[gcc r13-3848] c++: Minimal handling of carries_dependency attribute

[gcc r13-3848] c++: Minimal handling of carries_dependency attribute

[Jakub Jelinek]

https://gcc.gnu.org/g:05119c345797bc04cc34e6973ebd921b4dcb0326

commit r13-3848-g05119c345797bc04cc34e6973ebd921b4dcb0326
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Wed Nov 9 11:39:22 2022 +0100

    c++: Minimal handling of carries_dependency attribute
    
    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.

Diff:
---
 gcc/cp/parser.cc                                   |  5 ++--
 gcc/cp/tree.cc                                     | 28 ++++++++++++++++++++++
 .../g++.dg/cpp0x/attr-carries_dependency1.C        | 17 +++++++++++++
 3 files changed, 47 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 9523f73d9a3..e4021835ed5 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -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))
diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index c30bbeb0839..954c6de2fcd 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -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 }
 };
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/attr-carries_dependency1.C b/gcc/testsuite/g++.dg/cpp0x/attr-carries_dependency1.C
new file mode 100644
index 00000000000..bf0f9cb3645
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/attr-carries_dependency1.C
@@ -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