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From: "Pedro F. Giffuni" <pfg@FreeBSD.org>
Date: Thu, 14 May 2015 15:49:49 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: svn commit: r282907 - head/sys/sys
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Author: pfg
Date: Thu May 14 15:49:48 2015
New Revision: 282907
URL: https://svnweb.freebsd.org/changeset/base/282907

Log:
  Add new __unreachable() builtin
  
  This is one of the few post gcc4.2 builtins that has been implemented by
  clang:
  
  __builtin_unreachable is used to indicate that a specific point in the
  program cannot be reached, even if the compiler might otherwise think it
  can. This is useful to improve optimization and eliminates certain
  warnings.
  
  Hinted by:	NetBSD
  Differential Revision:	https://reviews.freebsd.org/D2536

Modified:
  head/sys/sys/cdefs.h

Modified: head/sys/sys/cdefs.h
==============================================================================
--- head/sys/sys/cdefs.h	Thu May 14 15:14:03 2015	(r282906)
+++ head/sys/sys/cdefs.h	Thu May 14 15:49:48 2015	(r282907)
@@ -388,6 +388,12 @@
 #define	__alloc_size(x)
 #endif
 
+#if __has_builtin(__builtin_unreachable) || __GNUC_PREREQ__(4, 6)
+#define	__unreachable()	__builtin_unreachable()
+#else
+#define	__unreachable()	do {} while (/*CONSTCOND*/0)
+#endif
+
 #if __has_attribute(alloc_align) || __GNUC_PREREQ__(4, 9)
 #define	__alloc_align(x)	__attribute__((__alloc_align__(x)))
 #else