Changes in v5:
* Static-assert movable RNG object correctly.
* Add a more comprehensive test gencanon.cc
Changes in v4:
* Static-assert that arg is floating-point, coercible from bigger unsigned.
* Static-assert that 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.
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. A new test,
gencanon.cc, is more thorough.
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.
*
testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon.cc:
Test for float and double from 32-bit RNG, including 1.0 do-over.
---
libstdc++-v3/include/bits/random.tcc | 48 +++++++++++
libstdc++-v3/include/std/random | 1 +
.../operators/64351.cc | 8 +-
.../operators/gencanon.cc | 85 +++++++++++++++++++
4 files changed, 140 insertions(+), 2 deletions(-)
create mode 100644
libstdc++-v3/testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon.cc
diff --git a/libstdc++-v3/include/bits/random.tcc
b/libstdc++-v3/include/bits/random.tcc
index 53ccacb2e38..65f457a6659 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 +3388,51 @@ 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) {}(
+ static_cast<std::remove_cv_t<_URBG>&&>(__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
diff --git
a/libstdc++-v3/testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon.cc
b/libstdc++-v3/testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon.cc
new file mode 100644
index 00000000000..ba781315e0d
--- /dev/null
+++
b/libstdc++-v3/testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon.cc
@@ -0,0 +1,85 @@
+// { dg-do run { target { c++26 && { ! simulator } } } }
+
+#include <random>
+#include <limits>
+#include <type_traits>
+#include <cmath>
+#include <testsuite_hooks.h>
+
+// Verify P0952R9 implementation requiring a second round-trip
+// if first yields exactly 1. In this test, the RNG delivering
+// 32 bits per call is seeded such that this occurs once on the
+// sixth iteration for float, and not at all for double.
+// However, each double iteration requires two calls to the RNG.
+
+template <typename T, typename RNG>
+void
+test01(RNG& rng, int& deviation, int& max, int& rms, int& zero, int& skips)
+{
+ const auto size = 1'000'000, buckets = 100;
+ std::array<number, buckets> histo{};
+ int zero = 0, skips = 0;
+ for (auto i = 0; i != size; ++i) {
+ T sample = std::generate_canonical<T, 100>(rng);
+ VERIFY(sample >= 0.0);
+ VERIFY(sample < 1.0)
+ if (sample == 0.0) {
+ ++zero;
+ }
+ auto bucket = static_cast<int>(std::floor(sample * buckets));
+ ++histo[bucket];
+ rng2.discard(1);
+ if (rng != rng2) {
+ ++skips;
+ rng2.discard(1);
+ VERIFY(rng == rng2)
+ }
+ }
+ int devsquare = 0;
+ for (int i = 0; i < buckets; ++i) {
+ const auto expected = size / buckets;
+ auto count = histo[i];
+ auto diff = count - expected;
+ if (diff < 0) diff = -diff;
+ deviation += diff;
+ devsquare += diff * diff;
+ if (diff > max) max = diff;
+ }
+ rms = std::sqrt(devsquare);
+}
+
+int
+main()
+{
+ std::mt19937 rng(8890);
+ std::seed_seq sequence{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
+ rng.seed(sequence);
+ rng.discard(12 * 629143);
+
+ {
+ int deviation{}, max{}, rms{}, zero{}, skips{};
+ auto rng2{rng};
+ test01<float>(rng2, deviation, max, rms, zero, skips);
+
+ if (std::numeric_limits<float>::is_iec559) {
+ VERIFY(deviation == 7032);
+ VERIFY(max == 276);
+ VERIFY(rms == 906);
+ VERIFY(zero == 0);
+ }
+ VERIFY(skips == 1);
+ }
+ {
+ auto rng3{rng}
+ test01<double>(rng2, deviation, max, rms, zero, skips);
+
+ if (std::numeric_limits<double>::is_iec559) {
+ VERIFY(deviation == 7650);
+ VERIFY(max == 259);
+ VERIFY(rms == 975);
+ VERIFY(zero == 0);
+ }
+ VERIFY(skips == 1'000'000);
+ }
+ return 0;
+}
--
2.50.0
