Verify optimized brctz - #2383
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klinashka
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August 17, 2026 12:10
miriampolzer
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Thanks! Looks good overall, just some detail comments. Let me know when you rebased and updated so that I can trigger the CI and merge.
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This PR implements an implementation of
br_ctz, which takes as input a 64-bit unsigned word and returns the number of trailing Zeros.The implementation repeatedly shifts the input left by 1, until the input becomes 0. We have opted for this implementation, rather than the more intuitive alternative, because the current implementation targets optimizations performed in the
LoopIdiomRecognizepass of the LLVM compiler. This results in efficient compilations by bothclangandgcc(even though we did not specifically targetgccinitially). More details on this can be found here.The PR is currently a draft, as it should be merged after mit-plv/coqutil#177 propagates to fiat-crypto.