[RFC] arm: use built-in byte swap function

[Kim Phillips]

Enable the compiler intrinsic for byte swapping on arch ARM.  This
allows the compiler to detect and be able to optimize out byte
swappings, e.g. in big endian to big endian moves.

AFAICT, arm gcc got __builtin_bswap{32,64} support in 4.6,
and for the 16-bit version in 4.8.

Signed-off-by: Kim Phillips <kim.phillips at freescale.com>
---
akin to: http://comments.gmane.org/gmane.linux.kernel.cross-arch/16016

based on linux-next.  Depends on commit "compiler-gcc{3,4}.h: Use
GCC_VERSION macro" by Daniel Santos <daniel.santos at pobox.com>,
currently in the akpm branch.

RFC because of unfamiliarity with arch ARM, and that at91sam9rl,
at91rm9200, and lpd270 (so far, at least) builds fail with:

include/uapi/linux/swab.h:60: undefined reference to `__bswapsi2'

I'm using eldk-5.2.1/armv7a's arm-linux-gnueabi-gcc (GCC) 4.6.4
20120303 (prerelease)

 arch/arm/Kconfig              |    1 +
 include/linux/compiler-gcc4.h |    3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index eda8711..437d11a 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -3,6 +3,7 @@ config ARM
 	default y
 	select ARCH_BINFMT_ELF_RANDOMIZE_PIE
 	select ARCH_HAS_ATOMIC64_DEC_IF_POSITIVE
+	select ARCH_USE_BUILTIN_BSWAP
 	select ARCH_HAVE_CUSTOM_GPIO_H
 	select ARCH_WANT_IPC_PARSE_VERSION
 	select BUILDTIME_EXTABLE_SORT if MMU
diff --git a/include/linux/compiler-gcc4.h b/include/linux/compiler-gcc4.h
index 68b162d..da5f728 100644
--- a/include/linux/compiler-gcc4.h
+++ b/include/linux/compiler-gcc4.h
@@ -67,7 +67,8 @@
 
 
 #ifdef CONFIG_ARCH_USE_BUILTIN_BSWAP
-#if GCC_VERSION >= 40400
+#if (!defined(__arm__) && GCC_VERSION >= 40400) || \
+    (defined(__arm__) && GCC_VERSION >= 40600)
 #define __HAVE_BUILTIN_BSWAP32__
 #define __HAVE_BUILTIN_BSWAP64__
 #endif
-- 
1.7.9.7