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<random>: Fast discard() for minstd_rand and minstd_rand0 via fast exponentiation #5412

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Merged
Stephan T. Lavavej (StephanTLavavej) merged 5 commits into from
Apr 22, 2025
Merged

<random>: Fast discard() for minstd_rand and minstd_rand0 via fast exponentiation #5412

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99084bd
Fast discard() for minstd_ramd amd minstd_ramd0 via fast exponentiation
mirounga Apr 17, 2025
f1f1a07
clang formatting
mirounga Apr 17, 2025
1ce1a09
Delete mistakenly added CSV file.
StephanTLavavej Apr 17, 2025
843520a
Add newline.
StephanTLavavej Apr 17, 2025
3ee32da
Add discard() benchmarks.
StephanTLavavej Apr 17, 2025
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17 changes: 17 additions & 0 deletions benchmarks/src/random_integer_generation.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,23 @@ void BM_lcg(benchmark::State& state) {
}
BENCHMARK(BM_lcg);

/// Test discard()

template <class Engine>
void BM_discard(benchmark::State& state) {
Engine gen;
const auto n = static_cast<unsigned long long>(state.range(0));
for (auto _ : state) {
gen.discard(n);
benchmark::DoNotOptimize(gen());
}
}
BENCHMARK(BM_discard<std::mt19937>)->Range(0, 1 << 18);
BENCHMARK(BM_discard<std::mt19937_64>)->Range(0, 1 << 18);
BENCHMARK(BM_discard<std::minstd_rand>)->Range(0, 1 << 18);

/// Support machinery for testing _Rng_from_urng and _Rng_from_urng_v2

std::uint32_t GetMax() {
std::mt19937 gen;
std::uniform_int_distribution<std::uint32_t> dist(10'000'000, 20'000'000);
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36 changes: 31 additions & 5 deletions stl/inc/random
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Expand Up @@ -617,12 +617,38 @@ public:

void discard(unsigned long long _Nskip) noexcept /* strengthened */ {
// discard _Nskip elements
auto _Temp = _Prev;
for (; 0 < _Nskip; --_Nskip) {
_Temp = _Next_linear_congruential_value<_Uint, _Ax, _Cx, _Mx>(_Temp);
}
if constexpr (_Cx == 0 && _Mx == 2147483647) {
// for minstd_rand and minstd_rand0 we can improve performance by
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// performing fast exponentiation and avoiding constant divisions
auto _Temp = static_cast<unsigned long long>(_Prev);
auto _Mul = static_cast<unsigned long long>(_Ax);

for (;;) {
if (_Nskip & 1) {
_Temp = _Temp * _Mul;
_Temp = (_Temp >> 31) + (_Temp & _Mx);
_Temp = _Temp < _Mx ? _Temp : _Temp - _Mx;
}

_Prev = _Temp;
if (_Nskip >>= 1) {
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_Mul = _Mul * _Mul;
_Mul = (_Mul >> 31) + (_Mul & _Mx);
_Mul = _Mul < _Mx ? _Mul : _Mul - _Mx;
} else {
break;
}
}

_Prev = static_cast<_Uint>(_Temp);
} else {
auto _Temp = _Prev;

for (; 0 < _Nskip; --_Nskip) {
_Temp = _Next_linear_congruential_value<_Uint, _Ax, _Cx, _Mx>(_Temp);
}

_Prev = _Temp;
}
}

_NODISCARD friend bool operator==(
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