Messages in this thread | | | Date | Tue, 22 Nov 2022 17:24:51 +0100 | From | Willy Tarreau <> | Subject | Re: Optimising csum_fold() |
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On Tue, Nov 22, 2022 at 01:08:23PM +0000, David Laight wrote: > There are currently 20 copies of csum_fold(), some in C some in assembler. > The default C version (in asm-generic/checksum.h) is pretty horrid. > Some of the asm versions (including x86 and x86-64) aren't much better. > > There are 3 pretty good C versions: > 1: (~sum - rol32(sum, 16)) >> 16 > 2: ~(sum + rol32(sum, 16)) >> 16 > 3: (u16)~((sum + rol32(sum, 16)) >> 16) > All three are (usually) 4 arithmetic instructions. > > The first two have the advantage that the high bits are zero. > Relevant when the value is being checked rather than set. > > The first one can generate better instruction scheduling (the rotate > and invert can be executed in the same clock). > > The 3rd one saves an instruction on arm, but may need masking. > (I've not compiled an arm kernel to see how often that happens.) > > The only architectures where (I think) the current asm code is better > than the C above are sparc and sparc64. > Sparc doesn't have a rotate instruction, but does have a carry flag. > This makes the current asm version one instruction shorter. > > For architectures like mips and risc-v which have neither rotate > instructions nor carry flags the C is as good as the current asm. > The rotate is 3 instructions - the same as the extra cmp+add. > > Changing everything to use [1] would improve quite a few architectures > while only adding 1 clock to some paths in arm/arm64 and sparc. > > Unfortunately it is all currently a mess. > Most architectures don't include asm-generic/checksum.h at all. > > Thoughts?
Then why not just have one version per arch, the most efficient one, and use it everywhere ? The simple fact that we're discussing the tradeoffs means that if we don't want to compromise performance here (which I assume to be the case), then it needs to be per-arch and that's all. At least that's the way I understand it.
Regards, Willy
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