Changes in v4:
* Static-assert arg is floating-point, coercible from bigger unsigned.
* Static-assert arg satisfies uniform_random_bit_generator, movable.
* Include uniform_int_dist.h for concept uniform_random_bit_generator
* Coerce floating consts from unsigned literals, matching other usage.
* Remove redundant type decorations static and -> .
* Use __builtin_floor for broader applicability than std::floor.
* Use __builtin_pow in place of immediate lambda.
* Rename, rearrange local objects for clarity.
* Add redundant "if constexpr (k > 1)" for clarity.
* #if-guard new impl against Clang, which lacks constexpr floor() etc.
Changes in v3:
* Implement exactly as specified in the WP, for C++26 and up.
* Eliminate massive overhead seen in present implementation.
Implement P0952R2 "A new specification for std::generate_canonical".
It has us start over if the naive generation of a float in 0..1
comes up exactly equal to 1, which occurs too rarely for the change
to measurably affect performance.
A test for the case already appears in 64351.cc. This change amounts
to a different resolution for PR64351 and LWG 2524.
libstdc++-v3/ChangeLog:
PR libstdc++/119739
* include/bits/random.tcc: Add generate_canonical impl for C++26.
* include/std/random: Include uniform_int_dist.h for concept.
*
testsuite/26_numerics/random/uniform_real_distribution/operators/64351.cc:
Adapt test for both pre- and post- C++26.
---
libstdc++-v3/include/bits/random.tcc | 47 +++++++++++++++++++
libstdc++-v3/include/std/random | 1 +
.../operators/64351.cc | 8 +++-
3 files changed, 54 insertions(+), 2 deletions(-)
diff --git a/libstdc++-v3/include/bits/random.tcc
b/libstdc++-v3/include/bits/random.tcc
index 53ccacb2e38..7babb5c7826 100644
--- a/libstdc++-v3/include/bits/random.tcc
+++ b/libstdc++-v3/include/bits/random.tcc
@@ -3349,6 +3349,9 @@ namespace __detail
}
}
+#if __cplusplus < 202400 || defined(__clang__)
+ // Clang lacks constexpr __builtin_floor and __builtin_pow.
+
template<typename _RealType, size_t __bits,
typename _UniformRandomNumberGenerator>
_RealType
@@ -3385,6 +3388,50 @@ namespace __detail
}
return __ret;
}
+#else
+ template<typename _RealT, size_t __digits, typename _URBG>
+ _RealT
+ generate_canonical(_URBG& __urng)
+ {
+ static_assert(requires {
+ [](uniform_random_bit_generator auto&&) {}(std::move(__urng)); },
+ "argument must meet uniform_random_bit_generator requirements");
+ static_assert(
+ std::is_floating_point_v<_RealT> && requires { _RealT(_URBG::max()); },
+ "template argument must be floating point, coercible from unsigned");
+
+ // Names are chosen to match the description in the Standard.
+ const size_t __r = std::numeric_limits<_RealT>::radix;
+ const auto __sample = [](auto __high) constexpr
+ { return _RealT(__high - _URBG::min()); };
+ constexpr _RealT __R = _RealT(1) + __sample(_URBG::max());
+ const size_t __d_max = std::numeric_limits<_RealT>::digits;
+ const size_t __d = __digits < __d_max ? __digits : __d_max;
+ constexpr _RealT __rd = __builtin_pow(_RealT(__r), __d);
+ const unsigned __k = [](_RealT __R, _RealT __rd) consteval
+ {
+ unsigned __i = 1;
+ for (auto __Ri = __R; __Ri < __rd; __Ri *= __R)
+ ++__i;
+ return __i;
+ } (__R, __rd);
+ constexpr _RealT __Rk = __builtin_pow(__R, __k);
+ constexpr _RealT __x = __builtin_floor(__Rk / __rd);
+ constexpr _RealT __xrd = __x * __rd;
+
+ _RealT __sum;
+ do
+ {
+ _RealT __Ri = _RealT(1);
+ __sum = __sample(__urng());
+ if constexpr (__k > 1)
+ for (int __i = 1; __i != __k; ++__i)
+ __Ri *= __R, __sum += __sample(__urng()) * __Ri;
+ } while (__builtin_expect(__sum >= __xrd, false));
+ _RealT __ret = __builtin_floor(__sum / __x) / __rd;
+ return __ret;
+ }
+#endif
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
diff --git a/libstdc++-v3/include/std/random b/libstdc++-v3/include/std/random
index 0e058a04bd9..86d4f0a8380 100644
--- a/libstdc++-v3/include/std/random
+++ b/libstdc++-v3/include/std/random
@@ -49,6 +49,7 @@
#include <type_traits>
#include <bits/random.h>
#include <bits/opt_random.h>
+#include <bits/uniform_int_dist.h>
#include <bits/random.tcc>
#endif // C++11
diff --git
a/libstdc++-v3/testsuite/26_numerics/random/uniform_real_distribution/operators/64351.cc
b/libstdc++-v3/testsuite/26_numerics/random/uniform_real_distribution/operators/64351.cc
index 3c0cb8bbd49..6212a32a364 100644
---
a/libstdc++-v3/testsuite/26_numerics/random/uniform_real_distribution/operators/64351.cc
+++
b/libstdc++-v3/testsuite/26_numerics/random/uniform_real_distribution/operators/64351.cc
@@ -45,16 +45,20 @@ test02()
rng.seed(sequence);
rng.discard(12 * 629143);
std::mt19937 rng2{rng};
+ int mismatch = 0;
for (int i = 0; i < 20; ++i)
{
float n =
std::generate_canonical<float, std::numeric_limits<float>::digits>(rng);
VERIFY( n != 1.f );
- // PR libstdc++/80137
rng2.discard(1);
- VERIFY( rng == rng2 );
}
+ // PR libstdc++/80137
+#if __cplusplus >= 202400
+ rng2.discard(1); // account for a 1.0 generated and discarded.
+#endif
+ VERIFY( rng == rng2 );
}
int
--
2.50.0
