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@chfast, let me know what you find out please :-) |
Owner
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Preliminary benchmarks: So this looks solid for compare operators directly and seems to help the shifts as well, although this may be code layout effect. |
chfast
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Author
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Thanks, glad to hear :-) |
Owner
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Confirmed, this does not affect assembly of shift operators. Proof: https://godbolt.org/z/4b6cT4rje. |
Author
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Yes, makes sense. When I played with this repo a while back, I noticed that the benchmarks had variability from one run to the next, so I'm not sure how reliable the comparison truly is. |
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Collaborator
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@chfast should this be merged? |
Owner
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On the latest compilers I don't see much difference so I'm not very much in favor because this implementation has 4 branches while the original one is branchless. But people are welcome to drop their own benchmarking results (use "compare" filter). |
Author
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Why does it matter that it is branchless if it is slower? |
Owner
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I will try to clean up the situation.
Here you can see the assembly output for each: https://godbolt.org/z/WY3qqzrET AnalysisIn the worst case every variant need to load all 8 words. Additionally, "sub" executes 4 subtract instructions (independent of input). The "ne" executes 4 BenchmarksI added all variants of implementations in #277 and benchmarked them on two CPUs. There are 5 benchmark cases:
HaswellSkylakeZen3This is noisy machine with AMD CPU. For this case I used Xeon Platinum 8272CLFrom the results we should rather consider switching back to "sub" implementation. I may do some other benchmarks if I have time. |
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