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<random>: Optimized divisionless implementation of minstd_rand, minstd_rand0 #5256

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Merged
merged 6 commits into from
Mar 13, 2025
Merged

<random>: Optimized divisionless implementation of minstd_rand, minstd_rand0 #5256

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2b58429
Optimized divisionless implementation of minstd
mirounga Jan 27, 2025
33dcc0a
Formatting updates
mirounga Jan 28, 2025
7165215
switched to if constexpr
mirounga Jan 31, 2025
bb4b32a
Merge branch 'main' into mirounga/main
StephanTLavavej Mar 3, 2025
468a296
We don't need to widen `_Ax` and `_Mx`.
StephanTLavavej Mar 3, 2025
7b0905f
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StephanTLavavej Mar 3, 2025
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7 changes: 7 additions & 0 deletions stl/inc/random
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Original file line number Diff line number Diff line change
Expand Up @@ -490,6 +490,13 @@ _NODISCARD _Uint _Next_linear_congruential_value(_Uint _Prev) noexcept {
// numeric_limits<result_type>::max() plus 1." That is: Just do the multiply
// and let normal unsigned modulo take care of it
return static_cast<_Uint>(static_cast<_Uint>(_Ax * _Prev) + _Cx);
} else if constexpr (_Cx == 0 && _Mx == 2147483647) {
// for minstd_rand and minstd_rand0 we can improve performance by avoiding constant divisions
auto _Mul = static_cast<unsigned long long>(_Prev) * _Ax;
_Mul = (_Mul >> 31) + (_Mul & _Mx);
_Mul = _Mul < _Mx ? _Mul : _Mul - _Mx;

return static_cast<_Uint>(_Mul);
} else if constexpr (_Cx <= UINT_MAX && static_cast<_Uint>(_Mx - 1) <= (UINT_MAX - _Cx) / _Ax) {
// unsigned int is sufficient to store intermediate calculation
const auto _Mul =
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