| Message ID | ZeIresptVf4uV5-T@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | [v5] c++: implement [[gnu::non_owning]] [PR110358] | expand |
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
On Fri, 1 Mar 2024, Jason Merrill wrote: > 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)) Sorry for not asking this sooner, but does it matter whether we attach the attribute to the function type rather than the function declaration? I noticed e.g. nodiscard gets attached to the decl. > > + { > > + 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 And we document it as a function attribute despite attaching it to the function type. > > +@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 > >
On 3/1/24 16:23, Patrick Palka wrote: > > Sorry for not asking this sooner, but does it matter whether we attach > the attribute to the function type rather than the function declaration? > I noticed e.g. nodiscard gets attached to the decl. > > And we document it as a function attribute despite attaching it to the > function type. I think it doesn't matter much, some attributes are represented on the type and some on the decl. Might be a bit better on the decl but I wasn't worrying about it. Jason
On 01/03/24 15:38 -0500, Jason Merrill wrote: >On 3/1/24 14:24, Marek Polacek wrote: >>+@smallexample >>+template <typename T> >>+[[gnu::no_dangling(std::is_reference_v<T>)]] int foo (T& t) @{ > >I think this function should return a reference. The condition in the attribute can only ever be true if you call this function with an explicit template argument list: foo<int&>(i). Is that intentional? And if T is non-const it can't be called with a temporary and so dangling seems less of a problem for this function anyway, right? Would it make more sense as something like this? template <typename T> [[gnu::no_dangling(std::is_lvalue_reference_v<T>)]] decltype(auto) foo(T&& t) { ... } Or is this getting too complex/subtle for a simple example?
On Mon, Mar 04, 2024 at 11:00:18AM +0000, Jonathan Wakely wrote: > On 01/03/24 15:38 -0500, Jason Merrill wrote: > > On 3/1/24 14:24, Marek Polacek wrote: > > > +@smallexample > > > +template <typename T> > > > +[[gnu::no_dangling(std::is_reference_v<T>)]] int foo (T& t) @{ > > > > I think this function should return a reference. > > The condition in the attribute can only ever be true if you call this > function with an explicit template argument list: foo<int&>(i). Is > that intentional? Not intentional. I just wanted to make it clear that the user can use something like std::is_reference as the attribute argument, but I didn't think about it very long. > And if T is non-const it can't be called with a temporary and so > dangling seems less of a problem for this function anyway, right? Right. > Would it make more sense as something like this? > > template <typename T> > [[gnu::no_dangling(std::is_lvalue_reference_v<T>)]] > decltype(auto) foo(T&& t) { > ... > } > > Or is this getting too complex/subtle for a simple example? I like your example; it's only slightly more complex than the original one and most likely more realistic. I'm pushing the following patch. Thanks! [pushed] doc: update [[gnu::no_dangling]] ...to offer a more realistic example. gcc/ChangeLog: * doc/extend.texi: Update [[gnu::no_dangling]]. --- gcc/doc/extend.texi | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi index f679c81acf2..df0982fdfda 100644 --- a/gcc/doc/extend.texi +++ b/gcc/doc/extend.texi @@ -29370,7 +29370,8 @@ Or: @smallexample template <typename T> -[[gnu::no_dangling(std::is_reference_v<T>)]] int& foo (T& t) @{ +[[gnu::no_dangling(std::is_lvalue_reference_v<T>)]] +decltype(auto) foo(T&& t) @{ @dots{} @}; @end smallexample base-commit: 77eb86be8841989651b3150a020dd1a95910cc00
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: + +@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) @{ + @dots{} +@}; +@end smallexample + @cindex @code{warn_unused} type attribute @item warn_unused diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi index dc5fd863ca4..bdf05be387d 100644 --- a/gcc/doc/invoke.texi +++ b/gcc/doc/invoke.texi @@ -3908,6 +3908,9 @@ const T& foo (const T&) @{ @dots{} @} #pragma GCC diagnostic pop @end smallexample +The @code{#pragma} can also surround the class; in that case, the warning +will be disabled for all the member functions. + @option{-Wdangling-reference} also warns about code like @smallexample @@ -3932,6 +3935,9 @@ 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 +(@pxref{C++ Attributes}). + 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..dff34e89436 --- /dev/null +++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling1.C @@ -0,0 +1,40 @@ +// { dg-do compile { target c++11 } } +// { dg-options "-Wdangling-reference" } + +int g = 42; + +struct [[gnu::no_dangling]] A { + ~A(); + int *i; + int &foo() { return *i; } +}; + +struct A2 { + ~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" } +} diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling2.C b/gcc/testsuite/g++.dg/ext/attr-no-dangling2.C new file mode 100644 index 00000000000..4cdc97ea7c4 --- /dev/null +++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling2.C @@ -0,0 +1,29 @@ +// { dg-do compile { target c++11 } } +// Negative tests. + +struct [[no_dangling]] A { // { dg-warning "ignored" } + [[no_dangling]] int &foo (int &); // { dg-warning "ignored" } +}; + +[[no_dangling]] int &bar (int &); // { dg-warning "ignored" } + +[[gnu::no_dangling]] int i; // { dg-warning "ignored" } +[[gnu::no_dangling]] double d; // { dg-warning "ignored" } +[[gnu::no_dangling]] typedef int T; // { dg-warning "ignored" } + +[[gnu::no_dangling()]] int &fn1 (int &); // { dg-error "parentheses" } +[[gnu::no_dangling("a")]] int &fn2 (int &); // { dg-error "must be an expression" } +[[gnu::no_dangling(true, true)]] int &fn3 (int &); // { dg-error "wrong number of arguments" } + +enum [[gnu::no_dangling]] E { // { dg-warning "ignored" } + X [[gnu::no_dangling]] // { dg-warning "ignored" } +}; + +[[gnu::no_dangling]]; // { dg-warning "ignored" } + +void +g () +{ + goto L; +[[gnu::no_dangling]] L:; // { dg-warning "ignored" } +} diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling3.C b/gcc/testsuite/g++.dg/ext/attr-no-dangling3.C new file mode 100644 index 00000000000..764b104fd3c --- /dev/null +++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling3.C @@ -0,0 +1,24 @@ +// { dg-do compile { target c++11 } } +// { dg-options "-Wdangling-reference" } + +template <typename T> +struct [[gnu::no_dangling]] Span { + T* data_; + int len_; + // So that our heuristic doesn't suppress the warning anyway. + ~Span(); + + [[nodiscard]] constexpr auto operator[](int n) const noexcept -> T& { return data_[n]; } + [[nodiscard]] constexpr auto front() const noexcept -> T& { return data_[0]; } + [[nodiscard]] constexpr auto back() const noexcept -> T& { return data_[len_ - 1]; } +}; + +auto get() -> Span<int>; + +auto f() -> int { + int const& a = get().front(); // { dg-bogus "dangling" } + int const& b = get().back(); // { dg-bogus "dangling" } + int const& c = get()[0]; // { dg-bogus "dangling" } + + return a + b + c; +} diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling4.C b/gcc/testsuite/g++.dg/ext/attr-no-dangling4.C new file mode 100644 index 00000000000..e910723d985 --- /dev/null +++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling4.C @@ -0,0 +1,14 @@ +// { dg-do compile { target c++11 } } + +#if !__has_attribute(no_dangling) +#error unsupported +#endif + +#ifdef __has_cpp_attribute +# if !__has_cpp_attribute(no_dangling) +# error no_dangling +# endif +#endif + +struct [[gnu::no_dangling]] S { }; +static_assert (__builtin_has_attribute (S, no_dangling), ""); diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling5.C b/gcc/testsuite/g++.dg/ext/attr-no-dangling5.C new file mode 100644 index 00000000000..ec5075482c4 --- /dev/null +++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling5.C @@ -0,0 +1,31 @@ +// PR c++/110358 +// { dg-do compile { target c++20 } } +// { dg-options "-Wdangling-reference" } + +template <typename T> +struct Span { + T* data_; + int len_; + ~Span(); + + [[nodiscard]] constexpr auto operator[](int n) const noexcept -> T& { return data_[n]; } +}; + +template <> +struct [[gnu::no_dangling]] Span<int> { + int* data_; + int len_; + ~Span(); + + [[nodiscard]] constexpr auto operator[](int n) const noexcept -> int& { return data_[n]; } +}; + +auto getch() -> Span<char>; +auto geti() -> Span<int>; + +void +f () +{ + [[maybe_unused]] const auto &a = getch()[0]; // { dg-warning "dangling reference" } + [[maybe_unused]] const auto &b = geti()[0]; // { dg-bogus "dangling reference" } +} diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling6.C b/gcc/testsuite/g++.dg/ext/attr-no-dangling6.C new file mode 100644 index 00000000000..235a5fd86c5 --- /dev/null +++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling6.C @@ -0,0 +1,65 @@ +// PR c++/110358 +// { dg-do compile { target c++20 } } +// { dg-options "-Wdangling-reference" } + +class X { }; +const X x1; +const X x2; + +constexpr bool val () { return true; } +struct ST { static constexpr bool value = true; }; +struct SF { static constexpr bool value = false; }; + +template<typename T> +[[gnu::no_dangling(T::value)]] +const X& get (const int& i) +{ + return i == 0 ? x1 : x2; +} + +template<bool B = true> +[[gnu::no_dangling(B)]] +const X& foo (const int& i) +{ + return i == 0 ? x1 : x2; +} + +[[gnu::no_dangling(val ())]] +const X& bar (const int& i) +{ + return i == 0 ? x1 : x2; +} + +[[gnu::no_dangling(!val ())]] +const X& baz (const int& i) +{ + return i == 0 ? x1 : x2; +} + +template <typename T> +struct [[gnu::no_dangling(T::value)]] +Span { + T* data_; + int len_; + ~Span(); + + [[nodiscard]] constexpr auto operator[](int n) const noexcept -> T& { return data_[n]; } +}; + +auto geti() -> Span<ST>; +auto gety() -> Span<SF>; + +void +test () +{ + [[maybe_unused]] const X& x1 = get<ST> (10); // { dg-bogus "dangling" } + [[maybe_unused]] const X& x2 = get<SF> (10); // { dg-warning "dangling" } + [[maybe_unused]] const X& x3 = foo<true> (10); // { dg-bogus "dangling" } + [[maybe_unused]] const X& x4 = foo<false> (10); // { dg-warning "dangling" } + [[maybe_unused]] const X& x7 = foo<> (10); // { dg-bogus "dangling" } + [[maybe_unused]] const X& x5 = bar (10); // { dg-bogus "dangling" } + [[maybe_unused]] const X& x6 = baz (10); // { dg-warning "dangling" } + + [[maybe_unused]] const auto &b1 = geti()[0]; // { dg-bogus "dangling" } + [[maybe_unused]] const auto &b2 = gety()[0]; // { dg-warning "dangling" } +} diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling7.C b/gcc/testsuite/g++.dg/ext/attr-no-dangling7.C new file mode 100644 index 00000000000..3c392ed409f --- /dev/null +++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling7.C @@ -0,0 +1,31 @@ +// PR c++/110358 +// { dg-do compile { target c++20 } } +// { dg-options "-Wdangling-reference" } + +class X { }; +const X x1; +const X x2; + +template<bool... N> +[[gnu::no_dangling(N)]] const X& get(const int& i); // { dg-error "parameter packs not expanded" } + +template<typename T> +[[gnu::no_dangling(T::x)]] // { dg-error "member" } +const X& foo(const int& i); + +bool val () { return true; } + +[[gnu::no_dangling(val ())]] // { dg-error "call" } +const X& bar (const int& i); + +[[gnu::no_dangling(20)]] const X& fn1 (const int &); + +void +test () +{ + [[maybe_unused]] const X& x1 = bar (10); // { dg-warning "dangling" } + [[maybe_unused]] const X& x2 = foo<int> (10); // { dg-error "no matching" } + [[maybe_unused]] const X& x3 // { dg-warning "dangling" } + = fn1 (10); // { dg-error "narrowing" } +} + diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling8.C b/gcc/testsuite/g++.dg/ext/attr-no-dangling8.C new file mode 100644 index 00000000000..8208d751a4b --- /dev/null +++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling8.C @@ -0,0 +1,30 @@ +// PR c++/110358 +// { dg-do compile { target c++20 } } +// { dg-options "-Wdangling-reference" } + +template<class T> constexpr bool is_reference_v = false; +template<class T> constexpr bool is_reference_v<T&> = true; +template<class T> constexpr bool is_reference_v<T&&> = true; + +template <typename T> +struct [[gnu::no_dangling(is_reference_v<T>)]] S { + int &foo (const int &); +}; + +template <typename T1, typename T2> +struct X { + template <typename U1 = T1, typename U2 = T2> + struct [[gnu::no_dangling(is_reference_v<U1> && is_reference_v<U2>)]] Y { + int &foo (const int &); + }; +}; + +void +g () +{ + [[maybe_unused]] const int &x0 = S<int&>().foo (42); // { dg-bogus "dangling" } + [[maybe_unused]] const int &x1 = S<int>().foo (42); // { dg-warning "dangling" } + [[maybe_unused]] const auto &x2 = X<int, int&>::Y<>().foo (42); // { dg-warning "dangling" } + [[maybe_unused]] const auto &x3 = X<int&, int&>::Y<>().foo (42); // { dg-bogus "dangling" } +} + diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling9.C b/gcc/testsuite/g++.dg/ext/attr-no-dangling9.C new file mode 100644 index 00000000000..65b4f7145a9 --- /dev/null +++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling9.C @@ -0,0 +1,25 @@ +// PR c++/110358 +// { dg-do compile { target c++20 } } +// { dg-options "-Wdangling-reference" } + +template<bool B> +struct bool_constant { + static constexpr bool value = B; + constexpr operator bool() const { return value; } +}; + +using true_type = bool_constant<true>; +using false_type = bool_constant<false>; + +struct S { + template<bool B> + [[gnu::no_dangling(B)]] int &foo (const int &); +}; + +void +g () +{ + [[maybe_unused]] const int &x0 = S().foo<false_type{}> (42); // { dg-warning "dangling" } + [[maybe_unused]] const int &x1 = S().foo<true_type{}> (42); // { dg-bogus "dangling" } +} +