Message-ID: <5b3c9b61-7fd5-f50c-32ba-e857090b71bc@xen0n.name>
Date:   Wed, 8 Feb 2023 21:48:08 +0800
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Subject: Re: [PATCH] LoongArch: add checksum optimization for 64-bit system
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To:     David Laight <David.Laight@ACULAB.COM>,
        'Bibo Mao' <maobibo@loongson.cn>,
        Huacai Chen <chenhuacai@kernel.org>
Cc:     Jiaxun Yang <jiaxun.yang@flygoat.com>,
        "loongarch@lists.linux.dev" <loongarch@lists.linux.dev>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <20230207040148.1801169-1-maobibo@loongson.cn>
 <8fa91bca5e624861b190917933951c7e@AcuMS.aculab.com>
From:   WANG Xuerui <kernel@xen0n.name>
In-Reply-To: <8fa91bca5e624861b190917933951c7e@AcuMS.aculab.com>
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On 2023/2/8 21:12, David Laight wrote:
> 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.

Yeah LoongArch can do rotates, and your suggestion can indeed reduce one 
insn from every invocation of csum_fold.

 From this:

000000000000096c <csum_fold>:
         sum += (sum >> 16) | (sum << 16);
  96c:   004cc08c        rotri.w         $t0, $a0, 0x10
  970:   00101184        add.w           $a0, $t0, $a0
         return ~(__force __sum16)(sum >> 16);
  974:   0044c084        srli.w          $a0, $a0, 0x10
  978:   00141004        nor             $a0, $zero, $a0
}
  97c:   006f8084        bstrpick.w      $a0, $a0, 0xf, 0x0
  980:   4c000020        jirl            $zero, $ra, 0

To:

0000000000000984 <csum_fold2>:
         return (~sum - rol32(sum, 16)) >> 16;
  984:   0014100c        nor             $t0, $zero, $a0
         return (x << amt) | (x >> (32 - amt));
  988:   004cc084        rotri.w         $a0, $a0, 0x10
         return (~sum - rol32(sum, 16)) >> 16;
  98c:   00111184        sub.w           $a0, $t0, $a0
}
  990:   00df4084        bstrpick.d      $a0, $a0, 0x1f, 0x10
  994:   4c000020        jirl            $zero, $ra, 0

I guess Bibo would take this suggestion and check the other arches 
afterwards, okay? ;-)

-- 
WANG "xen0n" Xuerui

Linux/LoongArch mailing list: https://lore.kernel.org/loongarch/