On Fri, 2011-09-09 at 14:10 +0200, Joakim Tjernlund wrote:
> PPC __arch_swab32 and __arch_swab16 generates non optimal code.
> They do not schedule very well, need to copy its input register
> and swab16 needs an extra insn to clear its upper bits.
> Fix this with better inline ASM.
>
> Signed-off-by: Joakim Tjernlund <joakim.tjernl...@transmode.se>
> ---
> arch/powerpc/include/asm/swab.h | 28 ++++++++++++++--------------
> 1 files changed, 14 insertions(+), 14 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/swab.h b/arch/powerpc/include/asm/swab.h
> index c581e3e..3b9a200 100644
> --- a/arch/powerpc/include/asm/swab.h
> +++ b/arch/powerpc/include/asm/swab.h
> @@ -61,25 +61,25 @@ static inline void __arch_swab32s(__u32 *addr)
>
> static inline __attribute_const__ __u16 __arch_swab16(__u16 value)
> {
> - __u16 result;
> -
> - __asm__("rlwimi %0,%1,8,16,23"
> - : "=r" (result)
> - : "r" (value), "0" (value >> 8));
> - return result;
> + __asm__("rlwimi %0,%0,16,0x00ff0000\n\t"
> + "rlwinm %0,%0,24,0x0000ffff"
> + : "+r"(value));
> + return value;
> }
> #define __arch_swab16 __arch_swab16
I don't quite get the thing about needing to clear the high bits.
Value is a u16 to start with, %0 is pre-filled with value >> 8 which
won't add anything to the upper bits, neither will rlwimi, so why would
you need to clear upper bits ?
Now I do see why gcc might generate something sub-optimal here, but can
you provide examples of asm output before/after in the patch commit ?
> static inline __attribute_const__ __u32 __arch_swab32(__u32 value)
> {
> - __u32 result;
> -
> - __asm__("rlwimi %0,%1,24,16,23\n\t"
> - "rlwimi %0,%1,8,8,15\n\t"
> - "rlwimi %0,%1,24,0,7"
> - : "=r" (result)
> - : "r" (value), "0" (value >> 24));
> - return result;
> + __u32 tmp;
> +
> + __asm__("rlwimi %0,%1,24,0xffffffff"
> + : "=r" (value) : "r" (value));
> + tmp = value;
> + __asm__("rlwimi %0,%1,16,0x00ff0000"
> + : "+r" (value) : "r" (tmp));
> + __asm__("rlwimi %0,%1,16,0x000000ff"
> + : "+r" (value) : "r" (tmp));
> + return value;
> }
> #define __arch_swab32 __arch_swab32
Cheers,
Ben.
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