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As always, this page is indespensable.
https://graphics.stanford.edu/~seander/bithacks.html#IntegerLogObvious
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Bernie's comment shows he doesn't really understand what's going on here. Neither do many of the commenters. Basically the algorithm is doing a binary search over the bits of the input to find the highest 1 with just 5 comparisons. It looks ugly but it's pretty much the most efficient way to implement this function.