FUNCTION_DECL.  Returns true if the attributes
> +   are successfully decoded, false otherwise.  */
>   
>   static bool
> -decode_format_attr (const_tree fntype, const_tree fndecl, tree atname,
> -		    tree args, function_format_info *info, bool validated_p)
> +decode_format_attr (const_tree fn, tree atname, tree args,
> +		    function_format_info *info, bool validated_p)
>   {
> +  const_tree fndecl = TYPE_P (fn) ? NULL_TREE : fn;
> +  const_tree fntype = TYPE_P (fn) ? fn : TREE_TYPE (fn);
>     tree format_type_id = TREE_VALUE (args);
>     /* Note that TREE_VALUE (args) is changed in place below.  Ditto
>        for the value of the next element on the list.  */
> @@ -1170,7 +1172,7 @@ decode_format_type (const char *s, bool *is_raw /* = NULL */)
>      attribute themselves.  */
>   
>   void
> -check_function_format (const_tree fntype, const_tree fndecl, tree attrs,
> +check_function_format (const_tree fndecl, tree attrs,
>   		       int nargs, tree *argarray, vec<location_t> *arglocs)
>   {
>     tree a;
> @@ -1184,7 +1186,7 @@ check_function_format (const_tree fntype, const_tree fndecl, tree attrs,
>   	{
>   	  /* Yup; check it.  */
>   	  function_format_info info;
> -	  decode_format_attr (fntype, fndecl, atname, TREE_VALUE (a), &info,
> +	  decode_format_attr (fndecl, atname, TREE_VALUE (a), &info,
>   			      /*validated=*/true);
>   	  if (warn_format)
>   	    {
> @@ -5190,7 +5192,7 @@ handle_format_attribute (tree node[3], tree atname, tree args,
>     if (TREE_CODE (TREE_VALUE (args)) == IDENTIFIER_NODE)
>       TREE_VALUE (args) = canonicalize_attr_name (TREE_VALUE (args));
>   
> -  if (!decode_format_attr (type, fndecl, atname, args, &info,
> +  if (!decode_format_attr (fndecl ? fndecl : type, atname, args, &info,
>   			   /* validated_p = */false))
>       {
>         *no_add_attrs = true;
> 
> 
> Marek
>