RE: [PATCH v5 1/4] asm-generic: Improve csum_fold

[David Laight <David.Laight@ACULAB.COM>]

From: Charlie Jenkins
> Sent: 15 September 2023 04:50
> 
> This csum_fold implementation introduced into arch/arc by Vineet Gupta
> is better than the default implementation on at least arc, x86, arm, and
> riscv. Using GCC trunk and compiling non-inlined version, this
> implementation has 41.6667%, 25%, 16.6667% fewer instructions on
> riscv64, x86-64, and arm64 respectively with -O3 optimization.

Nit-picking the commit message...
Some of those architectures have their own asm implementation.
The arm one is better than the C code below, the x86 ones aren't.

I think that only sparc32 (carry flag but no rotate) and
arm/arm64 (barrel shifter on every instruction) have versions
that are better than the one here.

Since I suggested it to Charlie:

Reviewed-by: David Laight <david.laight@aculab.com>

> 
> Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> ---
>  include/asm-generic/checksum.h | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/include/asm-generic/checksum.h b/include/asm-generic/checksum.h
> index 43e18db89c14..adab9ac4312c 100644
> --- a/include/asm-generic/checksum.h
> +++ b/include/asm-generic/checksum.h
> @@ -30,10 +30,7 @@ extern __sum16 ip_fast_csum(const void *iph, unsigned int ihl);
>   */
>  static inline __sum16 csum_fold(__wsum csum)
>  {
> -	u32 sum = (__force u32)csum;

You'll need to re-instate that line to stop sparse complaining.

> -	sum = (sum & 0xffff) + (sum >> 16);
> -	sum = (sum & 0xffff) + (sum >> 16);
> -	return (__force __sum16)~sum;
> +	return (__force __sum16)((~csum - ror32(csum, 16)) >> 16);
>  }
>  #endif
> 
> 
> --
> 2.42.0

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)