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Avoid using wider types in generate_canonical when the range of the random generator is a power of 2 #4850

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Aug 8, 2024
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Avoid using wider types in generate_canonical when the range of the random generator is a power of 2 #4850

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30 changes: 15 additions & 15 deletions stl/inc/random
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Original file line number Diff line number Diff line change
Expand Up @@ -310,32 +310,32 @@ _NODISCARD _Real generate_canonical(_Gen& _Gx) { // build a floating-point value
constexpr auto _Gxmax = (_Gen::max)();
constexpr auto _Params = _Generate_canonical_params(_Minbits, _Gxmax - _Gxmin);

using _Sx_type = conditional_t<_Params._Smax_bits <= 32, uint32_t,
conditional_t<_Params._Smax_bits <= 64, uint64_t, _Unsigned128>>;
_Sx_type _Sx;
_STL_INTERNAL_STATIC_ASSERT(_Params._Kx >= 1);

if constexpr (_Params._Rx_is_pow2) {
// Always needs only one attempt. Multiplications can be replaced with shift/add. Optimize k=1 case.
if constexpr (_Params._Kx == 1) {
_Sx = static_cast<_Sx_type>(_Gx() - _Gxmin);
} else if constexpr (_Params._Kx > 1) {
// Always needs only one attempt. Multiplications can be replaced with shift/add.
constexpr int _Discarded_bits = _Params._Smax_bits - _Minbits;
_Result_uint_type _Sx = static_cast<_Result_uint_type>((_Gx() - _Gxmin) >> _Discarded_bits);

if constexpr (_Params._Kx > 1) {
constexpr int _Rx_bits = _Params._Smax_bits / _Params._Kx;
_Sx = 0;
int _Shift = 0;
for (int _Idx = 0; _Idx < _Params._Kx; ++_Idx) {
_Sx += static_cast<_Sx_type>(_Gx() - _Gxmin) << _Shift;
int _Shift = -_Discarded_bits;
for (int _Idx = 1; _Idx < _Params._Kx; ++_Idx) {
_Shift += _Rx_bits;
_Sx += static_cast<_Result_uint_type>(_Gx() - _Gxmin) << _Shift;
}
} else {
_STL_INTERNAL_STATIC_ASSERT(false); // unexpected k
}

_Sx >>= _Params._Smax_bits - _Minbits;
return static_cast<_Real>(static_cast<_Result_uint_type>(_Sx)) * static_cast<_Real>(_Params._Scale);
return static_cast<_Real>(_Sx) * static_cast<_Real>(_Params._Scale);
} else {
using _Sx_type = conditional_t<_Params._Smax_bits <= 32, uint32_t,
conditional_t<_Params._Smax_bits <= 64, uint64_t, _Unsigned128>>;

constexpr auto _Rx = _Sx_type{_Gxmax - _Gxmin} + 1u;
constexpr _Sx_type _Limit = static_cast<_Sx_type>(_Params._Xx) * (_Sx_type{1} << _Minbits);

_Sx_type _Sx;

do {
// unroll first two iterations to avoid unnecessary multiplications
_Sx = static_cast<_Sx_type>(_Gx() - _Gxmin);
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4 changes: 2 additions & 2 deletions tests/std/tests/P0952R2_new_generate_canonical/test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -128,7 +128,7 @@ int main() {
const auto expected3 = ldexpf(0xDE'55'2C >> (3 * 8 - 24), -24);

assert((generate_with_ibe<float, 24, Engine>) () == expected1);
assert((generate_with_ibe<float, 20, Engine>) () == expected2); // needs a 64 bit accumulator for $S$
assert((generate_with_ibe<float, 20, Engine>) () == expected2);
assert((generate_with_ibe<float, 8, Engine>) () == expected3);
}

Expand All @@ -149,7 +149,7 @@ int main() {

assert((generate_with_ibe<double, 64, Engine>) () == expected1);
assert((generate_with_ibe<double, 32, Engine>) () == expected2);
assert((generate_with_ibe<double, 24, Engine>) () == expected3); // needs a 128 bit accumulator for $S$
assert((generate_with_ibe<double, 24, Engine>) () == expected3);
}

{
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