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c++: Minimal handling of carries_dependency attribute

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A comment in D2552R1:
"The only questionable (but still conforming) case we found was
[[carries_dependency(some_argument)]] on GCC, where the emitted diagnostic said that the
carries_dependency attribute is not supported, but did not specifically call out the syntax error
in the argument clause."
made me try the following patch, where we'll error at least
for arguments to the attribute and for some uses of the attribute
appertaining to something not mentioned in the standard warn
with different diagnostics (or should that be an error?; clang++
does that, but I think we never do for any attribute, standard or not).
The diagnostics on toplevel attribute declaration is still an
attribute ignored warning and on empty statement different wording.

The paper additionally mentions
struct X { [[nodiscard]]; }; // no diagnostic on GCC
and 2 cases of missing diagnostics on [[fallthrough]] (guess I should
file a PR about those; one problem is that do { ... } while (0); there
is replaced during genericization just by ... and another that
[[fallthrough]] there is followed by a label, but not user/case/default
label, but an artificial one created from while loop genericization.

2022-11-09  Jakub Jelinek  <jakub@redhat.com>

	* tree.cc (handle_carries_dependency_attribute): New function.
	(std_attribute_table): Add carries_dependency attribute.
	* parser.cc (cp_parser_check_std_attribute): Add carries_dependency
	attribute.

	* g++.dg/cpp0x/attr-carries_dependency1.C: New test.
This commit is contained in:
Jakub Jelinek 2022-11-09 11:39:22 +01:00
parent cecc039fbb
commit 05119c3457
3 changed files with 47 additions and 3 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

5
gcc/cp/parser.cc
View file

@ -29239,8 +29239,7 @@ cp_parser_std_attribute (cp_parser *parser, tree attr_ns)
/* Warn if the attribute ATTRIBUTE appears more than once in the
attribute-list ATTRIBUTES. This used to be enforced for certain
attributes, but the restriction was removed in P2156. Note that
carries_dependency ([dcl.attr.depend]) isn't implemented yet in GCC.
attributes, but the restriction was removed in P2156.
LOC is the location of ATTRIBUTE. Returns true if ATTRIBUTE was not
found in ATTRIBUTES. */
@ -29249,7 +29248,7 @@ cp_parser_check_std_attribute (location_t loc, tree attributes, tree attribute)
{
static auto alist = { "noreturn", "deprecated", "nodiscard", "maybe_unused",
"likely", "unlikely", "fallthrough",
"no_unique_address" };
"no_unique_address", "carries_dependency" };
if (attributes)
for (const auto &a : alist)
if (is_attribute_p (a, get_attribute_name (attribute))

28
gcc/cp/tree.cc
View file

@ -4923,6 +4923,32 @@ structural_type_p (tree t, bool explain)
return true;
}
/* Partially handle the C++11 [[carries_dependency]] attribute.
Just emit a different diagnostics when it is used on something the
spec doesn't allow vs. where it allows and we just choose to ignore
it. */
static tree
handle_carries_dependency_attribute (tree *node, tree name,
tree ARG_UNUSED (args),
int ARG_UNUSED (flags),
bool *no_add_attrs)
{
if (TREE_CODE (*node) != FUNCTION_DECL
&& TREE_CODE (*node) != PARM_DECL)
{
warning (OPT_Wattributes, "%qE attribute can only be applied to "
"functions or parameters", name);
*no_add_attrs = true;
}
else
{
warning (OPT_Wattributes, "%qE attribute ignored", name);
*no_add_attrs = true;
}
return NULL_TREE;
}
/* Handle the C++17 [[nodiscard]] attribute, which is similar to the GNU
warn_unused_result attribute. */
@ -5036,6 +5062,8 @@ const struct attribute_spec std_attribute_table[] =
handle_likeliness_attribute, attr_cold_hot_exclusions },
{ "noreturn", 0, 0, true, false, false, false,
handle_noreturn_attribute, attr_noreturn_exclusions },
{ "carries_dependency", 0, 0, true, false, false, false,
handle_carries_dependency_attribute, NULL },
{ NULL, 0, 0, false, false, false, false, NULL, NULL }
};

17
gcc/testsuite/g++.dg/cpp0x/attr-carries_dependency1.C Normal file
View file

@ -0,0 +1,17 @@
// { dg-do compile { target c++11 } }
[[carries_dependency]] int *f1 (); // { dg-warning "attribute ignored" }
int f2 (int *x [[carries_dependency]]); // { dg-warning "attribute ignored" }
[[carries_dependency]] int f3 (); // { dg-warning "attribute ignored" }
int f4 (int x [[carries_dependency]]); // { dg-warning "attribute ignored" }
[[carries_dependency(1)]] int f5 (); // { dg-error "'carries_dependency' attribute does not take any arguments" }
[[carries_dependency]] int v; // { dg-warning "'carries_dependency' attribute can only be applied to functions or parameters" }
[[carries_dependency]]; // { dg-warning "attribute ignored" }
void
f6 ()
{
[[carries_dependency]]; // { dg-warning "attributes at the beginning of statement are ignored" }
}
#if __has_cpp_attribute(carries_dependency)
#error carries_dependency attribute is not actually implemented
#endif