Add __generic(), to be able to use a very simple _Generic().

Already introducing this allows us to be forward compatible with C11
compilers. By implementing <tgmath.h> on top of this interface, it
becomes trivial to support both our existing GCC and newer compilers.

1 files changed, 18 insertions, 0 deletions

diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
index 7dc04dceec98..373503f3c0d2 100644
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -248,6 +248,24 @@
 #endif
 #endif
 
+/*
+ * Emulation of C11 _Generic().  Unlike the previously defined C11
+ * keywords, it is not possible to implement this using exactly the same
+ * syntax.  Therefore implement something similar under the name
+ * __generic().  Unlike _Generic(), this macro can only distinguish
+ * between a single type, so it requires nested invocations to
+ * distinguish multiple cases.
+ */
+
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
+#define	__generic(expr, t, yes, no)					\
+	_Generic(expr, t: yes, default: no)
+#elif __GNUC_PREREQ__(3, 1)
+#define	__generic(expr, t, yes, no)					\
+	__builtin_choose_expr(						\
+	    __builtin_types_compatible_p(__typeof(expr), t), yes, no)
+#endif
+
 #if __GNUC_PREREQ__(2, 96)
 #define	__malloc_like	__attribute__((__malloc__))
 #define	__pure		__attribute__((__pure__))