linux-kernel - Re: [PATCH v6 1/4] asm-generic: Improve csum

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Date:   Sat, 16 Sep 2023 09:50:36 +0100
From:   Conor Dooley <conor@...nel.org>
To:     Charlie Jenkins <charlie@...osinc.com>
Cc:     Palmer Dabbelt <palmer@...belt.com>,
        Samuel Holland <samuel.holland@...ive.com>,
        David Laight <David.Laight@...lab.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v6 1/4] asm-generic: Improve csum_fold

On Fri, Sep 15, 2023 at 10:01:17AM -0700, Charlie Jenkins wrote:
> This csum_fold implementation introduced into arch/arc by Vineet Gupta
> is better than the default implementation on at least arc, x86, and
> riscv. Using GCC trunk and compiling non-inlined version, this
> implementation has 41.6667%, 25% fewer instructions on riscv64, x86-64
> respectively with -O3 optimization. Most implmentations override this
> default in asm, but this should be more performant than all of those
> other implementations except for arm which has barrel shifting and
> sparc32 which has a carry flag.
> 
> Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
> Reviewed-by: David Laight <david.laight@...lab.com>
> ---
>  include/asm-generic/checksum.h | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/include/asm-generic/checksum.h b/include/asm-generic/checksum.h
> index 43e18db89c14..37f5ec70ac93 100644
> --- a/include/asm-generic/checksum.h
> +++ b/include/asm-generic/checksum.h
> @@ -31,9 +31,7 @@ extern __sum16 ip_fast_csum(const void *iph, unsigned int ihl);
>  static inline __sum16 csum_fold(__wsum csum)
>  {
>  	u32 sum = (__force u32)csum;
> -	sum = (sum & 0xffff) + (sum >> 16);
> -	sum = (sum & 0xffff) + (sum >> 16);
> -	return (__force __sum16)~sum;
> +	return (__force __sum16)((~sum - ror32(sum, 16)) >> 16);

Breaks the build on RISC-V in a way that is repaired by later patches in
the series, so you likely did not notice:

./include/asm-generic/checksum.h:34:35: error: call to undeclared function 'ror32'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
../include/linux/bitops.h:134:21: error: static declaration of 'ror32' follows non-static declaration

Cheers,
Conor.

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