bitops: fastlog2 to use a single bsr instruction on x86 with GCC #7415
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Currently with `31 - builtin_clz`, this compiles into bsr + xor (from clz) + sub. To massage GCC into producing a single bsr instruction, it is better to use xor than sub. Those are equivalent since 31 has the form `2^n -1`
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Who can confirm this is correct? |
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Please add some comments in the code for such optimisations, not everyone has the time to dig through Git history to learn about these :) |
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Currently with
31 - builtin_clz, this compiles into bsr + xor (from clz) + sub.To massage GCC into producing a single bsr instruction, it is better to use xor than sub.
Those are equivalent since 31 has the form
2^n -1