[PATCH 01/10] Add support for GCC-4.5's __builtin_unreachable() to compiler.h

[David Daney]

Starting with version 4.5, GCC has a new built-in function
__builtin_unreachable() that can be used in places like the kernel's
BUG() where inline assembly is used to transfer control flow.  This
eliminated the need for an endless loop in these places.

The patch adds a new macro 'unreachable()' that will expand to either
__builtin_unreachable() or an endless loop depending on the compiler
version.

Signed-off-by: David Daney <ddaney at caviumnetworks.com>
CC: Thomas Gleixner <tglx at linutronix.de>
CC: Ingo Molnar <mingo at redhat.com>
CC: "H. Peter Anvin" <hpa at zytor.com>
CC: x86 at kernel.org
CC: ralf at linux-mips.org
CC: linux-mips at linux-mips.org
CC: Martin Schwidefsky <schwidefsky at de.ibm.com>
CC: Heiko Carstens <heiko.carstens at de.ibm.com>
CC: linux390 at de.ibm.com
CC: linux-s390 at vger.kernel.org
CC: David Howells <dhowells at redhat.com>
CC: Koichi Yasutake <yasutake.koichi at jp.panasonic.com>
CC: linux-am33-list at redhat.com
CC: Kyle McMartin <kyle at mcmartin.ca>
CC: Helge Deller <deller at gmx.de>
CC: linux-parisc at vger.kernel.org
CC: Benjamin Herrenschmidt <benh at kernel.crashing.org>
CC: Paul Mackerras <paulus at samba.org>
CC: linuxppc-dev at ozlabs.org
CC: Richard Henderson <rth at twiddle.net>
CC: Ivan Kokshaysky <ink at jurassic.park.msu.ru>
CC: linux-alpha at vger.kernel.org
CC: Haavard Skinnemoen <hskinnemoen at atmel.com>
CC: Mike Frysinger <vapier at gentoo.org>
CC: uclinux-dist-devel at blackfin.uclinux.org
---
 include/linux/compiler-gcc4.h |   14 ++++++++++++++
 include/linux/compiler.h      |    5 +++++
 2 files changed, 19 insertions(+), 0 deletions(-)

diff --git a/include/linux/compiler-gcc4.h b/include/linux/compiler-gcc4.h
index 450fa59..ab3af40 100644
--- a/include/linux/compiler-gcc4.h
+++ b/include/linux/compiler-gcc4.h
@@ -36,4 +36,18 @@
    the kernel context */
 #define __cold			__attribute__((__cold__))
 
+
+#if __GNUC_MINOR__ >= 5
+/*
+ * Mark a position in code as unreachable.  This can be used to
+ * suppress control flow warnings after asm blocks that transfer
+ * control elsewhere.
+ *
+ * Early snapshots of gcc 4.5 don't support this and we can't detect
+ * this in the preprocessor, but we can live with this because they're
+ * unreleased.  Really, we need to have autoconf for the kernel.
+ */
+#define unreachable() __builtin_unreachable()
+#endif
+
 #endif
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 04fb513..7efd73f 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -144,6 +144,11 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
 # define barrier() __memory_barrier()
 #endif
 
+/* Unreachable code */
+#ifndef unreachable
+# define unreachable() do { for (;;) ; } while (0)
+#endif
+
 #ifndef RELOC_HIDE
 # define RELOC_HIDE(ptr, off)					\
   ({ unsigned long __ptr;					\
-- 
1.6.2.5