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<random>: Improve performance of poisson_distribution constructor #5411

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Merged
merged 3 commits into from
Apr 22, 2025
Merged

<random>: Improve performance of poisson_distribution constructor #5411

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bc46765
Avoid useless call to sqrt, log and _XLgamma on poisson_distrubution …
gcerretani Apr 17, 2025
1bbe839
Extract `_Small_threshold`.
StephanTLavavej Apr 17, 2025
283f4da
Add data member initializers: zero is a strict subset of garbage.
StephanTLavavej Apr 17, 2025
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19 changes: 12 additions & 7 deletions stl/inc/random
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Original file line number Diff line number Diff line change
Expand Up @@ -2740,7 +2740,7 @@ public:
}

private:
_Ty1 _Gx0;
_Ty1 _Gx0 = 0.0;
};

_EXPORT_STD template <class _Ty = int>
Expand Down Expand Up @@ -2779,19 +2779,24 @@ public:
return _Mean;
}

static constexpr double _Small_threshold = 12.0;

void _Init(_Ty1 _Mean0) noexcept { // set internal state
_STL_ASSERT(0.0 < _Mean0, "invalid mean argument for poisson_distribution");
_Mean = _Mean0;
if (_Mean0 < _Small_threshold) {
_Small._Init(_Mean0);
return;
}
_Sqrt = _CSTD sqrt(2.0 * _Mean0);
_Logm = _CSTD log(_Mean0);
_Gx1 = _Mean0 * _Logm - _XLgamma(_Mean0 + 1.0);
_Small._Init(_Mean0);
}

_Ty1 _Mean;
_Ty1 _Sqrt;
_Ty1 _Logm;
_Ty1 _Gx1;
_Ty1 _Mean = 0.0;
_Ty1 _Sqrt = 0.0;
_Ty1 _Logm = 0.0;
_Ty1 _Gx1 = 0.0;
Comment on lines +2797 to +2799
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No change requested. Now it seems clear that the member group {_Sqrt, _Logm, _Gx1} and the _Small member never logically coexist. So perhaps we can store them into a union with _Mean being the tag, which makes param_type smaller. Although such change is ABI breaking and can only be done since vNext.


_Small_poisson_distribution<_Ty> _Small;
};
Expand Down Expand Up @@ -2868,7 +2873,7 @@ public:
private:
template <class _Engine>
result_type _Eval(_Engine& _Eng, const param_type& _Par0) const {
if (_Par0._Mean < 12.0) {
if (_Par0._Mean < param_type::_Small_threshold) {
return _Par0._Small(_Eng);
}

Expand Down