[gcc r13-2324] c: C2x attributes fixes and updates

[gcc r13-2324] c: C2x attributes fixes and updates

[Joseph Myers]

https://gcc.gnu.org/g:2eca4ff4e867eb994e5110f0637c8762b6cfddc6

commit r13-2324-g2eca4ff4e867eb994e5110f0637c8762b6cfddc6
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Wed Aug 31 22:22:07 2022 +0000

    c: C2x attributes fixes and updates
    
    Implement some changes to the currently supported C2x standard
    attributes that have been made to the specification since they were
    first implemented in GCC, and some consequent changes:
    
    * maybe_unused is now supported on labels.  In fact that was already
      accidentally supported in GCC as a result of sharing the
      implementation with __attribute__ ((unused)), but needed to be
      covered in the tests.
    
    * As part of the support for maybe_unused on labels, its
      __has_c_attribute value changed.
    
    * The issue of maybe_unused accidentally being already supported on
      labels showed up the lack of tests for other standard attributes
      being incorrectly applied to labels; add such tests.
    
    * Use of fallthrough or nodiscard attributes on labels already
      properly resulted in a pedwarn.  For the deprecated attribute,
      however, there was only a warning, and the wording "'deprecated'
      attribute ignored for 'void'" included an unhelpful "for 'void'".
      Arrange for the case of the deprecated attribute on a label to be
      checked for separately and result in a pedwarn.  As with
      inappropriate uses of fallthrough (see commit
      6c80b1b56dec2691436f3e2676e3d1b105b01b89), it seems reasonable for
      this pedwarn to apply regardless of whether [[]] or __attribute__
      was used and regardless of whether C or C++ is being compiled.
    
    * Attributes on case or default labels (the standard syntax supports
      attributes on all kinds of labels) were quietly ignored, whether or
      not appropriate for use in such a context, because they weren't
      passed to decl_attributes at all.  (Note where I'm changing the
      do_case prototype that such a function is actually only defined in
      the C front end, not for C++, despite the declaration being in
      c-common.h.)
    
    * A recent change as part of the editorial review in preparation for
      the C2x CD ballot has changed the __has_c_attribute value for
      fallthrough to 201910 to reflect when that attribute was actually
      voted into the working draft.
    
    Bootstrapped with no regressions for x86_64-pc-linux-gnu.
    
    gcc/c-family/
            * c-attribs.cc (handle_deprecated_attribute): Check and pedwarn
            for LABEL_DECL.
            * c-common.cc (c_add_case_label): Add argument ATTRS.  Call
            decl_attributes.
            * c-common.h (do_case, c_add_case_label): Update declarations.
            * c-lex.cc (c_common_has_attribute): For C, produce a result of
            201910 for fallthrough and 202106 for maybe_unused.
    
    gcc/c/
            * c-parser.cc (c_parser_label): Pass attributes to do_case.
            * c-typeck.cc (do_case): Add argument ATTRS.  Pass it to
            c_add_case_label.
    
    gcc/testsuite/
            * gcc.dg/c2x-attr-deprecated-2.c, gcc.dg/c2x-attr-fallthrough-2.c,
            gcc.dg/c2x-attr-maybe_unused-1.c, gcc.dg/c2x-attr-nodiscard-2.c:
            Add tests of attributes on labels.
            * gcc.dg/c2x-has-c-attribute-2.c: Update expected results for
            maybe_unused and fallthrough.

Diff:
---
 gcc/c-family/c-attribs.cc                      |  7 +++++++
 gcc/c-family/c-common.cc                       |  6 ++++--
 gcc/c-family/c-common.h                        |  5 +++--
 gcc/c-family/c-lex.cc                          |  8 +++++---
 gcc/c/c-parser.cc                              |  6 +++---
 gcc/c/c-typeck.cc                              |  6 +++---
 gcc/testsuite/gcc.dg/c2x-attr-deprecated-2.c   |  9 ++++++++-
 gcc/testsuite/gcc.dg/c2x-attr-fallthrough-2.c  |  4 ++++
 gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-1.c | 13 +++++++++++++
 gcc/testsuite/gcc.dg/c2x-attr-nodiscard-2.c    |  6 ++++++
 gcc/testsuite/gcc.dg/c2x-has-c-attribute-2.c   |  8 ++++----
 11 files changed, 60 insertions(+), 18 deletions(-)

diff --git a/gcc/c-family/c-attribs.cc b/gcc/c-family/c-attribs.cc
index e4f1d3542f3..8bb80e251dc 100644
--- a/gcc/c-family/c-attribs.cc
+++ b/gcc/c-family/c-attribs.cc
@@ -4163,6 +4163,13 @@ handle_deprecated_attribute (tree *node, tree name,
 	  || TREE_CODE (decl) == CONST_DECL
 	  || objc_method_decl (TREE_CODE (decl)))
 	TREE_DEPRECATED (decl) = 1;
+      else if (TREE_CODE (decl) == LABEL_DECL)
+	{
+	  pedwarn (input_location, OPT_Wattributes, "%qE attribute ignored",
+		   name);
+	  *no_add_attrs = true;
+	  return NULL_TREE;
+	}
       else
 	warn = 1;
     }
diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
index 71fe7305369..1eb842e1c7b 100644
--- a/gcc/c-family/c-common.cc
+++ b/gcc/c-family/c-common.cc
@@ -5068,11 +5068,12 @@ case_compare (splay_tree_key k1, splay_tree_key k2)
    CASES is a tree containing all the case ranges processed so far;
    COND is the condition for the switch-statement itself.
    Returns the CASE_LABEL_EXPR created, or ERROR_MARK_NODE if no
-   CASE_LABEL_EXPR is created.  */
+   CASE_LABEL_EXPR is created.  ATTRS are the attributes to be applied
+   to the label.  */
 
 tree
 c_add_case_label (location_t loc, splay_tree cases, tree cond,
-		  tree low_value, tree high_value)
+		  tree low_value, tree high_value, tree attrs)
 {
   tree type;
   tree label;
@@ -5081,6 +5082,7 @@ c_add_case_label (location_t loc, splay_tree cases, tree cond,
 
   /* Create the LABEL_DECL itself.  */
   label = create_artificial_label (loc);
+  decl_attributes (&label, attrs, 0);
 
   /* If there was an error processing the switch condition, bail now
      before we get more confused.  */
diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
index e7b0fd1309d..64fe14b66fe 100644
--- a/gcc/c-family/c-common.h
+++ b/gcc/c-family/c-common.h
@@ -1018,7 +1018,7 @@ extern void c_parse_final_cleanups (void);
 /* True iff TYPE is cv decltype(nullptr).  */
 #define NULLPTR_TYPE_P(TYPE) (TREE_CODE (TYPE) == NULLPTR_TYPE)
 
-extern tree do_case (location_t, tree, tree);
+extern tree do_case (location_t, tree, tree, tree);
 extern tree build_stmt (location_t, enum tree_code, ...);
 extern tree build_real_imag_expr (location_t, enum tree_code, tree);
 
@@ -1046,7 +1046,8 @@ extern tree boolean_increment (enum tree_code, tree);
 
 extern int case_compare (splay_tree_key, splay_tree_key);
 
-extern tree c_add_case_label (location_t, splay_tree, tree, tree, tree);
+extern tree c_add_case_label (location_t, splay_tree, tree, tree, tree,
+			      tree = NULL_TREE);
 extern bool c_switch_covers_all_cases_p (splay_tree, tree);
 extern bool c_block_may_fallthru (const_tree);
 
diff --git a/gcc/c-family/c-lex.cc b/gcc/c-family/c-lex.cc
index 0b6f94e18a8..417ba3e38ba 100644
--- a/gcc/c-family/c-lex.cc
+++ b/gcc/c-family/c-lex.cc
@@ -381,12 +381,14 @@ c_common_has_attribute (cpp_reader *pfile, bool std_syntax)
 	    }
 	  else
 	    {
-	      if (is_attribute_p ("deprecated", attr_name)
-		  || is_attribute_p ("maybe_unused", attr_name)
-		  || is_attribute_p ("fallthrough", attr_name))
+	      if (is_attribute_p ("deprecated", attr_name))
 		result = 201904;
+	      else if (is_attribute_p ("fallthrough", attr_name))
+		result = 201910;
 	      else if (is_attribute_p ("nodiscard", attr_name))
 		result = 202003;
+	      else if (is_attribute_p ("maybe_unused", attr_name))
+		result = 202106;
 	    }
 	  if (result)
 	    attr_name = NULL_TREE;
diff --git a/gcc/c/c-parser.cc b/gcc/c/c-parser.cc
index 0fe2ff55040..95f4ead54a0 100644
--- a/gcc/c/c-parser.cc
+++ b/gcc/c/c-parser.cc
@@ -5912,14 +5912,14 @@ c_parser_label (c_parser *parser, tree std_attrs)
       if (c_parser_next_token_is (parser, CPP_COLON))
 	{
 	  c_parser_consume_token (parser);
-	  label = do_case (loc1, exp1, NULL_TREE);
+	  label = do_case (loc1, exp1, NULL_TREE, std_attrs);
 	}
       else if (c_parser_next_token_is (parser, CPP_ELLIPSIS))
 	{
 	  c_parser_consume_token (parser);
 	  exp2 = c_parser_expr_no_commas (parser, NULL).value;
 	  if (c_parser_require (parser, CPP_COLON, "expected %<:%>"))
-	    label = do_case (loc1, exp1, exp2);
+	    label = do_case (loc1, exp1, exp2, std_attrs);
 	}
       else
 	c_parser_error (parser, "expected %<:%> or %<...%>");
@@ -5928,7 +5928,7 @@ c_parser_label (c_parser *parser, tree std_attrs)
     {
       c_parser_consume_token (parser);
       if (c_parser_require (parser, CPP_COLON, "expected %<:%>"))
-	label = do_case (loc1, NULL_TREE, NULL_TREE);
+	label = do_case (loc1, NULL_TREE, NULL_TREE, std_attrs);
     }
   else
     {
diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
index ee891ee33c2..e4d58e318f8 100644
--- a/gcc/c/c-typeck.cc
+++ b/gcc/c/c-typeck.cc
@@ -11172,10 +11172,10 @@ c_start_switch (location_t switch_loc,
   return add_stmt (cs->switch_stmt);
 }
 
-/* Process a case label at location LOC.  */
+/* Process a case label at location LOC, with attributes ATTRS.  */
 
 tree
-do_case (location_t loc, tree low_value, tree high_value)
+do_case (location_t loc, tree low_value, tree high_value, tree attrs)
 {
   tree label = NULL_TREE;
 
@@ -11211,7 +11211,7 @@ do_case (location_t loc, tree low_value, tree high_value)
 
   label = c_add_case_label (loc, c_switch_stack->cases,
 			    SWITCH_STMT_COND (c_switch_stack->switch_stmt),
-			    low_value, high_value);
+			    low_value, high_value, attrs);
   if (label == error_mark_node)
     label = NULL_TREE;
   return label;
diff --git a/gcc/testsuite/gcc.dg/c2x-attr-deprecated-2.c b/gcc/testsuite/gcc.dg/c2x-attr-deprecated-2.c
index 44f2cc9bd13..7c01317d8fa 100644
--- a/gcc/testsuite/gcc.dg/c2x-attr-deprecated-2.c
+++ b/gcc/testsuite/gcc.dg/c2x-attr-deprecated-2.c
@@ -3,7 +3,8 @@
 /* { dg-options "-std=c2x -pedantic-errors" } */
 
 /* This attribute is not valid in most cases on types other than their
-   definitions, or on statements, or as an attribute-declaration.  */
+   definitions, or on labels, or on statements, or as an
+   attribute-declaration.  */
 
 [[deprecated]]; /* { dg-error "ignored" } */
 
@@ -21,4 +22,10 @@ f (void)
   int a;
   [[deprecated]]; /* { dg-error "ignored" } */
   [[deprecated]] a = 1; /* { dg-error "ignored" } */
+  [[deprecated]] label: ; /* { dg-error "ignored" } */
+  switch (var)
+    {
+    [[deprecated]] case 1: ; /* { dg-error "ignored" } */
+    [[deprecated]] default: ; /* { dg-error "ignored" } */
+    }
 }
diff --git a/gcc/testsuite/gcc.dg/c2x-attr-fallthrough-2.c b/gcc/testsuite/gcc.dg/c2x-attr-fallthrough-2.c
index 9d6995938cd..b65bcbef709 100644
--- a/gcc/testsuite/gcc.dg/c2x-attr-fallthrough-2.c
+++ b/gcc/testsuite/gcc.dg/c2x-attr-fallthrough-2.c
@@ -33,6 +33,10 @@ f (int a)
     case 5:
       b += 5;
       break;
+    [[fallthrough]] case 6: break; /* { dg-error "ignored" } */
+    [[fallthrough]] default: break; /* { dg-error "ignored" } */
     }
   [[fallthrough]] return b; /* { dg-error "ignored" } */
+ [[fallthrough]] label: ; /* { dg-error "ignored" } */
+  goto label;
 }
diff --git a/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-1.c b/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-1.c
index 477f30dbd44..7090a3f30e1 100644
--- a/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-1.c
+++ b/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-1.c
@@ -14,7 +14,9 @@ g ([[maybe_unused]] int x, int y)
   [[maybe_unused]] int a;
   int b [[__maybe_unused__]];
   int c [[maybe_unused]];
+ [[__maybe_unused__]] label1:
   c = y;
+ [[maybe_unused]] label2:
   return y;
 }
 
@@ -29,3 +31,14 @@ union [[maybe_unused]] u { int x; };
 enum [[maybe_unused]] eu { E2 };
 
 union u2 { [[maybe_unused]] int a; int b [[maybe_unused]]; } y;
+
+void
+g2 (int x)
+{
+  switch (x)
+    {
+      [[maybe_unused]] case 1: ;
+      [[__maybe_unused__]] case 2: ;
+      [[maybe_unused]] default: ;
+    }
+}
diff --git a/gcc/testsuite/gcc.dg/c2x-attr-nodiscard-2.c b/gcc/testsuite/gcc.dg/c2x-attr-nodiscard-2.c
index 45c4d50dee0..0ed2ebe92ff 100644
--- a/gcc/testsuite/gcc.dg/c2x-attr-nodiscard-2.c
+++ b/gcc/testsuite/gcc.dg/c2x-attr-nodiscard-2.c
@@ -39,4 +39,10 @@ f (void)
   [[nodiscard ("reason")]] int b = 1; /* { dg-error "can only be applied" } */
   [[nodiscard]]; /* { dg-error "ignored" } */
   [[nodiscard]] a = 1; /* { dg-error "ignored" } */
+  [[nodiscard]] label: ; /* { dg-error "can only be applied" } */
+  switch (var)
+    {
+      [[nodiscard]] case 1: ; /* { dg-error "can only be applied" } */
+      [[nodiscard]] default: ; /* { dg-error "can only be applied" } */
+    }
 }
diff --git a/gcc/testsuite/gcc.dg/c2x-has-c-attribute-2.c b/gcc/testsuite/gcc.dg/c2x-has-c-attribute-2.c
index d6c4c6de509..6a379e9db4f 100644
--- a/gcc/testsuite/gcc.dg/c2x-has-c-attribute-2.c
+++ b/gcc/testsuite/gcc.dg/c2x-has-c-attribute-2.c
@@ -10,11 +10,11 @@
 #error "bad result for __nodiscard__"
 #endif
 
-#if __has_c_attribute(maybe_unused) != 201904L
+#if __has_c_attribute(maybe_unused) != 202106L
 #error "bad result for maybe_unused"
 #endif
 
-#if __has_c_attribute(__maybe_unused__) != 201904L
+#if __has_c_attribute(__maybe_unused__) != 202106L
 #error "bad result for __maybe_unused__"
 #endif
 
@@ -26,11 +26,11 @@
 #error "bad result for __deprecated__"
 #endif
 
-#if __has_c_attribute (fallthrough) != 201904L
+#if __has_c_attribute (fallthrough) != 201910L
 #error "bad result for fallthrough"
 #endif
 
-#if __has_c_attribute (__fallthrough__) != 201904L
+#if __has_c_attribute (__fallthrough__) != 201910L
 #error "bad result for __fallthrough__"
 #endif