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From: Ed Schouten <ed@FreeBSD.org>
Date: Thu, 5 Jan 2012 10:43:04 +0000 (UTC)
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Subject: svn commit: r229574 - head/sys/sys
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Author: ed
Date: Thu Jan  5 10:43:03 2012
New Revision: 229574
URL: http://svn.freebsd.org/changeset/base/229574

Log:
  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.

Modified:
  head/sys/sys/cdefs.h

Modified: head/sys/sys/cdefs.h
==============================================================================
--- head/sys/sys/cdefs.h	Thu Jan  5 10:42:19 2012	(r229573)
+++ head/sys/sys/cdefs.h	Thu Jan  5 10:43:03 2012	(r229574)
@@ -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__))