Optimise ll::base::num_base_digits for 2 & other powers of it.
#53
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The old code would end up with an actual division when inlined into another crate, because the `BASES` symbol isn't inlined and hence the compiler can't tell that the division will be by a power of two. This caused `bit_length` to be very high in the profiles of my `float` crate with the `div` instruction taking the majority of the time. It's especially unfortunate because the divison for `bit_length` is a division by 1. (A simple benchmark of `Int::bit_length` shows the performance dropping from 9 ns/iter to 1-2 ns/iter, with `div` taking ~80% of the time in the former case.) cc Aatch#52
Aatch
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Nov 12, 2015
Optimise `ll::base::num_base_digits` for 2 & other powers of it.
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The old code would end up with an actual division when inlined into
another crate, because the
BASESsymbol isn't inlined and hence thecompiler can't tell that the division will be by a power of two. This
caused
bit_lengthto be very high in the profiles of myfloatcratewith the
divinstruction taking the majority of the time. It'sespecially unfortunate because the divison for
bit_lengthis adivision by 1.
(A simple benchmark of
Int::bit_lengthshows the performance droppingfrom 9 ns/iter to 1-2 ns/iter, with
divtaking ~80% of the time in theformer case.)
cc #52