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fun post showing how LLVM optimizes a very bad is-even function blog.matthieud.me/2020/exploring

is-even function with runtime exponential in the input bitwidth

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John Regehr

@johnregehr

Aug 18, 2021

oops-- I just noticed the author is here!

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God I wish there was an easier way to do this

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that version is awesome because it has been optimized by aggressive loop unrolling

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Every program should ship with a lookup table with 1 bit for each 32-bit integer value based on whether it's odd or even. That's pretty much how Unicode works.

6:49 PM · Aug 18, 2021Twitter Web App

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Can then write a simple macro to iterate through the table until you get to the right index, then check the bit, the compiler will optimise it

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This tweet pretty much sums up how gratuitously bad most Unicode library implementations are...