ARM: 7173/1: Add optimised swahb32() byteswap helper for v6 and above

ARMv6 and later processors have the REV16 instruction, which swaps
the bytes within each halfword of a register value.

This is already used to implement swab16(), but since the native
operation performaed by REV16 is actually swahb32(), this patch
renames the existing swab16() helper accordingly and defines
__arch_swab16() in terms of it.  This allows calls to both swab16()
and swahb32() to be optimised.

The compiler's generated code might improve someday, but as of
4.5.2 the code generated for pure C implementing these 16-bit
bytesswaps remains pessimal.

swahb32() is useful for converting 32-bit Thumb instructions
between integer and memory representation on BE8 platforms (among
other uses).

Signed-off-by: Dave Martin <dave.martin@linaro.org>
Reviewed-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/include/asm/swab.h

@@ -24,12 +24,13 @@
 
 #if defined(__KERNEL__) && __LINUX_ARM_ARCH__ >= 6
 
-static inline __attribute_const__ __u16 __arch_swab16(__u16 x)
+static inline __attribute_const__ __u32 __arch_swahb32(__u32 x)
 {
 	__asm__ ("rev16 %0, %1" : "=r" (x) : "r" (x));
 	return x;
 }
-#define __arch_swab16 __arch_swab16
+#define __arch_swahb32 __arch_swahb32
+#define __arch_swab16(x) ((__u16)__arch_swahb32(x))
 
 static inline __attribute_const__ __u32 __arch_swab32(__u32 x)
 {