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<bit>: Use popcount for has_single_bit() if it's always available #5534

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Merged
merged 4 commits into from
May 28, 2025
Merged

<bit>: Use popcount for has_single_bit() if it's always available #5534

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53ce129
`<bit>`: Use popcount for `has_single_bit()` if it's always available
pps83 May 23, 2025
dda8819
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pps83 May 23, 2025
420c391
Always define `_POPCNT_INTRINSICS_ALWAYS_AVAILABLE`, leave `_HAS_POPC…
StephanTLavavej May 28, 2025
9f7ee26
Avoid code duplication.
StephanTLavavej May 28, 2025
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1 change: 0 additions & 1 deletion stl/inc/__msvc_bit_utils.hpp
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Expand Up @@ -386,7 +386,6 @@ _CONSTEXPR20 decltype(auto) _Select_popcount_impl(_Fn _Callback) {

#undef _HAS_POPCNT_INTRINSICS
#undef _HAS_TZCNT_BSF_INTRINSICS
#undef _POPCNT_INTRINSICS_ALWAYS_AVAILABLE

_STD_END

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7 changes: 7 additions & 0 deletions stl/inc/bit
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Expand Up @@ -84,7 +84,14 @@ _NODISCARD constexpr int countl_zero(_Ty _Val) noexcept;

_EXPORT_STD template <_Standard_unsigned_integral _Ty>
_NODISCARD constexpr bool has_single_bit(const _Ty _Val) noexcept {
#if defined(_POPCNT_INTRINSICS_ALWAYS_AVAILABLE) && _POPCNT_INTRINSICS_ALWAYS_AVAILABLE
if (_STD is_constant_evaluated()) {
return (_Val ^ (_Val - 1)) > _Val - 1;
}
return _Unchecked_popcount(_Val) == 1;
#else
return (_Val ^ (_Val - 1)) > _Val - 1;
#endif // ! _POPCNT_INTRINSICS_ALWAYS_AVAILABLE
}

inline void _Precondition_violation_in_bit_ceil() noexcept {}
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