bpo-38436: Improved performance for list addition. - #16705
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brandtbucher wants to merge 2 commits into
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Where the benchmarks executed with CPU isolation? I am a bit suspicious of the If not, can you repeat them with CPU isolation and affinity? Check this for more info |
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@pablogsal No isolation (I'm on a MacBook) but I can rerun with affinity on two cores. |
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I can try to run the benchmarks on my machine or the speed.python.org server with isolation, as this is a very sensitive operation. |
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Thanks @pablogsal - that would probably be best! |
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This PR adds a fast path for
BINARY_ADDinstructions involving two lists, where the left list has a refcount of exactly 1.In this case, we instead do a
PySequence_InPlaceConcatoperation. This has the affect of avoiding quadratic complexity for list summations, by keeping only one intermediate result and extending it in-place.For (potentially large) lists:
Currently executed as:
With this change:
Here are thepyperformanceresults (optimizations enabled):EDIT: there are better measurements in the BPO discussion.
This is related to my earlier work in bpo-36229, however this is a much less invasive change that's limited to
ceval.c, where our knowledge of context is much better.https://bugs.python.org/issue38436