Re: [PATCH v5] c++: implement [[gnu::non_owning]] [PR110358]

[Jason Merrill <jason@redhat.com>]

On 3/1/24 14:24, Marek Polacek wrote:
> On Fri, Mar 01, 2024 at 01:19:40PM -0500, Jason Merrill wrote:
>> On 3/1/24 12:39, Marek Polacek wrote:
>>>    @option{-Wdangling-reference} also warns about code like
>>>    @smallexample
>>> @@ -3932,6 +3935,10 @@ struct Span @{
>>>    as @code{std::span}-like; that is, the class is a non-union class
>>>    that has a pointer data member and a trivial destructor.
>>> +The warning can be disabled by using the @code{gnu::no_dangling} attribute
>>> +on a function (@pxref{Common Function Attributes}), or a class type
>>> +(@pxref{C++ Attributes}).
>>
>> It seems surprising that one is in a generic attributes section and the
>> other in the C++-specific section.  Maybe both uses could be covered in the
>> C++ attributes section?
> 
> Arg yes, definitely.  Done here.
>   
>>>    This warning is enabled by @option{-Wall}.
>>>    @opindex Wdelete-non-virtual-dtor
>>> diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling1.C b/gcc/testsuite/g++.dg/ext/attr-no-dangling1.C
>>> new file mode 100644
>>> index 00000000000..02eabbc5003
>>> --- /dev/null
>>> +++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling1.C
>>> @@ -0,0 +1,38 @@
>>> +// { dg-do compile { target c++11 } }
>>> +// { dg-options "-Wdangling-reference" }
>>> +
>>> +int g = 42;
>>> +
>>> +struct [[gnu::no_dangling]] A {
>>> +  int *i;
>>> +  int &foo() { return *i; }
>>> +};
>>> +
>>> +struct A2 {
>>> +  int *i;
>>> +  [[gnu::no_dangling]] int &foo() { return *i; }
>>> +  [[gnu::no_dangling]] static int &bar (const int &) { return *&g; }
>>> +};
>>> +
>>> +union [[gnu::no_dangling]] U { };
>>> +
>>> +A a() { return A{&g}; }
>>> +A2 a2() { return A2{&g}; }
>>> +
>>> +class X { };
>>> +const X x1;
>>> +const X x2;
>>> +
>>> +[[gnu::no_dangling]] const X& get(const int& i)
>>> +{
>>> +   return i == 0 ? x1 : x2;
>>> +}
>>> +
>>> +void
>>> +test ()
>>> +{
>>> +  [[maybe_unused]] const X& x = get (10);	// { dg-bogus "dangling" }
>>> +  [[maybe_unused]] const int &i = a().foo();	// { dg-bogus "dangling" }
>>> +  [[maybe_unused]] const int &j = a2().foo();	// { dg-bogus "dangling" }
>>> +  [[maybe_unused]] const int &k = a2().bar(10);	// { dg-bogus "dangling" }
>>> +}
>>
>> Do you want to add destructors to A/A2 like you did in other tests?
> 
> Added.  I think this test predates the recent heuristic.
> 
> Ok for trunk?
> 
> -- >8 --
> Since -Wdangling-reference has false positives that can't be
> prevented, we should offer an easy way to suppress the warning.
> Currently, that is only possible by using a #pragma, either around the
> enclosing class or around the call site.  But #pragma GCC diagnostic tend
> to be onerous.  A better solution would be to have an attribute.
> 
> To that end, this patch adds a new attribute, [[gnu::no_dangling]].
> This attribute takes an optional bool argument to support cases like:
> 
>    template <typename T>
>    struct [[gnu::no_dangling(std::is_reference_v<T>)]] S {
>       // ...
>    };
> 
> 	PR c++/110358
> 	PR c++/109642
> 
> gcc/cp/ChangeLog:
> 
> 	* call.cc (no_dangling_p): New.
> 	(reference_like_class_p): Use it.
> 	(do_warn_dangling_reference): Use it.  Don't warn when the function
> 	or its enclosing class has attribute gnu::no_dangling.
> 	* tree.cc (cxx_gnu_attributes): Add gnu::no_dangling.
> 	(handle_no_dangling_attribute): New.
> 
> gcc/ChangeLog:
> 
> 	* doc/extend.texi: Document gnu::no_dangling.
> 	* doc/invoke.texi: Mention that gnu::no_dangling disables
> 	-Wdangling-reference.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/ext/attr-no-dangling1.C: New test.
> 	* g++.dg/ext/attr-no-dangling2.C: New test.
> 	* g++.dg/ext/attr-no-dangling3.C: New test.
> 	* g++.dg/ext/attr-no-dangling4.C: New test.
> 	* g++.dg/ext/attr-no-dangling5.C: New test.
> 	* g++.dg/ext/attr-no-dangling6.C: New test.
> 	* g++.dg/ext/attr-no-dangling7.C: New test.
> 	* g++.dg/ext/attr-no-dangling8.C: New test.
> 	* g++.dg/ext/attr-no-dangling9.C: New test.
> ---
>   gcc/cp/call.cc                               | 38 ++++++++++--
>   gcc/cp/tree.cc                               | 26 ++++++++
>   gcc/doc/extend.texi                          | 47 ++++++++++++++
>   gcc/doc/invoke.texi                          |  6 ++
>   gcc/testsuite/g++.dg/ext/attr-no-dangling1.C | 40 ++++++++++++
>   gcc/testsuite/g++.dg/ext/attr-no-dangling2.C | 29 +++++++++
>   gcc/testsuite/g++.dg/ext/attr-no-dangling3.C | 24 ++++++++
>   gcc/testsuite/g++.dg/ext/attr-no-dangling4.C | 14 +++++
>   gcc/testsuite/g++.dg/ext/attr-no-dangling5.C | 31 ++++++++++
>   gcc/testsuite/g++.dg/ext/attr-no-dangling6.C | 65 ++++++++++++++++++++
>   gcc/testsuite/g++.dg/ext/attr-no-dangling7.C | 31 ++++++++++
>   gcc/testsuite/g++.dg/ext/attr-no-dangling8.C | 30 +++++++++
>   gcc/testsuite/g++.dg/ext/attr-no-dangling9.C | 25 ++++++++
>   13 files changed, 400 insertions(+), 6 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling1.C
>   create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling2.C
>   create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling3.C
>   create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling4.C
>   create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling5.C
>   create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling6.C
>   create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling7.C
>   create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling8.C
>   create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling9.C
> 
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index c40ef2e3028..9e4c8073600 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -14033,11 +14033,7 @@ std_pair_ref_ref_p (tree t)
>     return true;
>   }
>   
> -/* Return true if a class CTYPE is either std::reference_wrapper or
> -   std::ref_view, or a reference wrapper class.  We consider a class
> -   a reference wrapper class if it has a reference member.  We no
> -   longer check that it has a constructor taking the same reference type
> -   since that approach still generated too many false positives.  */
> +/* Return true if a class T has a reference member.  */
>   
>   static bool
>   class_has_reference_member_p (tree t)
> @@ -14061,12 +14057,41 @@ class_has_reference_member_p_r (tree binfo, void *)
>   	  ? integer_one_node : NULL_TREE);
>   }
>   
> +
> +/* Return true if T (either a class or a function) has been marked as
> +   not-dangling.  */
> +
> +static bool
> +no_dangling_p (tree t)
> +{
> +  t = lookup_attribute ("no_dangling", TYPE_ATTRIBUTES (t));
> +  if (!t)
> +    return false;
> +
> +  t = TREE_VALUE (t);
> +  if (!t)
> +    return true;
> +
> +  t = build_converted_constant_bool_expr (TREE_VALUE (t), tf_warning_or_error);
> +  t = cxx_constant_value (t);
> +  return t == boolean_true_node;
> +}
> +
> +/* Return true if a class CTYPE is either std::reference_wrapper or
> +   std::ref_view, or a reference wrapper class.  We consider a class
> +   a reference wrapper class if it has a reference member.  We no
> +   longer check that it has a constructor taking the same reference type
> +   since that approach still generated too many false positives.  */
> +
>   static bool
>   reference_like_class_p (tree ctype)
>   {
>     if (!CLASS_TYPE_P (ctype))
>       return false;
>   
> +  if (no_dangling_p (ctype))
> +    return true;
> +
>     /* Also accept a std::pair<const T&, const T&>.  */
>     if (std_pair_ref_ref_p (ctype))
>       return true;
> @@ -14173,7 +14198,8 @@ do_warn_dangling_reference (tree expr, bool arg_p)
>   	       but probably not to one of its arguments.  */
>   	    || (DECL_OBJECT_MEMBER_FUNCTION_P (fndecl)
>   		&& DECL_OVERLOADED_OPERATOR_P (fndecl)
> -		&& DECL_OVERLOADED_OPERATOR_IS (fndecl, INDIRECT_REF)))
> +		&& DECL_OVERLOADED_OPERATOR_IS (fndecl, INDIRECT_REF))
> +	    || no_dangling_p (TREE_TYPE (fndecl)))
>   	  return NULL_TREE;
>   
>   	tree rettype = TREE_TYPE (TREE_TYPE (fndecl));
> diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
> index ad312710f68..e75be9a4e66 100644
> --- a/gcc/cp/tree.cc
> +++ b/gcc/cp/tree.cc
> @@ -47,6 +47,7 @@ static tree verify_stmt_tree_r (tree *, int *, void *);
>   static tree handle_init_priority_attribute (tree *, tree, tree, int, bool *);
>   static tree handle_abi_tag_attribute (tree *, tree, tree, int, bool *);
>   static tree handle_contract_attribute (tree *, tree, tree, int, bool *);
> +static tree handle_no_dangling_attribute (tree *, tree, tree, int, bool *);
>   
>   /* If REF is an lvalue, returns the kind of lvalue that REF is.
>      Otherwise, returns clk_none.  */
> @@ -5102,6 +5103,8 @@ static const attribute_spec cxx_gnu_attributes[] =
>       handle_init_priority_attribute, NULL },
>     { "abi_tag", 1, -1, false, false, false, true,
>       handle_abi_tag_attribute, NULL },
> +  { "no_dangling", 0, 1, false, true, false, false,
> +    handle_no_dangling_attribute, NULL },
>   };
>   
>   const scoped_attribute_specs cxx_gnu_attribute_table =
> @@ -5391,6 +5394,29 @@ handle_contract_attribute (tree *ARG_UNUSED (node), tree ARG_UNUSED (name),
>     return NULL_TREE;
>   }
>   
> +/* Handle a "no_dangling" attribute; arguments as in
> +   struct attribute_spec.handler.  */
> +
> +tree
> +handle_no_dangling_attribute (tree *node, tree name, tree args, int,
> +			      bool *no_add_attrs)
> +{
> +  if (args && TREE_CODE (TREE_VALUE (args)) == STRING_CST)
> +    {
> +      error ("%qE attribute argument must be an expression that evaluates "
> +	     "to true or false", name);
> +      *no_add_attrs = true;
> +    }
> +  else if (!FUNC_OR_METHOD_TYPE_P (*node)
> +	   && !RECORD_OR_UNION_TYPE_P (*node))
> +    {
> +      warning (OPT_Wattributes, "%qE attribute ignored", name);
> +      *no_add_attrs = true;
> +    }
> +
> +  return NULL_TREE;
> +}
> +
>   /* Return a new PTRMEM_CST of the indicated TYPE.  The MEMBER is the
>      thing pointed to by the constant.  */
>   
> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> index 6c2c7ae5d8a..8e1751eae6c 100644
> --- a/gcc/doc/extend.texi
> +++ b/gcc/doc/extend.texi
> @@ -29327,6 +29327,53 @@ Some_Class  B  __attribute__ ((init_priority (543)));
>   Note that the particular values of @var{priority} do not matter; only their
>   relative ordering.
>   
> +@cindex @code{no_dangling} type attribute
> +@cindex @code{no_dangling} function attribute
> +@item no_dangling
> +
> +This attribute can be applied on a class type, function, or member
> +function.  Dangling references to classes marked with this attribute
> +will have the @option{-Wdangling-reference} diagnostic suppressed; so
> +will the @code{gnu::no_dangling}-marked functions.  For example:

...; so will references returned from...

> +@smallexample
> +class [[gnu::no_dangling]] S @{ @dots{} @};
> +@end smallexample
> +
> +Or:
> +
> +@smallexample
> +class A @{
> +  int *p;
> +  [[gnu::no_dangling]] int &foo() @{ return *p; @}
> +@};
> +
> +[[gnu::no_dangling]] const int &
> +foo (const int &i)
> +@{
> +  @dots{}
> +@}
> +@end smallexample
> +
> +This attribute takes an optional argument, which must be an expression that
> +evaluates to true or false:
> +
> +@smallexample
> +template <typename T>
> +struct [[gnu::no_dangling(std::is_reference_v<T>)]] S @{
> +  @dots{}
> +@};
> +@end smallexample
> +
> +Or:
> +
> +@smallexample
> +template <typename T>
> +[[gnu::no_dangling(std::is_reference_v<T>)]] int foo (T& t) @{

I think this function should return a reference.

OK with those changes, thanks.

Jason