[PATCH v2] libstdc++: Change piecewise distribution densities to return vector<result_type> [PR82749]
Tomasz Kaminski
tkaminsk@redhat.com
Tue Jul 14 06:46:35 GMT 2026
On Mon, Jul 13, 2026 at 7:23 PM Jonathan Wakely <jwakely@redhat.com> wrote:
> On Mon, 01 Jun 2026 at 09:21 +0200, Tomasz Kamiński wrote:
> >This implements the interface changes from
> >LWG1439, "Return from densities() functions?".
> >
> >Due the ABI concerns, we cannot change the the element_type of member
> >(_M_den, _M_cp, and _M_m) vectors, and we convert the _M_den values
> >to result_type when function is invoked. However, as the usage of
> >distributions with types other than float, double, and long double
> >is either undefined (pre C++26) or implementaiton-defined (post C++26),
> >this limitation de-fact applies to float, and long double (if it's
> >different size than double).
> >
> >To handle above we introduce the __piecewise_distributions_storage<_Tp>
> >struct, whose type nested typedef points to double in above two cases,
> >and _Tp otherwise. The _StorageType is defined in terms of
> >__piecewise_distributions_storage_t alias to above, and is then used
> >in defintion of _M_den, _M_cp, and _M_m members. In consequence for
> >extended floating point types (__float128, std::float16, ...) for which
> >ABI can be changed, we store densitiees (and other values) as result_type.
> >
> >To avoid symbol conflicts, the param_type::densities member functions
> >are marked with "__rt" ABI tag. The corresponding members of the
> >distributions are modified to invoke param_type method, and marked
> >as always_inline. For consistency intervals methods are adjusted same
> >way.
> >
> >To preserve current behavior, the operators>> load densities
> >as values of _StorageType.
> >
> >Finally, we introduce _GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS, that
> >revers above changes, restoring old behavior for all floating point
> >types. Additionally for the users accept above ABI change,
> >we introduce a macro _GLIBCXX_USE_RESULT_TYPE_FOR_PIECEWISE_DENSITIES,
> >that result in __piecewise_distributions_storage_t being unconditionally
> >defined as _Tp, and thus adjust internal storage also for float and long
> >double.
> >
> > libstdc++ PR/82749
> >
> >libstdc++-v3/ChangeLog:
> >
> > * doc/xml/manual/using.xml
> > (_GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS): Add entry.
> > (_GLIBCXX_USE_RESULT_TYPE_FOR_PIECEWISE_DENSITIES): Add entry.
> > * doc/html/manual/using_macros.html: Regenerate.
> > * include/bits/random.h
> > (__detail::__piecewise_distributions_storage)
> > (__detail::__piecewise_distributions_storage_t)
> > (piecewise_constant_distribution::_StorageType)
> > (piecewise_linear_distribution::_StorageType): Define.
> > (piecewise_constant_distribution::param_type::_M_den)
> > (piecewise_constant_distribution::param_type::_M_cp)
> > (piecewise_linear_distribution::param_type::_M_den)
> > (piecewise_linear_distribution::param_type::_M_cp)
> > (piecewise_linear_distribution::param_type::_M_m): Use _StorageType
> > as element type.
> > (piecewise_constant_distribution::param_type::densities)
> > (piecewise_linear_distribution::param_type::densities)
> > [!_GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS]: Change return type
> > to vector<result_type>, mark with "__rt" ABI tag, and adjust
> > implementation.
> > (piecewise_constant_distribution::param_type::intervals)
> > (piecewise_linear_distribution::param_type::intervals): Use
> > result_type alias in return type.
> > (piecewise_constant_distribution::densities)
> > (piecewise_linear_distribution::densities): Delegate to param_type
> > method, additional change return type and mark as always_inline
> > if _GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS is not defined.
> > (piecewise_constant_distribution::intervals)
> > (piecewise_linear_distribution::intervals): Delegate to param_type
> > method and use result_type alias in return type.
> > * include/bits/random.tcc
> > (operator<<(basic_ostream&, const
> piecewise_constant_distribution&))
> > (operator<<(basic_ostream&, const piecewise_linear_distribution&)):
> > Parse vector of _StorageType for densities, to preserve behavior.
> > *
> testsuite/26_numerics/random/piecewise_constant_distribution/operators/accessors.cc:
> > New test.
> > *
> testsuite/26_numerics/random/piecewise_linear_distribution/operators/accessors.cc:
> > New test.
> > *
> testsuite/26_numerics/random/piecewise_constant_distribution/operators/accessors_exact.cc:
> > Run accessors.cc with
> _GLIBCXX_USE_RESULT_TYPE_FOR_PIECEWISE_DENSITIES.
> > *
> testsuite/26_numerics/random/piecewise_linear_distribution/operators/accessors_exact.cc:
> > Likewise.
> > *
> testsuite/26_numerics/random/piecewise_constant_distribution/operators/accessors_fallback.cc:
> > Run accessors.cc with _GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS.
> > *
> testsuite/26_numerics/random/piecewise_linear_distribution/operators/accessors_fallback.cc:
> > Likewise.
> >---
> >v2 adds also _GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS, that
> >reverts the changes. I plan to use it for all changes made
> >to this distributions in GCC-17.
> >Additionally
> >* updates and add PR link to commit description
> >* add accessorrs_fallback.cc tests
> >* use result_type in return consistently on new path
> >
> >Resated *piecewise* test. Testing on x86_64-linux.
> >OK for trunk?
> >
> > .../doc/html/manual/using_macros.html | 19 +++
> > libstdc++-v3/doc/xml/manual/using.xml | 29 ++++
> > libstdc++-v3/include/bits/random.h | 154 +++++++++++++-----
> > libstdc++-v3/include/bits/random.tcc | 8 +-
> > .../operators/accessors.cc | 83 ++++++++++
> > .../operators/accessors_exact.cc | 5 +
> > .../operators/accessors_fallback.cc | 5 +
> > .../operators/accessors.cc | 85 ++++++++++
> > .../operators/accessors_exact.cc | 5 +
> > .../operators/accessors_fallback.cc | 5 +
> > 10 files changed, 359 insertions(+), 39 deletions(-)
> > create mode 100644
> libstdc++-v3/testsuite/26_numerics/random/piecewise_constant_distribution/operators/accessors.cc
> > create mode 100644
> libstdc++-v3/testsuite/26_numerics/random/piecewise_constant_distribution/operators/accessors_exact.cc
> > create mode 100644
> libstdc++-v3/testsuite/26_numerics/random/piecewise_constant_distribution/operators/accessors_fallback.cc
> > create mode 100644
> libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/operators/accessors.cc
> > create mode 100644
> libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/operators/accessors_exact.cc
> > create mode 100644
> libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/operators/accessors_fallback.cc
> >
> >diff --git a/libstdc++-v3/doc/html/manual/using_macros.html
> b/libstdc++-v3/doc/html/manual/using_macros.html
> >index f1d8492fb9e..4be2ab576dc 100644
> >--- a/libstdc++-v3/doc/html/manual/using_macros.html
> >+++ b/libstdc++-v3/doc/html/manual/using_macros.html
> >@@ -156,4 +156,23 @@
> > <code class="option">-fexec-charset=UTF-8</code> so might give
> incorrect results
> > if called from a source file that uses a non-Unicode encoding,
> > especially for format strings using non-ASCII fill characters.
> >+ </p></dd><dt><span class="term"><code
> class="code">_GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS</code></span></dt><dd><p>
> >+ Undefined by default.
> >+ When defined, the behaviour specializations of
>
> The grammar seems garbled here, "the behaviour specializations of foo
> and bar their behavior to the state prior to the implementation of
> ..."
>
> Also, two different spellings of "behaviour". The manual pages usually
> use the American spelling, "behavior" (there are a few places that
> were probably written by me and use the British spelling).
>
> >+ <code class="code">piecewise_constant_distribution</code> and
> >+ <code class="code">piecewise_linear_distribution</code> for
> >+ their behavior to the state prior the implementation
> >+ of <a class="link" href="
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82749" target="_top">
> >+ PR82749</a> (<a class="link" href="
> https://cplusplus.github.io/LWG/lwg-defects.html#1439" target="_top">
> >+ LWG1439</a>) in GCC-17.
> >+ </p></dd><dt><span class="term"><code
> class="code">_GLIBCXX_USE_RESULT_TYPE_FOR_PIECEWISE_DENSITIES</code></span></dt><dd><p>
> >+ Undefined by default. ABI-changing.
> >+ When defined, specializations of
> >+ <code class="code">piecewise_constant_distribution</code> and
> >+ <code class="code">piecewise_linear_distribution</code> with a
> >+ <code class="code">result_type</code> of either <code
> class="code">float</code> or
> >+ <code class="code">long double</code> will manage their internal
> state using
> >+ <code class="code">result_type</code> values (instead of <code
> class="code">double</code>).
> >+ This is required for standard conformance, but is incompatible with
> >+ the default implementation of those class templates
>
> Period at the end of the sentence.
>
> > </p></dd></dl></div></div><div class="navfooter"><hr /><table
> width="100%" summary="Navigation footer"><tr><td width="40%"
> align="left"><a accesskey="p" href="using_headers.html">Prev</a> </td><td
> width="20%" align="center"><a accesskey="u"
> href="using.html">Up</a></td><td width="40%" align="right"> <a
> accesskey="n" href="using_dual_abi.html">Next</a></td></tr><tr><td
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> >\ No newline at end of file
> >diff --git a/libstdc++-v3/doc/xml/manual/using.xml
> b/libstdc++-v3/doc/xml/manual/using.xml
> >index e2e75e8fe28..1759eb5a214 100644
> >--- a/libstdc++-v3/doc/xml/manual/using.xml
> >+++ b/libstdc++-v3/doc/xml/manual/using.xml
> >@@ -1379,6 +1379,35 @@ g++ -Winvalid-pch -I. -include stdc++.h -H -g -O2
> hello.cc -o test.exe
> > especially for format strings using non-ASCII fill characters.
> > </para>
> > </listitem></varlistentry>
> >+
> <varlistentry><term><code>_GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS</code></term>
> >+ <listitem>
> >+ <para>
> >+ Undefined by default.
> >+ When defined, the behaviour specializations of
> >+ <code>piecewise_constant_distribution</code> and
> >+ <code>piecewise_linear_distribution</code> for
> >+ their behavior to the state prior the implementation
> >+ of <link xmlns:xlink="http://www.w3.org/1999/xlink"
> >+ xlink:href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82749">
> >+ PR82749</link> (<link xmlns:xlink="http://www.w3.org/1999/xlink"
> >+ xlink:href="https://cplusplus.github.io/LWG/lwg-defects.html#1439
> ">
> >+ LWG1439</link>) in GCC-17.
> >+ </para>
> >+ </listitem></varlistentry>
> >+
> <varlistentry><term><code>_GLIBCXX_USE_RESULT_TYPE_FOR_PIECEWISE_DENSITIES</code></term>
> >+ <listitem>
> >+ <para>
> >+ Undefined by default. ABI-changing.
> >+ When defined, specializations of
> >+ <code>piecewise_constant_distribution</code> and
> >+ <code>piecewise_linear_distribution</code> with a
> >+ <code>result_type</code> of either <code>float</code> or
> >+ <code>long double</code> will manage their internal state using
> >+ <code>result_type</code> values (instead of <code>double</code>).
> >+ This is required for standard conformance, but is incompatible with
> >+ the default implementation of those class templates
> >+ </para>
> >+ </listitem></varlistentry>
> > </variablelist>
> >
> > </section>
> >diff --git a/libstdc++-v3/include/bits/random.h
> b/libstdc++-v3/include/bits/random.h
> >index a0592a00763..6b82d1120b7 100644
> >--- a/libstdc++-v3/include/bits/random.h
> >+++ b/libstdc++-v3/include/bits/random.h
> >@@ -6385,6 +6385,40 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
> > { return !(__d1 == __d2); }
> > #endif
> >
> >+ namespace __detail
> >+ {
> >+#ifdef _GLIBCXX_USE_RESULT_TYPE_FOR_PIECEWISE_DENSITIES
> >+ template<typename _Tp>
> >+ using __piecewise_distributions_storage_t = _Tp;
> >+#else
> >+ template<typename _Tp>
> >+ struct __piecewise_distributions_storage
> >+ { using type = _Tp; };
> >+
> >+ template<>
> >+ struct __piecewise_distributions_storage<float>
> >+ { using type = double; };
> >+
> >+# ifdef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
> >+ template<>
> >+ struct __piecewise_distributions_storage<__ibm128>
> >+ { using type = double; };
> >+
> >+ template<>
> >+ struct __piecewise_distributions_storage<__ieee128>
> >+ { using type = double; };
> >+# elif __LDBL_MANT_DIG__ != __DBL_MANT_DIG__
> >+ template<>
> >+ struct __piecewise_distributions_storage<long double>
> >+ { using type = double; };
> >+# endif
> >+
> >+ template<typename _Tp>
> >+ using __piecewise_distributions_storage_t
> >+ = typename __piecewise_distributions_storage<_Tp>::type;
> >+#endif // _GLIBCXX_USE_RESULT_TYPE_FOR_PIECEWISE_DENSITIES
> >+ }
> >+
> > /**
> > * @brief A piecewise_constant_distribution random number distribution.
> > *
> >@@ -6407,6 +6441,9 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
> > static_assert(std::is_floating_point<_RealType>::value,
> > "result_type must be a floating point type");
> >
> >+ using _StorageType
> >+ = __detail::__piecewise_distributions_storage_t<_RealType>;
> >+
> > public:
> > /** The type of the range of the distribution. */
> > typedef _RealType result_type;
> >@@ -6437,7 +6474,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
> > param_type(const param_type&) = default;
> > param_type& operator=(const param_type&) = default;
> >
> >- std::vector<_RealType>
> >+ std::vector<result_type>
> > intervals() const
> > {
> > if (_M_int.empty())
> >@@ -6450,9 +6487,34 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
> > return _M_int;
> > }
> >
> >+#ifdef _GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS
> > std::vector<double>
> > densities() const
> > { return _M_den.empty() ? std::vector<double>(1, 1.0) : _M_den; }
> >+#else
> >+ // _GLIBCXX_RESOLVE_LIB_DEFECTS
> >+ // 1439. Return from densities() functions?
> >+ [[__gnu__::__abi_tag__("__rt")]]
>
> I've been trying to think of a tag name that is a bit more
> self-documenting that "rt" but I don't have a better suggestion.
That is shorcut of result_type, also haven't tought about anything better.
>
> >+ std::vector<result_type>
> >+ densities() const
> >+ {
> >+#pragma GCC diagnostic push
> >+#pragma GCC diagnostic ignored "-Wc++17-extensions"
> >+ if (_M_den.empty())
> >+ return std::vector<_RealType>(1, _RealType(1));
> >+ else if constexpr (is_same<_RealType, _StorageType>::value)
> >+ return _M_den;
> >+ else
> >+ {
> >+ std::vector<_RealType> __res;
> >+ __res.reserve(_M_den.size());
> >+ for (_RealType __v : _M_den)
> >+ __res.push_back(__v);
> >+ return __res;
>
> Can this be simply return {_M_den.begin(), _M_den.end()} ?
> That will convert from _StorageType to _RealType.
>
Yes, leftover from the state were I was planning to store the values in
_M_cp.
Will do std::vector<_RealType>(_M_den.begin(), _M_den.end());
>
> >+ }
> >+#pragma GCC diagnostic pop
> >+ }
> >+#endif
> >
> > friend bool
> > operator==(const param_type& __p1, const param_type& __p2)
> >@@ -6472,8 +6534,8 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
> > _M_initialize2(const _RealType* __ints, _RealType __den);
> >
> > std::vector<_RealType> _M_int;
> >- std::vector<double> _M_den;
> >- std::vector<double> _M_cp;
> >+ std::vector<_StorageType> _M_den;
> >+ std::vector<_StorageType> _M_cp;
> >
> > template<typename _RealType1, typename _CharT, typename _Traits>
> > friend std::basic_ostream<_CharT, _Traits>&
> >@@ -6520,28 +6582,23 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
> > /**
> > * @brief Returns a vector of the intervals.
> > */
> >- std::vector<_RealType>
> >+ std::vector<result_type>
> > intervals() const
> >- {
> >- if (_M_param._M_int.empty())
> >- {
> >- std::vector<_RealType> __tmp(2);
> >- __tmp[1] = _RealType(1);
> >- return __tmp;
> >- }
> >- else
> >- return _M_param._M_int;
> >- }
> >+ { return _M_param.intervals(); }
> >
> > /**
> > * @brief Returns a vector of the probability densities.
> > */
> >+#ifdef _GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS
> > std::vector<double>
> > densities() const
> >- {
> >- return _M_param._M_den.empty()
> >- ? std::vector<double>(1, 1.0) : _M_param._M_den;
> >- }
> >+ { return _M_param.densities(); }
> >+#else
> >+ [[__gnu__::__always_inline__]]
> >+ std::vector<result_type>
> >+ densities() const
> >+ { return _M_param.densities(); }
> >+#endif
> >
> > /**
> > * @brief Returns the parameter set of the distribution.
> >@@ -6695,6 +6752,9 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
> > static_assert(std::is_floating_point<_RealType>::value,
> > "result_type must be a floating point type");
> >
> >+ using _StorageType
> >+ = __detail::__piecewise_distributions_storage_t<_RealType>;
> >+
> > public:
> > /** The type of the range of the distribution. */
> > typedef _RealType result_type;
> >@@ -6725,7 +6785,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
> > param_type(const param_type&) = default;
> > param_type& operator=(const param_type&) = default;
> >
> >- std::vector<_RealType>
> >+ std::vector<result_type>
> > intervals() const
> > {
> > if (_M_int.empty())
> >@@ -6738,9 +6798,34 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
> > return _M_int;
> > }
> >
> >+#ifdef _GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS
> > std::vector<double>
> > densities() const
> > { return _M_den.empty() ? std::vector<double>(2, 1.0) : _M_den; }
> >+#else
> >+ // _GLIBCXX_RESOLVE_LIB_DEFECTS
> >+ // 1439. Return from densities() functions?
> >+ [[__gnu__::__abi_tag__("__rt")]]
> >+ std::vector<result_type>
> >+ densities() const
> >+ {
> >+#pragma GCC diagnostic push
> >+#pragma GCC diagnostic ignored "-Wc++17-extensions"
> >+ if (_M_den.empty())
> >+ return std::vector<_RealType>(2, _RealType(1));
> >+ else if constexpr (is_same<_RealType, _StorageType>::value)
> >+ return _M_den;
> >+ else
> >+ {
> >+ std::vector<_RealType> __res;
> >+ __res.reserve(_M_den.size());
> >+ for (_RealType __v : _M_den)
> >+ __res.push_back(__v);
> >+ return __res;
> >+ }
> >+#pragma GCC diagnostic pop
> >+ }
> >+#endif
> >
> > friend bool
> > operator==(const param_type& __p1, const param_type& __p2)
> >@@ -6760,9 +6845,9 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
> > _M_initialize2(const _RealType* __ints, const _RealType* __dens);
> >
> > std::vector<_RealType> _M_int;
> >- std::vector<double> _M_den;
> >- std::vector<double> _M_cp;
> >- std::vector<double> _M_m;
> >+ std::vector<_StorageType> _M_den;
> >+ std::vector<_StorageType> _M_cp;
> >+ std::vector<_StorageType> _M_m;
> >
> > template<typename _RealType1, typename _CharT, typename _Traits>
> > friend std::basic_ostream<_CharT, _Traits>&
> >@@ -6809,29 +6894,24 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
> > /**
> > * @brief Return the intervals of the distribution.
> > */
> >- std::vector<_RealType>
> >+ std::vector<result_type>
> > intervals() const
> >- {
> >- if (_M_param._M_int.empty())
> >- {
> >- std::vector<_RealType> __tmp(2);
> >- __tmp[1] = _RealType(1);
> >- return __tmp;
> >- }
> >- else
> >- return _M_param._M_int;
> >- }
> >+ { return _M_param.intervals(); }
> >
> > /**
> > * @brief Return a vector of the probability densities of the
> > * distribution.
> > */
> >+#ifdef _GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS
> > std::vector<double>
> > densities() const
> >- {
> >- return _M_param._M_den.empty()
> >- ? std::vector<double>(2, 1.0) : _M_param._M_den;
> >- }
> >+ { return _M_param.densities(); }
> >+#else
> >+ [[__gnu__::__always_inline__]]
> >+ std::vector<result_type>
> >+ densities() const
> >+ { return _M_param.densities(); }
> >+#endif
> >
> > /**
> > * @brief Returns the parameter set of the distribution.
> >diff --git a/libstdc++-v3/include/bits/random.tcc
> b/libstdc++-v3/include/bits/random.tcc
> >index e581f912764..6c88d68d3e4 100644
> >--- a/libstdc++-v3/include/bits/random.tcc
> >+++ b/libstdc++-v3/include/bits/random.tcc
> >@@ -3199,6 +3199,8 @@ namespace __detail
> > piecewise_constant_distribution<_RealType>& __x)
> > {
> > using __ios_base = typename basic_istream<_CharT,
> _Traits>::ios_base;
> >+ using _StorageType
> >+ = typename
> piecewise_constant_distribution<_RealType>::_StorageType;
> >
> > const typename __ios_base::fmtflags __flags = __is.flags();
> > __is.flags(__ios_base::dec | __ios_base::skipws);
> >@@ -3209,7 +3211,7 @@ namespace __detail
> > std::vector<_RealType> __int_vec;
> > if (__detail::__extract_params(__is, __int_vec, __n + 1))
> > {
> >- std::vector<double> __den_vec;
> >+ std::vector<_StorageType> __den_vec;
> > if (__detail::__extract_params(__is, __den_vec, __n))
> > {
> > __x.param({ __int_vec.begin(), __int_vec.end(),
> >@@ -3466,6 +3468,8 @@ namespace __detail
> > piecewise_linear_distribution<_RealType>& __x)
> > {
> > using __ios_base = typename basic_istream<_CharT,
> _Traits>::ios_base;
> >+ using _StorageType
> >+ = typename piecewise_linear_distribution<_RealType>::_StorageType;
> >
> > const typename __ios_base::fmtflags __flags = __is.flags();
> > __is.flags(__ios_base::dec | __ios_base::skipws);
> >@@ -3476,7 +3480,7 @@ namespace __detail
> > vector<_RealType> __int_vec;
> > if (__detail::__extract_params(__is, __int_vec, __n + 1))
> > {
> >- vector<double> __den_vec;
> >+ vector<_StorageType> __den_vec;
> > if (__detail::__extract_params(__is, __den_vec, __n + 1))
> > {
> > __x.param({ __int_vec.begin(), __int_vec.end(),
> >diff --git
> a/libstdc++-v3/testsuite/26_numerics/random/piecewise_constant_distribution/operators/accessors.cc
> b/libstdc++-v3/testsuite/26_numerics/random/piecewise_constant_distribution/operators/accessors.cc
> >new file mode 100644
> >index 00000000000..219d1949040
> >--- /dev/null
> >+++
> b/libstdc++-v3/testsuite/26_numerics/random/piecewise_constant_distribution/operators/accessors.cc
> >@@ -0,0 +1,83 @@
> >+// { dg-do run { target c++11 } }
> >+// { dg-require-cstdint "" }
> >+
> >+#include <random>
> >+#include <testsuite_hooks.h>
> >+
> >+template<typename RealType>
> >+void
> >+test_exact()
> >+{
> >+ RealType x[6] = {0.0, 1.0, 2.0, 3.0, 4.0, 5.0};
> >+ RealType wt[5] = {0.5, 1.0, 3.5, 2.5, 0.5};
> >+
> >+ std::piecewise_constant_distribution<RealType>
> >+ u(std::begin(x), std::end(x), wt);
> >+
> >+ const std::vector<RealType>& interval = u.intervals();
> >+ VERIFY( interval.size() == 6 );
> >+ VERIFY( interval[0] == RealType(0.0) );
> >+ VERIFY( interval[5] == RealType(5.0) );
> >+
> >+#ifdef _GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS
> >+ const std::vector<double>& density = u.densities();
> >+#else
> >+ const std::vector<RealType>& density = u.densities();
> >+#endif
> >+
> >+ VERIFY( density.size() == 5 );
> >+ VERIFY( density[0] == RealType(0.5 / 8.0) );
> >+ VERIFY( density[1] == RealType(1.0 / 8.0) );
> >+ VERIFY( density[2] == RealType(3.5 / 8.0) );
> >+ VERIFY( density[3] == RealType(2.5 / 8.0) );
> >+ VERIFY( density[4] == RealType(0.5 / 8.0) );
> >+}
> >+
> >+template<typename RealType>
> >+void
> >+test_precision_depended()
> >+{
> >+ constexpr bool preserved
> >+#ifdef _GLIBCXX_USE_RESULT_TYPE_FOR_PIECEWISE_DENSITIES
> >+ = true;
> >+#else
> >+ = sizeof(RealType) <= sizeof(double);
> >+#endif
> >+
> >+ RealType x[3]{0.0, 0.5, 1.0};
> >+ constexpr RealType step
> >+ = std::numeric_limits<RealType>::epsilon();
> >+ RealType wt[2]{RealType(1) - step, RealType(1) + step};
> >+
> >+ std::piecewise_constant_distribution<RealType>
> >+ u(std::begin(x), std::end(x), wt);
> >+
> >+ const std::vector<RealType>& interval = u.intervals();
> >+ VERIFY( interval.size() == 3 );
> >+ VERIFY( interval[0] == RealType(0.0) );
> >+ VERIFY( interval[2] == RealType(1.0) );
> >+
> >+#ifdef _GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS
> >+ const std::vector<double>& density = u.densities();
> >+#else
> >+ const std::vector<RealType>& density = u.densities();
> >+#endif
> >+
> >+ VERIFY( density.size() == 2 );
> >+ VERIFY( density[0] == (preserved ? wt[0] : RealType(1)) );
> >+ VERIFY( density[1] == (preserved ? wt[1] : RealType(1)) );
> >+}
> >+
> >+int main()
> >+{
> >+ using namespace __gnu_test;
> >+ test_exact<float>();
> >+ test_exact<double>();
> >+ test_exact<long double>();
> >+
> >+ test_precision_depended<float>();
> >+ test_precision_depended<double>();
> >+ test_precision_depended<long double>();
> >+
> >+ return 0;
> >+}
> >diff --git
> a/libstdc++-v3/testsuite/26_numerics/random/piecewise_constant_distribution/operators/accessors_exact.cc
> b/libstdc++-v3/testsuite/26_numerics/random/piecewise_constant_distribution/operators/accessors_exact.cc
> >new file mode 100644
> >index 00000000000..469ed3fac46
> >--- /dev/null
> >+++
> b/libstdc++-v3/testsuite/26_numerics/random/piecewise_constant_distribution/operators/accessors_exact.cc
> >@@ -0,0 +1,5 @@
> >+// { dg-options "-D_GLIBCXX_USE_RESULT_TYPE_FOR_PIECEWISE_DENSITIES" }
> >+// { dg-do run { target c++11 } }
> >+// { dg-require-cstdint "" }
> >+#include "accessors.cc"
> >+
> >diff --git
> a/libstdc++-v3/testsuite/26_numerics/random/piecewise_constant_distribution/operators/accessors_fallback.cc
> b/libstdc++-v3/testsuite/26_numerics/random/piecewise_constant_distribution/operators/accessors_fallback.cc
> >new file mode 100644
> >index 00000000000..3e2fbf03430
> >--- /dev/null
> >+++
> b/libstdc++-v3/testsuite/26_numerics/random/piecewise_constant_distribution/operators/accessors_fallback.cc
> >@@ -0,0 +1,5 @@
> >+// { dg-options "-D_GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS" }
> >+// { dg-do run { target c++11 } }
> >+// { dg-require-cstdint "" }
> >+#include "accessors.cc"
> >+
> >diff --git
> a/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/operators/accessors.cc
> b/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/operators/accessors.cc
> >new file mode 100644
> >index 00000000000..d3855ee0291
> >--- /dev/null
> >+++
> b/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/operators/accessors.cc
> >@@ -0,0 +1,85 @@
> >+// { dg-do run { target c++11 } }
> >+// { dg-require-cstdint "" }
> >+
> >+#include <random>
> >+#include <testsuite_hooks.h>
> >+
> >+template<typename RealType>
> >+void
> >+test_exact()
> >+{
> >+ RealType x[6] = {0.0, 1.0, 2.0, 3.0, 4.0, 5.0};
> >+ RealType wt[6] = {0.0, 1.0, 1.0, 6.0, -1.0, 2.0};
> >+
> >+ std::piecewise_linear_distribution<RealType>
> >+ u(std::begin(x), std::end(x), wt);
> >+
> >+ const std::vector<RealType>& interval = u.intervals();
> >+ VERIFY( interval.size() == 6 );
> >+ VERIFY( interval[0] == RealType(0.0) );
> >+ VERIFY( interval[5] == RealType(5.0) );
> >+
> >+#ifdef _GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS
> >+ const std::vector<double>& density = u.densities();
> >+#else
> >+ const std::vector<RealType>& density = u.densities();
> >+#endif
> >+
> >+ VERIFY( density.size() == 6 );
> >+ VERIFY( density[0] == RealType(0.0 / 8.0) );
> >+ VERIFY( density[1] == RealType(1.0 / 8.0) );
> >+ VERIFY( density[2] == RealType(1.0 / 8.0) );
> >+ VERIFY( density[3] == RealType(6.0 / 8.0) );
> >+ VERIFY( density[4] == RealType(-1.0 / 8.0) );
> >+ VERIFY( density[5] == RealType(2.0 / 8.0) );
> >+}
> >+
> >+template<typename RealType>
> >+void
> >+test_precision_depended()
> >+{
> >+ constexpr bool preserved
> >+#ifdef _GLIBCXX_USE_RESULT_TYPE_FOR_PIECEWISE_DENSITIES
> >+ = true;
> >+#else
> >+ = sizeof(RealType) <= sizeof(double);
> >+#endif
> >+
> >+ RealType x[3]{0.0, 0.5, 1.0};
> >+ constexpr RealType step
> >+ = std::numeric_limits<RealType>::epsilon() * 2;
> >+ RealType wt[3]{RealType(1) - step, RealType(1), RealType(1) + step};
> >+
> >+ std::piecewise_linear_distribution<RealType>
> >+ u(std::begin(x), std::end(x), wt);
> >+
> >+ const std::vector<RealType>& interval = u.intervals();
> >+ VERIFY( interval.size() == 3 );
> >+ VERIFY( interval[0] == RealType(0.0) );
> >+ VERIFY( interval[2] == RealType(1.0) );
> >+
> >+#ifdef _GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS
> >+ const std::vector<double>& density = u.densities();
> >+#else
> >+ const std::vector<RealType>& density = u.densities();
> >+#endif
> >+
> >+ VERIFY( density.size() == 3 );
> >+ VERIFY( density[0] == (preserved ? wt[0] : RealType(1)) );
> >+ VERIFY( density[1] == RealType(1) );
> >+ VERIFY( density[2] == (preserved ? wt[2] : RealType(1)) );
> >+}
> >+
> >+int main()
> >+{
> >+ using namespace __gnu_test;
> >+ test_exact<float>();
> >+ test_exact<double>();
> >+ test_exact<long double>();
> >+
> >+ test_precision_depended<float>();
> >+ test_precision_depended<double>();
> >+ test_precision_depended<long double>();
> >+
> >+ return 0;
> >+}
> >diff --git
> a/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/operators/accessors_exact.cc
> b/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/operators/accessors_exact.cc
> >new file mode 100644
> >index 00000000000..469ed3fac46
> >--- /dev/null
> >+++
> b/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/operators/accessors_exact.cc
> >@@ -0,0 +1,5 @@
> >+// { dg-options "-D_GLIBCXX_USE_RESULT_TYPE_FOR_PIECEWISE_DENSITIES" }
> >+// { dg-do run { target c++11 } }
> >+// { dg-require-cstdint "" }
> >+#include "accessors.cc"
> >+
> >diff --git
> a/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/operators/accessors_fallback.cc
> b/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/operators/accessors_fallback.cc
> >new file mode 100644
> >index 00000000000..3e2fbf03430
> >--- /dev/null
> >+++
> b/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/operators/accessors_fallback.cc
> >@@ -0,0 +1,5 @@
> >+// { dg-options "-D_GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS" }
> >+// { dg-do run { target c++11 } }
> >+// { dg-require-cstdint "" }
> >+#include "accessors.cc"
> >+
> >--
> >2.54.0
> >
> >
>
>
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