Fastest way to strip trailing zeroes from an unsigned int

Question

Suppose we are trying to remove the trailing zeroes from some unsigned variable.

uint64_t a = ...
uint64_t last_bit = a & -a; // Two's complement trick: last_bit holds the trailing bit of a
a /= last_bit; // Removing all trailing zeroes from a.

I noticed that it's faster to manually count the bits and shift. (MSVC compiler with optimizations on)

uint64_t a = ...
uint64_t last_bit = a & -a;
size_t last_bit_index = _BitScanForward64( last_bit );
a >>= last_bit_index

Are there any further quick tricks that would make this even faster, assuming that the compiler intrinsic _BitScanForward64 is faster than any of the alternatives?

Maybe [this](https://stackoverflow.com/questions/62227706/how-to-remove-trailing-zeros-from-a-binary-number)? — NathanOliver, Jan 27 '22 at 14:06

score 4 · Accepted Answer · edited Jan 27 '22 at 14:16

4

On x86, _tzcnt_u64 is a faster alterative of _BitScanForward64, if it is available (it is available with BMI instruction set).

Also, you can directly use that on the input, you don't need to isolate lowest bit set, as pointed out by @AlanBirtles in a comment.

Other than that, noting can be done for a single variable. For an array of them, there may be a SIMD solution.

edited Jan 27 '22 at 14:16

Jérôme Richard

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answered Jan 27 '22 at 14:13

Alex Guteniev

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apple apple · Answer 2 · 2022-01-27T15:06:01.403

4

You can use std::countr_zero (c++20) and rely on the compiler to optimize it.

a >>= std::countr_zero(a);

^{(bonus: you don't need to specify the width and it works with any unsigned integer type)}

edited Jan 27 '22 at 15:06

answered Jan 27 '22 at 14:45

apple apple

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Fastest way to strip trailing zeroes from an unsigned int

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