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Date:   Sat, 28 Oct 2023 00:10:36 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Charlie Jenkins <charlie@rivosinc.com>
Cc:     Palmer Dabbelt <palmer@dabbelt.com>,
        Conor Dooley <conor@kernel.org>,
        Samuel Holland <samuel.holland@sifive.com>,
        David Laight <David.Laight@aculab.com>,
        Xiao Wang <xiao.w.wang@intel.com>,
        Evan Green <evan@rivosinc.com>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH v8 1/5] asm-generic: Improve csum_fold
Message-ID: <20231027231036.GM800259@ZenIV>
References: <20231027-optimize_checksum-v8-0-feb7101d128d@rivosinc.com>
 <20231027-optimize_checksum-v8-1-feb7101d128d@rivosinc.com>
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Sender: Al Viro <viro@ftp.linux.org.uk>
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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...