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

[Russell King - ARM Linux]

On Wed, Jan 30, 2013 at 08:09:00PM -0600, Kim Phillips wrote:
> The savings come mostly from device-tree related code, and some
> from drivers.

You forget that IP networking is all big endian, so these will be using
the byte swapping too (search it for htons/ntohs/htonl/ntohl).

> v2:
> - at91 and lpd270 builds fixed by limiting to ARMv6 and above
>   (i.e., ARM cores that have support for the 'rev' instruction).
>   Otherwise, the compiler emits calls to libgcc's __bswapsi2 on
>   these ARMv4/v5 builds (and arch ARM doesn't link with libgcc).

Which compiler version?  gcc 4.5.4 doesn't do this, except for the 16-bit
swap, so I doubt that any later compiler does.

> --- a/arch/arm/include/uapi/asm/swab.h
> +++ b/arch/arm/include/uapi/asm/swab.h
> @@ -50,4 +50,14 @@ static inline __attribute_const__ __u32 __arch_swab32(__u32 x)
>  
>  #endif
>  
> +#if defined(__KERNEL__) && __LINUX_ARM_ARCH__ >= 6
> +#if GCC_VERSION >= 40600
> +#define __HAVE_BUILTIN_BSWAP32__
> +#define __HAVE_BUILTIN_BSWAP64__
> +#endif
> +#if GCC_VERSION >= 40800
> +#define __HAVE_BUILTIN_BSWAP16__
> +#endif
> +#endif

If this is __KERNEL__ only, it should not be in a uapi header.  UAPI
headers get exported to userland, this is not userland interface code.
IT should be in arch/arm/include/asm/swab.h
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