linux-kernel - Re: [PATCH v8 1/5] asm-generic: Improve csum

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Date:   Sat, 28 Oct 2023 00:10:36 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Charlie Jenkins <charlie@...osinc.com>
Cc:     Palmer Dabbelt <palmer@...belt.com>,
        Conor Dooley <conor@...nel.org>,
        Samuel Holland <samuel.holland@...ive.com>,
        David Laight <David.Laight@...lab.com>,
        Xiao Wang <xiao.w.wang@...el.com>,
        Evan Green <evan@...osinc.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 v8 1/5] asm-generic: Improve csum_fold

On Fri, Oct 27, 2023 at 03:43:51PM -0700, Charlie Jenkins wrote:
>  /*
>   * computes the checksum of a memory block at buff, length len,
>   * and adds in "sum" (32-bit)
> @@ -31,9 +33,7 @@ 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);
>  }

Will (~(sum + ror32(sum, 16))>>16 produce worse code than that?
Because at least with recent gcc this will generate the exact thing
you get from arm inline asm...