[PATCH 2/2] libstdc++: Support any generator (up to 64bit) in generate_canonical [PR119739]
Tomasz Kaminski
tkaminsk@redhat.com
Mon Jul 20 13:19:28 GMT 2026
On Mon, Jul 20, 2026 at 2:32 PM Jonathan Wakely <jwakely@redhat.com> wrote:
> On Mon, 20 Jul 2026 at 13:02, Tomasz Kaminski <tkaminsk@redhat.com> wrote:
> >
> >
> >
> > On Mon, Jul 20, 2026 at 1:45 PM Jonathan Wakely <jwakely@redhat.com>
> 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;
>
We actually shift by 64 bits, per this comment, so that was real UB. I am
changing this in next revision
to: __bwRl > 96 ? 128 - __bwRl : 32, so we will do multiplication of chunks
of 32bits or smaller.
> >> >+ _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
> >> >
> >> >
> >>
>
>
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