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SubjectRe: [PATCH] LoongArch: add checksum optimization for 64-bit system
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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/

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