C front-end: fallthrough and __fallthrough__ are the same attribute

We already supported the former, but were missing the latter form.

Author: tautschnig

regression/ansi-c/gcc_attributes13/main.c

@@ -7,13 +7,20 @@ int main()
   int x;
   switch(x)
   {
-    case 1:
-      x = 2;
+  case 1:
+    x = 2;
 #ifdef __GNUC__
-      __attribute__((fallthrough));
+    __attribute__((fallthrough));
 #endif
-    case 2:
-      x = 3;
+  case 2:
+  {
+    x = 3;
+#ifdef __GNUC__
+    __attribute__((__fallthrough__));
+#endif
+  }
+  case 3:
+    break;
   }
 
   return 0;

src/ansi-c/parser.y

@@ -1634,6 +1634,11 @@ gcc_attribute:
         {
           init($$);
         }
+        | TOK_GCC_ATTRIBUTE_FALLTHROUGH
+        {
+          // attribute ignored
+          init($$);
+        }
         | gcc_type_attribute
         ;
 
@@ -2299,19 +2304,7 @@ declaration_statement:
         ;
 
 labeled_statement:
-          identifier_or_typedef_name ':' gcc_attribute_specifier ';'
-        {
-          /* Only semicolons permitted after the attribute:
-             https://gcc.gnu.org/onlinedocs/gcc/Label-Attributes.html */
-          $$=$2;
-          statement($$, ID_label);
-          irep_idt identifier=PARSER.lookup_label(parser_stack($1).get(ID_C_base_name));
-          parser_stack($$).set(ID_label, identifier);
-          // attribute ignored
-          statement($3, ID_skip);
-          mto($$, $3);
-        }
-        | identifier_or_typedef_name ':' statement
+        identifier_or_typedef_name ':' statement
         {
           $$=$2;
           statement($$, ID_label);
@@ -2346,10 +2339,14 @@ labeled_statement:
         ;
 
 statement_attribute:
-          TOK_GCC_ATTRIBUTE '(' '(' TOK_GCC_ATTRIBUTE_FALLTHROUGH ')' ')' ';' labeled_statement
+        gcc_attribute_specifier ';'
         {
-          // attribute ignored
-          $$=$8;
+          // Really should only be TOK_GCC_ATTRIBUTE_FALLTHROUGH or a label
+          // attribute. Only semicolons permitted after the attribute:
+          // https://gcc.gnu.org/onlinedocs/gcc/Label-Attributes.html
+          // We ignore all such attributes.
+          $$=$1;
+          statement($$, ID_skip);
         }
         ;
 

src/ansi-c/scanner.l

@@ -1697,7 +1697,8 @@ __decltype          { if(PARSER.cpp98 &&
 "destructor" |
 "__destructor__"   { BEGIN(GCC_ATTRIBUTE3); loc(); return TOK_GCC_ATTRIBUTE_DESTRUCTOR; }
 
-"fallthrough"       { BEGIN(GCC_ATTRIBUTE3); loc(); return TOK_GCC_ATTRIBUTE_FALLTHROUGH; }
+"fallthrough" |
+"__fallthrough__"   { BEGIN(GCC_ATTRIBUTE3); loc(); return TOK_GCC_ATTRIBUTE_FALLTHROUGH; }
 
 "used" |
 "__used__"          { BEGIN(GCC_ATTRIBUTE3); loc(); return TOK_GCC_ATTRIBUTE_USED; }