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<bit>: Remove tzcnt special case for small integers in countr_zero #1540

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Merged
StephanTLavavej merged 7 commits into from
Jan 15, 2021
Merged

<bit>: Remove tzcnt special case for small integers in countr_zero #1540

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190d2fd
Remove special case for small integers in countr_zero
sylveon Dec 17, 2020
490d4f4
Use _BitScanForward instead of tzcnt to reduce assembly output by 1 byte
sylveon Dec 17, 2020
bdf5182
Revert to tzcnt, fix 32 bits ints
sylveon Dec 17, 2020
52c7139
Resolve 32-bits build warnings
sylveon Dec 17, 2020
7785670
Delete superfluous newline
sylveon Dec 17, 2020
a2b3912
Merge branch 'master' of https://github.com/microsoft/STL into countr…
sylveon Jan 6, 2021
ec95804
Comment change
StephanTLavavej Jan 9, 2021
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11 changes: 8 additions & 3 deletions stl/inc/limits
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Original file line number Diff line number Diff line change
Expand Up @@ -1058,9 +1058,14 @@ _NODISCARD int _Checked_x86_x64_countr_zero(const _Ty _Val) noexcept {
constexpr _Ty _Max = (numeric_limits<_Ty>::max)();

#ifndef __AVX2__
const bool _Definitely_have_tzcnt = __isa_available >= __ISA_AVAILABLE_AVX2;
if (!_Definitely_have_tzcnt && _Val == 0) {
return _Digits;
// Because the widening done below will always give a non-0 value, checking for tzcnt
// is not required for 8-bit and 16-bit since the only difference in behavior between
// bsf and tzcnt is when the value is 0.
if constexpr (_Digits > 16) {
const bool _Definitely_have_tzcnt = __isa_available >= __ISA_AVAILABLE_AVX2;
if (!_Definitely_have_tzcnt && _Val == 0) {
return _Digits;
}
}
#endif // __AVX2__

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