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SubjectRE: [PATCH] x86: Run checksumming in parallel accross multiple alu's
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> and then I also wanted to try using both xmm and ymm registers and doing
> 64bit adds with 32bit numbers across multiple xmm/ymm registers as that
> should parallel nicely. David, you mentioned you've tried this, how did
> your experiment turn out and what was your method? I was planning on
> doing regular full size loads into one xmm/ymm register, then using
> pshufd/vshufd to move the data into two different registers, then
> summing into a fourth register, and possible running two of those
> pipelines in parallel.

It was a long time ago, and IIRC the code was just SSE so the
register length just wasn't going to give the required benefit.
I know I wrote the code, but I can't even remember whether I
actually got it working!
With the longer AVX words it might make enough difference.
Of course, this assumes that you have the fpu registers
available. If you have to do a fpu context switch it will
be a lot slower.

About the same time I did manage to an open coded copy
loop to run as fast as 'rep movs' - and without any unrolling
or any prefetch instructions.

Thinking about AVX you should be able to do (without looking up the
actual mnemonics):
load
add 32bit chunks to sum
compare sum with read value (equiv of carry)
add/subtract compare result (0 or ~0) to a carry-sum register
That is 4 instructions for 256 bits, so you can aim for 4 clocks.
You'd need to check the cpu book to see if any of those can
be scheduled at the same time (if not dependant).
(and also whether there is any result delay - don't think so.)

I'd try running two copies of the above - probably skewed so that
the memory accesses are separated, do the memory read for the
next iteration, and use the 3rd instruction unit for loop control.

David





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