linux-kernel - [PATCH 1/2] UNREACHABLE() macro

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Message-Id: <20090910020031.87DC68BF6E@magilla.sf.frob.com>
Date:	Wed,  9 Sep 2009 19:00:31 -0700 (PDT)
From:	Roland McGrath <roland@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jakub Jelinek <jakub@...hat.com>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, x86@...nel.org
Subject: [PATCH 1/2] UNREACHABLE() macro

This adds the UNREACHABLE() macro to <linux/compiler.h>.  The latest
GCC has __builtin_unreachable(), which makes it generate better code
(i.e. omit some unreachable instructions).

Signed-off-by: Roland McGrath <roland@...hat.com>
CC: Jakub Jelinek <jakub@...hat.com>
---
 include/linux/compiler-gcc4.h |   13 +++++++++++++
 include/linux/compiler.h      |    9 +++++++++
 2 files changed, 22 insertions(+), 0 deletions(-)

diff --git a/include/linux/compiler-gcc4.h b/include/linux/compiler-gcc4.h
index 450fa59..320a9ad 100644
--- a/include/linux/compiler-gcc4.h
+++ b/include/linux/compiler-gcc4.h
@@ -37,3 +37,16 @@
 #define __cold			__attribute__((__cold__))
 
 #endif
+
+/*
+ * __builtin_unreachable is available in GCC 4.5+ and
+ * also in Fedora/Red Hat GCC 4.4.1-10+.
+ */
+#if (__GNUC_MINOR__ > 4					\
+     || (__GNUC_MINOR__ == 4				\
+	 && defined __GNUC_RH_RELEASE__			\
+	 && (__GNUC_PATCHLEVEL__ > 1			\
+	     || (__GNUC_PATCHLEVEL__ == 1		\
+		 && __GNUC_RH_RELEASE__ >= 10))))
+#define UNREACHABLE()		__builtin_unreachable()
+#endif
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 04fb513..e3dc896 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -256,6 +256,15 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
 #define __cold
 #endif
 
+/*
+ * Tell the compiler following code is unreachable and should disappear.
+ * If nothing else, just make it obvious to avoid warnings about missing
+ * return or whatnot.  Some compilers define a way to optimize it better.
+ */
+#ifndef UNREACHABLE
+#define UNREACHABLE() for (;;)
+#endif
+
 /* Simple shorthand for a section definition */
 #ifndef __section
 # define __section(S) __attribute__ ((__section__(#S)))
--
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