linux-kernel - RE: [PATCH v5 1/4] asm-generic: Improve csum

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Message-ID: <b6bd38541255470b97bbaf3e6cdb4265@AcuMS.aculab.com>
Date:   Fri, 15 Sep 2023 07:29:23 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Charlie Jenkins' <charlie@...osinc.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Conor Dooley <conor@...nel.org>,
        Samuel Holland <samuel.holland@...ive.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>
Subject: RE: [PATCH v5 1/4] asm-generic: Improve csum_fold

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@...lab.com>

> 
> Signed-off-by: Charlie Jenkins <charlie@...osinc.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

-
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