[PATCH] ARM: Add optimised swahb32() byteswap helper for v6 and above
Nicolas Pitre
nicolas.pitre at linaro.org
Wed Nov 23 11:09:00 EST 2011
On Wed, 23 Nov 2011, Dave Martin wrote:
> 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 at linaro.org>
Reviewed-by: Nicolas Pitre <nico at linaro.org>
> ---
> arch/arm/include/asm/swab.h | 5 +++--
> 1 files changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/include/asm/swab.h b/arch/arm/include/asm/swab.h
> index 9997ad2..32ee164 100644
> --- a/arch/arm/include/asm/swab.h
> +++ b/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)
> {
> --
> 1.7.4.1
>
>
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