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<random>: Use _Unsigned128 for linear_congruential_engine #5436

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Merged
merged 3 commits into from
Apr 29, 2025
Merged

<random>: Use _Unsigned128 for linear_congruential_engine #5436

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2a96a0e
`_Uint _Cx` always satisfies `_Cx <= ULLONG_MAX`.
StephanTLavavej Apr 25, 2025
2d337f9
Use `_Unsigned128`, preserve multprec.cpp for bincompat.
StephanTLavavej Apr 25, 2025
a102151
Reduce multprec.cpp dependencies to slightly improve throughput.
StephanTLavavej Apr 25, 2025
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17 changes: 3 additions & 14 deletions stl/inc/random
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Original file line number Diff line number Diff line change
Expand Up @@ -461,14 +461,6 @@ _NODISCARD _Real _Nrand_impl(_Gen& _Gx) { // build a floating-point value from r
}
}

_INLINE_VAR constexpr int _MP_len = 5;
using _MP_arr = uint64_t[_MP_len];

extern "C++" _NODISCARD _CRTIMP2_PURE uint64_t __CLRCALL_PURE_OR_CDECL _MP_Get(_MP_arr) noexcept;
extern "C++" _CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL _MP_Add(_MP_arr, uint64_t) noexcept;
extern "C++" _CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL _MP_Mul(_MP_arr, uint64_t, uint64_t) noexcept;
extern "C++" _CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL _MP_Rem(_MP_arr, uint64_t) noexcept;

template <class _Uint, _Uint _Ax, _Uint _Cx, _Uint _Mx>
_NODISCARD _Uint _Next_linear_congruential_value(_Uint _Prev) noexcept {
// Choose intermediate type:
Expand Down Expand Up @@ -500,16 +492,13 @@ _NODISCARD _Uint _Next_linear_congruential_value(_Uint _Prev) noexcept {
const auto _Mul =
static_cast<unsigned int>(_Prev) * static_cast<unsigned int>(_Ax) + static_cast<unsigned int>(_Cx);
return static_cast<_Uint>(_Mul % _Mx);
} else if constexpr (_Cx <= ULLONG_MAX && static_cast<_Uint>(_Mx - 1) <= (ULLONG_MAX - _Cx) / _Ax) {
} else if constexpr (static_cast<_Uint>(_Mx - 1) <= (ULLONG_MAX - _Cx) / _Ax) {
// unsigned long long is sufficient to store intermediate calculation
const auto _Mul = static_cast<unsigned long long>(_Prev) * _Ax + _Cx;
return static_cast<_Uint>(_Mul % _Mx);
} else { // no intermediate integral type fits; fall back to multiprecision
_MP_arr _Wx;
_MP_Mul(_Wx, _Prev, _Ax);
_MP_Add(_Wx, _Cx);
_MP_Rem(_Wx, _Mx);
return static_cast<_Uint>(_MP_Get(_Wx));
const auto _Mul = _Unsigned128{_Prev} * _Ax + _Cx;
return static_cast<_Uint>(_Mul % _Mx);
}
}

Expand Down
20 changes: 13 additions & 7 deletions stl/src/multprec.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3,15 +3,18 @@

// implements multiprecision math for random number generators

#include <limits>
#include <random>
#include <yvals.h>

_STD_BEGIN
constexpr int shift = _STD numeric_limits<unsigned long long>::digits / 2;
constexpr int _MP_len = 5;
using _MP_arr = unsigned long long[_MP_len];

constexpr int shift = 64 / 2;
constexpr unsigned long long mask = ~(~0ULL << shift);
constexpr unsigned long long maxVal = mask + 1;

[[nodiscard]] unsigned long long __CLRCALL_PURE_OR_CDECL _MP_Get(
// TRANSITION, ABI: preserved for binary compatibility
[[nodiscard]] _CRTIMP2_PURE unsigned long long __CLRCALL_PURE_OR_CDECL _MP_Get(
_MP_arr u) noexcept { // convert multi-word value to scalar value
return (u[1] << shift) + u[0];
}
Expand All @@ -32,7 +35,8 @@ static void add(unsigned long long* u, int ulen, unsigned long long* v,
}
}

void __CLRCALL_PURE_OR_CDECL _MP_Add(
// TRANSITION, ABI: preserved for binary compatibility
_CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL _MP_Add(
_MP_arr u, unsigned long long v0) noexcept { // add scalar value to multi-word value
unsigned long long v[2];
v[0] = v0 & mask;
Expand All @@ -50,7 +54,8 @@ static void mul(
}
}

void __CLRCALL_PURE_OR_CDECL _MP_Mul(
// TRANSITION, ABI: preserved for binary compatibility
_CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL _MP_Mul(
_MP_arr w, unsigned long long u0, unsigned long long v0) noexcept { // multiply multi-word value by multi-word value
constexpr int m = 2;
constexpr int n = 2;
Expand Down Expand Up @@ -106,7 +111,8 @@ static void div(_MP_arr u,
return ulen;
}

void __CLRCALL_PURE_OR_CDECL _MP_Rem(
// TRANSITION, ABI: preserved for binary compatibility
_CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL _MP_Rem(
_MP_arr u, unsigned long long v0) noexcept { // divide multi-word value by value, leaving remainder in u
unsigned long long v[2];
v[0] = v0 & mask;
Expand Down