Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH] LoongArch: add checksum optimization for 64-bit system | Date | Wed, 8 Feb 2023 14:19:27 +0000 |
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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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