[gcc r12-5459] libcpp: Use [[likely]] conditionally

[gcc r12-5459] libcpp: Use [[likely]] conditionally

[Marek Polacek]

https://gcc.gnu.org/g:630686f93f0018fa1ef128aa673fddd302cc83e1

commit r12-5459-g630686f93f0018fa1ef128aa673fddd302cc83e1
Author: Marek Polacek <polacek@redhat.com>
Date:   Mon Nov 22 11:29:40 2021 -0500

    libcpp: Use [[likely]] conditionally
    
    Let's hide [[likely]] behind a macro, to suppress warnings if the
    compiler doesn't support it.
    
    Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
    
            PR preprocessor/103355
    
    libcpp/ChangeLog:
    
            * lex.c: Use ATTR_LIKELY instead of [[likely]].
            * system.h (ATTR_LIKELY): Define.

Diff:
---
 libcpp/lex.c    |  2 +-
 libcpp/system.h | 10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/libcpp/lex.c b/libcpp/lex.c
index 94c36f0d014..9c27d8b5a08 100644
--- a/libcpp/lex.c
+++ b/libcpp/lex.c
@@ -1286,7 +1286,7 @@ namespace bidi {
       case kind::RTL:
 	/* These aren't popped by a PDF/PDI.  */
 	break;
-      [[likely]] case kind::NONE:
+      ATTR_LIKELY case kind::NONE:
 	break;
       default:
 	abort ();
diff --git a/libcpp/system.h b/libcpp/system.h
index ee5fbe28889..f6fc583ab80 100644
--- a/libcpp/system.h
+++ b/libcpp/system.h
@@ -422,6 +422,16 @@ extern void fancy_abort (const char *, int, const char *) ATTRIBUTE_NORETURN;
 #define gcc_checking_assert(EXPR) ((void)(0 && (EXPR)))
 #endif
 
+#ifdef __has_cpp_attribute
+# if __has_cpp_attribute(likely)
+#  define ATTR_LIKELY [[likely]]
+# elif __has_cpp_attribute(__likely__)
+#  define ATTR_LIKELY [[__likely__]]
+# else
+#  define ATTR_LIKELY
+# endif
+#endif
+
 /* Poison identifiers we do not want to use.  */
 #if (GCC_VERSION >= 3000)
 #undef calloc