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

[WANG Xuerui]

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

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