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

[David Laight <David.Laight@ACULAB.COM>]

From: WANG Xuerui
> Sent: 08 February 2023 13:48
...
> 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

It is actually slightly better than that.
In the csum_fold2 version the first two instructions
are independent - so can execute in parallel on some cpu.

	David

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