On Mon, 20 Jul 2026 at 13:02, Tomasz Kaminski <[email protected]> wrote:
>
>
>
> On Mon, Jul 20, 2026 at 1:45 PM Jonathan Wakely <[email protected]> wrote:
>>
>> On Tue, 02 Jun 2026 at 16:01 +0200, Tomasz Kamiński wrote:
>> >This patch splits implementation of generate_cannonical_any, into tree
>>
>> "canonical"
>> "three"
>>
>> >different path depending on bits generated (R), required bits of entropy
>> >(b), and value k such that R^k > 2^d > R^(k-1):
>> >* log2(R) >= d: single call of generator is sufficient (k = 1),
>> > and computation can be performed in the generator value type (_UIntR)
>> >* bit_width(R^k) <= 128: computation can be performed using type
>> > __detail::_Select_uint_least_t; previous implementation is reused
>> >* bit_width(R^k) > 128: the computations required may overflow 128 bit;
>> > such generators were previously unsupported, as illustrated by
>> > r17-507-g5d17eec7d83445
>> >
>> >This patch implements the support for the last branch, by observing
>> >that while it is possible for computation to overflow 128 the sum:
>> > sum_[0, k) urng() * R^i
>> >The values of sum, such that (sum / x * 2^d) > 1, are rejected, and
>> >any value (sum / x) > 2^d can be rejected. As 2^d is guaranteed to fit
>> >in 128 integer, we can use above to reject the value, before overflow
>> >will occur.
>> >
>> >Firstly, we know that for l = k-1, the value R^l < 2^d, and
>> >sum_[0, l) urng() * R^i <= R^l (because urng() is alwayus < R),
>>
>> "always"
>>
>> >always fit in 128bit integer. We compute tha valuet (__lsum) using
>>
>> "value"
>>
>> >normal loop.
>> >
>> >This leaves us with final iteration of multiplying __kth * R^l,
>> >where __kth is result of invocation of urng. In that case we
>> >compute the bitwidth (__abits) of value that can be multiplied
>> >by _R^l without overflow 128bit integer. The __kth * R^l is
>> >performed by multiplying the loop of __abits of __kth in
>> >loop where:
>> >* __sumx (initially __lsum / __x) is the current value of sum / x
>> >* __remx (initially 2^d - __sumx) is the value that we can still
>> > add before overflowing (shifted with each iteration)
>> >* __modx (initially __modx % __x) are corrected reminders
>> >In each iteration the lowest __kth bits are multiplied with _Rl (__val),
>> >scaled (__valx) and checked against __rem. When fits the __sumx, __modx
>> >and __remx are updated, and both values of __remx and __kth are shifted
>> >right by __abits.
>> >
>> >If the multiplication finished without overflow (__kth is zero),
>> > we handle the collected reminders (__modx), and compute the __ret value.
>>
>> "remainders" (and there seems to be a stray space at the start of the
>> line)
>>
>> >The value greater or equal 1 (e.g. due floating point aproximation), are
>>
>> "approximation"
>>
>> >still rejected.
>> >
>> >Similar approach is applied when computing x = R^k / 2^d.
>> >Again while R^k will overflow 128 bits, we know that x < R (so fits
>> >in 64bits), as R^l < 2^d < R^k, then x < R. We again compute
>> >R * R^l / 2^d, by splitting the value of R into __abits chunks (_pR).
>> >For result of chunks multipliations (_pRk), the bits (_xp) remaining
>>
>> "multiplications"
>>
>> >after division by __shift (starts from __d and reduces by __abits),
>> >are collected into cumulative value of __x. While the lower bits
>> >are preserved for later iteration (__pRk).
>> >
>> > PR libstdc++/119739
>> >
>> >libstdc++-v3/ChangeLog:
>> >
>> > * include/bits/random.tcc (__generate_canonical_any):
>> > Rewrite to support more cases.
>> > *
>> > testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon.cc:
>> > Add test for previously unsupported generator.
>> > *
>> > testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon_eng.cc:
>> > Add test for previously unsuported-generator ranges.
>> > *
>> > testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon_eng_neg.cc:
>> > Moved (now-supported) generator test to gencanon_eng.cc, and removed
>> > the file.
>> >---
>> >Tested on x86_64-linux. Also tested a modified version, with __bits <= 128
>> >branch being removed and __abits hardoced to 10. This confirms that new
>> >implementation gives the same output as existing one.
>> >
>> >OK for trunk?
>> >
>> > libstdc++-v3/include/bits/random.tcc | 169 +++++++++++++++---
>> > .../operators/gencanon.cc | 96 ++++++++++
>> > .../operators/gencanon_eng.cc | 63 +++++++
>> > .../operators/gencanon_eng_neg.cc | 89 ---------
>> > 4 files changed, 300 insertions(+), 117 deletions(-)
>> > delete mode 100644
>> > libstdc++-v3/testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon_eng_neg.cc
>> >
>> >diff --git a/libstdc++-v3/include/bits/random.tcc
>> >b/libstdc++-v3/include/bits/random.tcc
>> >index a8fab79961d..aa1f4a83a66 100644
>> >--- a/libstdc++-v3/include/bits/random.tcc
>> >+++ b/libstdc++-v3/include/bits/random.tcc
>> >@@ -242,7 +242,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> > constexpr size_t
>> > mersenne_twister_engine<_UIntType, __w, __n, __m, __r, __a, __u, __d,
>> > __s, __b, __t, __c, __l, __f>::tempering_u;
>> >-
>> >+
>> > template<typename _UIntType,
>> > size_t __w, size_t __n, size_t __m, size_t __r,
>> > _UIntType __a, size_t __u, _UIntType __d, size_t __s,
>> >@@ -3753,38 +3753,151 @@ namespace __detail
>> > constexpr unsigned __log2R
>> > = sizeof(_UIntR) * __CHAR_BIT__ - __builtin_clzg(__R) - 1;
>> > // We overstimate number of required bits, by computing
>> >- // r such that l * log2(R) >= d, so:
>> >- // R^l >= (2 ^ log2(R)) ^ l == 2 ^ (log2(r) * l) >= 2^d
>> >- // And then requiring l * bit_width(R) bits.
>> >- constexpr unsigned __l = (__d + __log2R - 1) / __log2R;
>> >- constexpr unsigned __bits = (__log2R + 1) * __l;
>> >- using _UInt = typename __detail::_Select_uint_least_t<__bits>::type;
>> >-
>> >- _GLIBCXX_GEN_CANON_CONST _UInt __rd = _UInt(1) << __d;
>> >- _GLIBCXX_GEN_CANON_CONST auto __logRrd = __gen_canon_log(__rd, __R);
>> >- _GLIBCXX_GEN_CANON_CONST unsigned __k
>> >- = __logRrd.__floor_log + (__rd > __logRrd.__floor_pow);
>> >-
>> >- _GLIBCXX_GEN_CANON_CONST _UInt __Rk
>> >- = (__k > __logRrd.__floor_log)
>> >- ? _UInt(__logRrd.__floor_pow) * _UInt(__R)
>> >- : _UInt(__logRrd.__floor_pow);
>> >- _GLIBCXX_GEN_CANON_CONST _UInt __x = __Rk / __rd;
>> >+ // r such that m * log2(R) >= d, so:
>> >+ // R^m >= (2 ^ log2(R)) ^ m == 2 ^ (log2(r) * m) >= 2^d
>> >+ // And then requiring m * bit_width(R) bits.
>> >+ constexpr unsigned __m = (__d + __log2R - 1) / __log2R;
>> >+ constexpr unsigned __bits = (__log2R + 1) * __m;
>> >+ if constexpr (__log2R >= __d)
>> >+ {
>> >+ // range already provide required number of bits,
>> >+ // so in this case __k == 1, and single call to generator
>> >+ // is sufficient. Futhermore 2^d fits in _UIntR
>> >+ constexpr _UIntR __rd = _UIntR(1) << __d;
>> >+ constexpr _UIntR __x = __R >> __d;
>> >
>> >- while (true)
>> >+ while (true)
>> >+ {
>> >+ _UIntR __val(__urng() - _Urbg::min());
>> >+ const _RealT __ret = _RealT(__val / __x) / _RealT(__rd);
>> >+ if (__ret < _RealT(1.0))
>> >+ return __ret;
>> >+ }
>> >+ }
>> >+ // generator call produce less bits than __d, but R^k > rd fits
>>
>> s/less/fewer/
>>
>> >+ // into 128 bits.
>> >+ else if constexpr (__bits <= 128)
>> > {
>> >- _UInt __Ri{1};
>> >- _UInt __sum(__urng() - _Urbg::min());
>> >- for (int __i = __k - 1; __i > 0; --__i)
>> >+ using _UInt = typename
>> >__detail::_Select_uint_least_t<__bits>::type;
>> >+
>> >+ _GLIBCXX_GEN_CANON_CONST _UInt __rd = _UInt(1) << __d;
>> >+ _GLIBCXX_GEN_CANON_CONST auto __logRrd = __gen_canon_log(__rd,
>> >__R);
>> >+ // __rd is power of two, and __R is not so __floor_pow is never
>> >+ // equal to __rd.
>> >+ _GLIBCXX_GEN_CANON_CONST unsigned __k = __logRrd.__floor_log + 1;
>> >+ _GLIBCXX_GEN_CANON_CONST _UInt __Rk = __logRrd.__floor_pow *
>> >_UInt(__R);
>> >+ // x = floor(R^k / 2^d) = R^k >> d;
>> >+ _GLIBCXX_GEN_CANON_CONST _UInt __x = __Rk >> __d;
>> >+
>> >+ while (true)
>> > {
>> >- __Ri *= _UInt(__R);
>> >- __sum += _UInt(__urng() - _Urbg::min()) * __Ri;
>> >+ _UInt __Ri{1};
>> >+ _UInt __sum(__urng() - _Urbg::min());
>> >+ for (int __i = __k - 1; __i > 0; --__i)
>> >+ {
>> >+ __Ri *= __R;
>> >+ __sum += __Ri * _UIntR(__urng() - _Urbg::min());
>> >+ }
>> >+ const _RealT __ret = _RealT(__sum / __x) / _RealT(__rd);
>> >+ if (__ret < _RealT(1.0))
>> >+ return __ret;
>> > }
>> >- const _RealT __ret = _RealT(__sum / __x) / _RealT(__rd);
>> >- if (__ret < _RealT(1.0))
>> >- return __ret;
>> > }
>> >-#undef _GLIBCXX_GEN_CANON_CONST
>> >+ else
>> >+ {
>> >+#pragma GCC diagnostic ignored "-Wshift-count-overflow" // we shift by
>> >64bits
>>
>> There's no push/pop around this pragma, but why is it here at all?
>
> We are inside push/pop for if constexpr, so I didn't think I needed to add a
> new set.
Ah yes, I looked at the wrong place. That's right then, but I'd like a
better comment on the pragma to say what it's for. If it's a false
positive due to a compiler bug, say that.
>
>>
>> Doesn't this warning indicate undefined behaviour, so we don't want to
>> just suppress it?
>
> As far as I recall, I thought this is UB caused by compiler, but that was 6
> months ago,
> Let me double check to be certain.
>>
>>
>> >+ static_assert(__log2R < 64, "Only generators emitting up
>> >to 64 bits are supported");
>> >+ using _UInt = typename __detail::_Select_uint_least_t<128>::type;
>> >+ using _UInt64 = typename __detail::_Select_uint_least_t<64>::type;
>> >+
>> >+ _GLIBCXX_GEN_CANON_CONST _UInt __rd = _UInt(1) << __d;
>> >+ _GLIBCXX_GEN_CANON_CONST auto __logRrd = __gen_canon_log(__rd,
>> >__R);
>> >+ // __rd is power of two, and __R is not so __floor_pow is never
>> >+ // equal to __rd, and l == k - 1
>> >+ _GLIBCXX_GEN_CANON_CONST unsigned __l = __logRrd.__floor_log;
>> >+ _GLIBCXX_GEN_CANON_CONST _UInt __Rl = __logRrd.__floor_pow;
>> >+
>> >+ // __abits is maximum bit width of the value, that can
>> >+ // be multiplied by R^l without overflowing 128 bit integer
>> >+ // TODO: use bit_width(_Rl)
>> >+ _GLIBCXX_GEN_CANON_CONST unsigned __bwRl = (__log2R + 1) * __l;
>> >+ // For __R close to power of two, the actual __k may be smaller
>> >than __m,
>> >+ // and will use less than 128bits.
>> >+ _GLIBCXX_GEN_CANON_CONST unsigned __abits = __bwRl > 64 ? 128 -
>> >__bwRl : 64;
>> >+ _GLIBCXX_GEN_CANON_CONST _UInt64 __amask
>> >+ = (__abits == 64) ? ~_UInt64(0) : (_UInt64(1) << __abits) - 1;
>> >+
>> >+ // Compute __x = _Rk / __rd (_Rk >> __d), by spliting _R into
>> >chunks
>> >+ // (_pR) of __abits, so their multiplication does not overflow 128
>> >bits.
>> >+ // __x fits in 64bits, becuse __R^l < __rd < __R^k, thus __x < _R
>> >+ _UInt64 __x(0), __pR(__R); _UInt __pRk(0);
>> >+ for (unsigned __shift = __d; __pR; __shift -= __abits)
>> >+ {
>> >+ __pRk += __Rl * (__pR & __amask);
>> >+ __pR >>= __abits;
>> >+
>> >+ _UInt __xp(__pRk >> __shift);
>> >+ __x += _UInt64(__xp);
>> >+ __pRk -= (__xp << __shift);
>> >+ __pRk >>= __abits;
>> >+ }
>> >+
>> >+ while (true)
>> >+ {
>> >+ // Compute sum of __urng() * R^i for i in range [0, l).
>> >+ // The values is smaller than R^l which is smaller than
>>
>> "value"
>>
>> >+ // __rd so fits into 128 bit integer.
>> >+ _UInt __Ri{1};
>> >+ _UInt __lsum(__urng() - _Urbg::min());
>> >+ for (int __i = __l - 1; __i > 0; --__i)
>> >+ {
>> >+ __Ri *= _UInt64(__R);
>> >+ __lsum += __Ri * _UInt64(__urng() - _Urbg::min());
>> >+ }
>> >+
>> >+ // The last iteration __urng() * R^k may overflow 128bit
>> >+ // integer. We use the fact that we reject __sum / __x >= __rd,
>> >+ // and split __urng() value into chunks of __abits, so their
>> >+ // products with R^l does not overflow 128bit. These product
>>
>> "products"
>>
>> >+ // are then multiplied by 2^__shift, scaled by __x and added
>> >into
>> >+ // the __sumx, until they value is smaller than remaining value
>>
>> "the value" or "their value"?
>>
>> >+ // __rem [(__rd - __sumx) * 2^__shift], or whole value was
>> >+ // multiplied (__kth is zero). The reminder of division by
>>
>> "remainder"
>>
>> >+ // __x are accumulated into __modx value, and handled later.
>> >+ // In consequence this guarantees that __sumx never exceeded
>>
>> "exceeds"
>>
>> >+ // __rd value, and thus does not overflow 128 bit integer.
>> >+ _UInt __sumx = __lsum / __x, __modx = __lsum % __x;
>> >+ _UInt __remx = __rd - __sumx;
>> >+ _UInt64 __kth(__urng() - _Urbg::min());
>> >+ for (unsigned __shift = 0; __kth; __shift += __abits)
>> >+ {
>> >+ const _UInt __val = __Rl * (__kth & __amask);
>> >+ const _UInt __valx = __val / __x;
>> >+ if (__valx > __remx)
>> >+ break;
>> >+ __kth >>= __abits;
>> >+
>> >+ __sumx += (__valx << __shift);
>> >+ __modx += ((__val % __x) << __shift);
>> >+
>> >+ __remx -= __valx;
>> >+ __remx >>= __abits;
>> >+ }
>> >+ if (__kth) // __sum / __x > _rd after adding __kth iteration
>> >+ continue;
>> >+
>> >+ // Handle accumulated reminders
>>
>> "remainders"
>>
>> >+ const _UInt __valx = __modx / __x;
>> >+ if (__valx > __rd - __sumx) // avoids overflow
>> >+ continue;
>> >+
>> >+ __sumx += __valx;
>> >+ const _RealT __ret = _RealT(__sumx) / _RealT(__rd);
>> >+ if (__ret < _RealT(1.0))
>> >+ return __ret;
>> >+ }
>> >+ }
>> >+#undef _GLIBCXX_GEN_CANON_CONST
>> > }
>> >
>> > #if !defined(_GLIBCXX_GENERATE_CANONICAL_STRICT)
>> >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
>> >index ae93ec13b7a..4da53e9d4f2 100644
>> >---
>> >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
>> >@@ -234,6 +234,98 @@ test_2p31m1(const std::mt19937& rng)
>> > }
>> > }
>> >
>> >+template<std::uint64_t Max, typename Under = std::mt19937_64>
>> >+struct trimmed_engine
>> >+{
>> >+ using result_type = std::uint64_t;
>> >+
>> >+ static constexpr
>> >+ result_type min()
>> >+ { return result_type(0); }
>> >+
>> >+ static constexpr
>> >+ result_type max()
>> >+ { return result_type(Max); }
>> >+
>> >+ trimmed_engine() : dist(min(), max())
>> >+ {}
>> >+
>> >+ trimmed_engine(const Under& gen)
>> >+ : under(gen), dist(min(), max())
>> >+ {}
>> >+
>> >+ result_type operator()()
>> >+ { return dist(under); }
>> >+
>> >+ void discard(std::size_t n)
>> >+ {
>> >+ for (size_t i = 0; i < n; ++i)
>> >+ (void)dist(under);
>> >+ }
>> >+
>> >+ friend bool
>> >+ operator==(const trimmed_engine& lhs, const trimmed_engine& rhs)
>> >+ { return lhs.under == rhs.under; }
>> >+
>> >+private:
>> >+ Under under;
>> >+ std::uniform_int_distribution<result_type> dist;
>> >+};
>> >+
>> >+// Uses a generate that emits range of size 2^55+2.
>>
>> "generator"
>>
>> >+// To produce 128bit IEE float, we need 3 calls to above generator, that
>> >can
>> >+// produce value that have 155 bits, and uses separate path.
>> >+template <typename T>
>> >+void
>> >+test_2p55p1(const std::mt19937& rng)
>> >+{
>> >+ if (!std::numeric_limits<T>::is_iec559)
>> >+ return;
>> >+
>> >+ constexpr size_t mantissa = std::numeric_limits<T>::digits;
>> >+ constexpr size_t call_per_elem = (mantissa / 55) + 1;
>> >+ const trimmed_engine<(1ULL << 55) + 1ULL, std::mt19937> lrng{rng};
>> >+
>> >+ int deviation = 0, max = 0, rms = 0, zeros = 0;
>> >+ int skips = run_generator<T, -1u, call_per_elem, 1u>
>> >+ (lrng, deviation, max, rms, zeros);
>> >+
>> >+ switch (mantissa)
>> >+ {
>> >+ case 24: // ieee32
>> >+ VERIFY(skips == 0);
>> >+ VERIFY(deviation == 7682);
>> >+ VERIFY(max == 295);
>> >+ VERIFY(rms == 971);
>> >+ VERIFY(zeros == 0);
>> >+ break;
>> >+ case 53: // ieee64
>> >+ VERIFY(skips == 0);
>> >+ VERIFY(deviation == 7682);
>> >+ VERIFY(max == 295);
>> >+ VERIFY(rms == 971);
>> >+ VERIFY(zeros == 0);
>> >+ break;
>> >+ case 64: // ieee80
>> >+ VERIFY(skips == 0);
>> >+ VERIFY(deviation == 6788);
>> >+ VERIFY(max == 248);
>> >+ VERIFY(rms == 854);
>> >+ VERIFY(zeros == 0);
>> >+ break;
>> >+ case 113: // ieee128
>> >+ VERIFY(skips == 0);
>> >+ VERIFY(deviation == 8222);
>> >+ VERIFY(max == 257);
>> >+ VERIFY(rms == 999);
>> >+ VERIFY(zeros == 0);
>> >+ break;
>> >+ default:
>> >+ VERIFY(false);
>> >+ break;
>> >+ }
>> >+}
>> >+
>> > int main()
>> > {
>> > std::mt19937 rng(8890);
>> >@@ -244,20 +336,24 @@ int main()
>> > test_2p32<float>(rng);
>> > test_10p6<float>(rng);
>> > test_2p31m1<float>(rng);
>> >+ test_2p55p1<float>(rng);
>> >
>> > test_2p32<double>(rng);
>> > test_10p6<double>(rng);
>> > test_2p31m1<double>(rng);
>> >+ test_2p55p1<double>(rng);
>> >
>> > test_2p32<long double>(rng);
>> > test_10p6<long double>(rng);
>> > test_2p31m1<long double>(rng);
>> >+ test_2p55p1<long double>(rng);
>> >
>> > #ifndef _GLIBCXX_GENERATE_CANONICAL_STRICT
>> > # ifdef __SIZEOF_FLOAT128__
>> > test_2p32<__float128>(rng);
>> > test_10p6<__float128>(rng);
>> > test_2p31m1<__float128>(rng);
>> >+ test_2p55p1<__float128>(rng);
>> > # endif
>> > #endif
>> > }
>> >diff --git
>> >a/libstdc++-v3/testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon_eng.cc
>> >
>> >b/libstdc++-v3/testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon_eng.cc
>> >index 662e3568a30..f586ebc7d17 100644
>> >---
>> >a/libstdc++-v3/testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon_eng.cc
>> >+++
>> >b/libstdc++-v3/testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon_eng.cc
>> >@@ -10,6 +10,40 @@ test_engine()
>> > (void)std::generate_canonical<_Real, size_t(-1)>(__engine);
>> > }
>> >
>> >+template<std::uint64_t Max, typename Under = std::mt19937_64>
>> >+struct trimmed_engine
>> >+{
>> >+ using result_type = std::uint64_t;
>> >+
>> >+ static constexpr
>> >+ result_type min()
>> >+ { return result_type(0); }
>> >+
>> >+ static constexpr
>> >+ result_type max()
>> >+ { return result_type(Max); }
>> >+
>> >+ trimmed_engine() : dist(min(), max())
>> >+ {}
>> >+
>> >+ result_type operator()()
>> >+ { return dist(under); }
>> >+
>> >+private:
>> >+ Under under;
>> >+ std::uniform_int_distribution<result_type> dist;
>> >+};
>> >+
>> >+template<typename Real, size_t Bits>
>> >+void
>> >+test_bits()
>> >+{
>> >+ trimmed_engine<(std::uint64_t(1) << Bits) - 1> pow2_engine;
>> >+ (void)std::generate_canonical<Real, -1u>(pow2_engine);
>> >+ trimmed_engine<(std::uint64_t(1) << Bits) - 2> non_pow2_engine;
>> >+ (void)std::generate_canonical<Real, -1u>(non_pow2_engine);
>> >+}
>> >+
>> > template<typename _Real>
>> > void
>> > test_all_engines()
>> >@@ -29,6 +63,35 @@ test_all_engines()
>> > test_engine<_Real, std::philox4x32>();
>> > test_engine<_Real, std::philox4x64>();
>> > #endif
>> >+
>> >+ // For 128bit floating points, generator emitting a range, which size is
>> >+ // not power of two, but of width of B bits, such that for any N:
>> >+ // N * B < 113 (bits in ieee128)
>> >+ // (N+1) * B > 128
>> >+ // use >128bits patch, as they would otherwise require integer with more
>> >+ // than 128 bits.
>> >+ // N == 3: B in [43, 57)
>> >+ test_bits<_Real, 42>(); // 3 calls
>> >+ test_bits<_Real, 43>(); // 3 calls, >128bits path
>> >+ test_bits<_Real, 56>(); // 3 calls, >128bits path
>> >+ test_bits<_Real, 57>(); // 2 calls
>> >+
>> >+ // N == 4: B in [33, 38)
>> >+ test_bits<_Real, 32>(); // 4 calls
>> >+ test_bits<_Real, 33>(); // 4 calls, >128bits path
>> >+ test_bits<_Real, 37>(); // 4 calls, >128bits path
>> >+ test_bits<_Real, 38>(); // 3 calls
>> >+
>> >+ // N == 5: B in [26, 29)
>> >+ test_bits<_Real, 25>(); // 5 calls
>> >+ test_bits<_Real, 26>(); // 5 calls, >128bits path
>> >+ test_bits<_Real, 28>(); // 5 calls, >128bits path
>> >+ test_bits<_Real, 29>(); // 4 calls
>> >+
>> >+ // N == 6: B == 22
>> >+ test_bits<_Real, 21>(); // 6 calls
>> >+ test_bits<_Real, 22>(); // 6 calls, >128bits path
>> >+ test_bits<_Real, 23>(); // 5 calls
>> > }
>> >
>> > int main()
>> >diff --git
>> >a/libstdc++-v3/testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon_eng_neg.cc
>> >
>> >b/libstdc++-v3/testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon_eng_neg.cc
>> >deleted file mode 100644
>> >index e3b4540bf2b..00000000000
>> >---
>> >a/libstdc++-v3/testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon_eng_neg.cc
>> >+++ /dev/null
>> >@@ -1,89 +0,0 @@
>> >-// { dg-do compile { target { c++11 } } }
>> >-// { dg-require-effective-target __float128 }
>> >-// { dg-require-effective-target base_quadfloat_support }
>> >-// { dg-add-options __float128 }
>> >-
>> >-#include <random>
>> >-#include <cstdint>
>> >-
>> >-template<std::uint64_t Max, typename Under = std::mt19937_64>
>> >-struct trimmed_engine
>> >-{
>> >- using result_type = std::uint64_t;
>> >-
>> >- static constexpr
>> >- result_type min()
>> >- { return result_type(0); }
>> >-
>> >- static constexpr
>> >- result_type max()
>> >- { return result_type(Max); }
>> >-
>> >- trimmed_engine() : dist(min(), max())
>> >- {}
>> >-
>> >- result_type operator()()
>> >- { return dist(under); }
>> >-
>> >-private:
>> >- Under under;
>> >- std::uniform_int_distribution<result_type> dist;
>> >-};
>> >-
>> >-template<typename Real, size_t Bits>
>> >-void
>> >-test_non_pow2()
>> >-{
>> >- trimmed_engine<(std::uint64_t(1) << Bits) - 2> non_pow2_engine;
>> >- (void)std::generate_canonical<Real, -1u>(non_pow2_engine);
>> >-}
>> >-
>> >-template<typename Real, size_t Bits>
>> >-void
>> >-test_pow2()
>> >-{
>> >- trimmed_engine<(std::uint64_t(1) << Bits) - 1> pow2_engine;
>> >- (void)std::generate_canonical<Real, -1u>(pow2_engine);
>> >-}
>> >-
>> >-int main()
>> >-{
>> >- // For 128bit floating points, generator emitting a range, which size is
>> >- // not power of two, but of width of B bits, such that for any N:
>> >- // N * B < 113 (bits in ieee128)
>> >- // (N+1) * B > 128
>> >- // are not supported, as they would require integer with more than 127
>> >bits.
>> >-
>> >- // N == 3: B in [43, 57)
>> >- test_non_pow2<__float128, 42>(); // 3 calls
>> >- test_non_pow2<__float128, 43>(); // { dg-error "from here" }
>> >- test_non_pow2<__float128, 56>(); // { dg-error "from here" }
>> >- test_non_pow2<__float128, 57>(); // 2 calls
>> >- test_pow2<__float128, 43>();
>> >- test_pow2<__float128, 56>();
>> >-
>> >- // N == 4: B in [33, 38)
>> >- test_non_pow2<__float128, 32>(); // 4 calls
>> >- test_non_pow2<__float128, 33>(); // { dg-error "from here" }
>> >- test_non_pow2<__float128, 37>(); // { dg-error "from here" }
>> >- test_non_pow2<__float128, 38>(); // 3 calls
>> >- test_pow2<__float128, 33>();
>> >- test_pow2<__float128, 37>();
>> >-
>> >- // N == 5: B in [26, 29)
>> >- test_non_pow2<__float128, 25>(); // 5 calls
>> >- test_non_pow2<__float128, 26>(); // { dg-error "from here" }
>> >- test_non_pow2<__float128, 28>(); // { dg-error "from here" }
>> >- test_non_pow2<__float128, 29>(); // 4 calls
>> >- test_pow2<__float128, 26>();
>> >- test_pow2<__float128, 28>();
>> >-
>> >- // N == 6: B == 22
>> >- test_non_pow2<__float128, 21>(); // 6 calls
>> >- test_non_pow2<__float128, 22>(); // { dg-error "from here" }
>> >- test_non_pow2<__float128, 23>(); // 5 calls
>> >- test_pow2<__float128, 22>();
>> >-}
>> >-
>> >-// { dg-prune-output "no type named 'type' in 'struct
>> >std::__detail::_Select_uint_least_t" }
>> >-// { dg-prune-output "static assertion failed: sorry, would be too much
>> >trouble for a slow result" }
>> >--
>> >2.54.0
>> >
>> >
>>
