[PATCH] libstdc++: Fix weight computation for piecewise_linear_distribution(nw, xmin, xmax, fw) constructor [PR125548]
Jonathan Wakely
jwakely@redhat.com
Mon Jul 13 17:29:00 GMT 2026
On Mon, 01 Jun 2026 at 15:48 +0200, Tomasz Kamiński wrote:
>Compute kth weight using __fw(_M_int[k]) as required by standard,
>instead of __fw(_M_int[k] + __delta), i.e. __fw(_M_int[k + 1]).
>For __nw == 1, this was already corrected by r17-509-g7bed7d9276c11b.
>
>In both cases, previous behavior can be restored by defining
>_GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS.
"PIECEWISE"
>
> PR libstdc++/125548
>
>libstdc++-v3/ChangeLog:
>
> * doc/html/manual/using_macros.html:
> (_GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS): Mention PR125548
"PIECEWISE"
> * doc/xml/manual/using.xml: Regenerate
> * include/bits/random.tcc
> (piecewise_linear_distribution::param_type::param_type(size_t, _RealType, ...)):
> Update weight computation.
> * testsuite/26_numerics/random/piecewise_linear_distribution/operators/serialize2.cc:
> Updated expected values.
> * testsuite/26_numerics/random/piecewise_linear_distribution/cons/pr125548.cc:
> New test.
> * testsuite/26_numerics/random/piecewise_linear_distribution/cons/pr125548_fallback.cc:
> Run pr125548.cc with _GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS.
"PIECEWISE"
OK with those typos fixed in the commit log (the patch itself is
fine). This has to wait for the densities() return type patch to be
committed though.
>---
>
>Testing on x86_64-linux. The *piecewise* test already passed.
>
> .../doc/html/manual/using_macros.html | 5 +--
> libstdc++-v3/doc/xml/manual/using.xml | 4 ++-
> libstdc++-v3/include/bits/random.tcc | 15 +++++++--
> .../cons/pr125548.cc | 33 +++++++++++++++++++
> .../cons/pr125548_fallback.cc | 5 +++
> .../operators/serialize2.cc | 6 ++--
> 6 files changed, 59 insertions(+), 9 deletions(-)
> create mode 100644 libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/cons/pr125548.cc
> create mode 100644 libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/cons/pr125548_fallback.cc
>
>diff --git a/libstdc++-v3/doc/html/manual/using_macros.html b/libstdc++-v3/doc/html/manual/using_macros.html
>index 4be2ab576dc..2f54bf259fd 100644
>--- a/libstdc++-v3/doc/html/manual/using_macros.html
>+++ b/libstdc++-v3/doc/html/manual/using_macros.html
>@@ -156,7 +156,7 @@
> <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>
>+ </p></dd><dt><span class="term"><code class="code">_GLIBCXX_USE_OLD_PIECEWISE_DISTRIBUTIONS</code></span></dt><dd><p>
> Undefined by default.
> When defined, the behaviour specializations of
> <code class="code">piecewise_constant_distribution</code> and
>@@ -164,7 +164,8 @@
> 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.
>+ LWG1439</a>), and <a class="link" href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125548" target="_top">
>+ PR125548</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
>diff --git a/libstdc++-v3/doc/xml/manual/using.xml b/libstdc++-v3/doc/xml/manual/using.xml
>index 71fa23c24ee..1d4b32de30c 100644
>--- a/libstdc++-v3/doc/xml/manual/using.xml
>+++ b/libstdc++-v3/doc/xml/manual/using.xml
>@@ -1391,7 +1391,9 @@ g++ -Winvalid-pch -I. -include stdc++.h -H -g -O2 hello.cc -o test.exe
> 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.
>+ LWG1439</link>), and <link xmlns:xlink="http://www.w3.org/1999/xlink"
>+ xlink:href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125548">
>+ PR125548</link>) in GCC-17.
> </para>
> </listitem></varlistentry>
> <varlistentry><term><code>_GLIBCXX_USE_RESULT_TYPE_FOR_PIECEWISE_DENSITIES</code></term>
>diff --git a/libstdc++-v3/include/bits/random.tcc b/libstdc++-v3/include/bits/random.tcc
>index 6c88d68d3e4..6e1ad1ef31d 100644
>--- a/libstdc++-v3/include/bits/random.tcc
>+++ b/libstdc++-v3/include/bits/random.tcc
>@@ -3355,12 +3355,21 @@ namespace __detail
> {
> const size_t __n = __nw == 0 ? 1 : __nw;
> const _RealType __delta = (__xmax - __xmin) / __n;
>+ const auto __cfw = [&] (_RealType __v)
>+ {
>+#ifdef _GLIBCXX_USE_OLD_PIECEWISE_DISTRIBUTIONS
>+ return __fw(__v + __delta);
>+#else
>+ return __fw(__v);
>+#endif
>+ };
>+
> if (__n == 1)
> {
> _RealType __ints[2] = { __xmin, __xmin + __delta };
> _RealType __dens[2];
>- __dens[0] = __fw(__ints[0]);
>- __dens[1] = __fw(__ints[1]);
>+ __dens[0] = __cfw(__ints[0]);
>+ __dens[1] = __cfw(__ints[1]);
> _M_initialize2(__ints, __dens);
> return;
> }
>@@ -3370,7 +3379,7 @@ namespace __detail
> for (size_t __k = 0; __k <= __nw; ++__k)
> {
> _M_int.push_back(__xmin + __k * __delta);
>- _M_den.push_back(__fw(_M_int[__k] + __delta));
>+ _M_den.push_back(__cfw(_M_int[__k]));
> }
>
> _M_configure();
>diff --git a/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/cons/pr125548.cc b/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/cons/pr125548.cc
>new file mode 100644
>index 00000000000..2d7ae555eaa
>--- /dev/null
>+++ b/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/cons/pr125548.cc
>@@ -0,0 +1,33 @@
>+// { dg-do run { target c++11 } }
>+// { dg-require-cstdint "" }
>+
>+#include <random>
>+#include <testsuite_hooks.h>
>+
>+void
>+test_functor(int n)
>+{
>+ const double step = 1.0/n;
>+#ifdef _GLIBCXX_USE_OLD_PIECEWISE_DISTRIBUTIONS
>+ double expected = step;
>+#else
>+ double expected = 0.0;
>+#endif
>+
>+ auto check_val = [&] (double value) mutable {
>+ VERIFY( value == expected );
>+ expected += step;
>+ return value;
>+ };
>+
>+ std::piecewise_linear_distribution<> d(n, 0.0, 1.0, check_val);
>+}
>+
>+int
>+main()
>+{
>+ test_functor(1);
>+ test_functor(2);
>+ test_functor(4);
>+ test_functor(8);
>+}
>diff --git a/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/cons/pr125548_fallback.cc b/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/cons/pr125548_fallback.cc
>new file mode 100644
>index 00000000000..4b8e6658570
>--- /dev/null
>+++ b/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/cons/pr125548_fallback.cc
>@@ -0,0 +1,5 @@
>+// { dg-options "-D_GLIBCXX_USE_OLD_PIECEWISE_DISTRIBUTIONS" }
>+// { dg-do run { target c++11 } }
>+// { dg-require-cstdint "" }
>+#include "pr125548.cc"
>+
>diff --git a/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/operators/serialize2.cc b/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/operators/serialize2.cc
>index e8ab174eb70..21365057f9e 100644
>--- a/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/operators/serialize2.cc
>+++ b/libstdc++-v3/testsuite/26_numerics/random/piecewise_linear_distribution/operators/serialize2.cc
>@@ -77,19 +77,19 @@ test_custom()
> case 24: // ieee32
> expected =
> "3 0.000000000e+00 3.333333433e-01 6.666666865e-01 1.000000000e+00"
>- " 7.272727292e-01 9.090909278e-01 1.090909061e+00 1.272727325e+00";
>+ " 6.666666534e-01 8.888888977e-01 1.111111142e+00 1.333333307e+00";
> VERIFY( res == expected );
> break;
> case 53: // ieee64
> expected =
> "3 0.00000000000000000e+00 3.33333333333333315e-01 6.66666666666666630e-01 1.00000000000000000e+00"
>- " 7.27272727272727182e-01 9.09090909090908950e-01 1.09090909090909083e+00 1.27272727272727249e+00";
>+ " 6.66666666666666630e-01 8.88888888888888840e-01 1.11111111111111094e+00 1.33333333333333326e+00";
> VERIFY( res == expected );
> break;
> case 64: // ieee80
> expected =
> "3 0.000000000000000000000e+00 3.333333333333333333424e-01 6.666666666666666666847e-01 1.000000000000000000000e+00"
>- " 7.272727272727271818908e-01 9.090909090909090606303e-01 1.090909090909090828347e+00 1.272727272727272707087e+00";
>+ " 6.666666666666666296592e-01 8.888888888888888395456e-01 1.111111111111111160454e+00 1.333333333333333259318e+00";
> VERIFY( res == expected );
> break;
> default:
>--
>2.54.0
>
>
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