From:   David Laight <David.Laight@ACULAB.COM>
To:     'Bibo Mao' <maobibo@loongson.cn>,
        Huacai Chen <chenhuacai@kernel.org>
CC:     WANG Xuerui <kernel@xen0n.name>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        "loongarch@lists.linux.dev" <loongarch@lists.linux.dev>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH] LoongArch: add checksum optimization for 64-bit system
Thread-Topic: [PATCH] LoongArch: add checksum optimization for 64-bit system
Thread-Index: AQHZOqjw3I588J9fP0akyPjC04Hova7FBpIA
Date:   Wed, 8 Feb 2023 13:12:47 +0000
Message-ID: <8fa91bca5e624861b190917933951c7e@AcuMS.aculab.com>
References: <20230207040148.1801169-1-maobibo@loongson.cn>
In-Reply-To: <20230207040148.1801169-1-maobibo@loongson.cn>
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From: Bibo Mao
> Sent: 07 February 2023 04:02
> 
> loongArch platform is 64-bit system, which supports 8 bytes memory
> accessing, generic checksum function uses 4 byte memory access.
> This patch adds 8-bytes memory access optimization for checksum
> function on loongArch. And the code comes from arm64 system.
> 
> When network hw checksum is disabled, iperf performance improves
> about 10% with this patch.
> 
...
> +static inline __sum16 csum_fold(__wsum csum)
> +{
> +	u32 sum = (__force u32)csum;
> +
> +	sum += (sum >> 16) | (sum << 16);
> +	return ~(__force __sum16)(sum >> 16);
> +}

Does LoongArch have a rotate instruction?
But for everything except arm (which has a rotate+add instruction)
the best is (probably):
	(~sum - rol32(sum, 16)) >> 16

To the point where it is worth killing all the asm
versions and just using that one.

	David

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