[PATCH] libstdc++: Fix weight computation for piecewise_linear_distribution(nw, xmin, xmax, fw) constructor [PR125548]
Tomasz Kamiński
tkaminsk@redhat.com
Mon Jun 1 13:48:26 GMT 2026
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.
PR libstdc++/125548
libstdc++-v3/ChangeLog:
* doc/html/manual/using_macros.html:
(_GLIBCXX_USE_OLD_PICEWISE_DISTRIBUTIONS): Mention PR125548
* 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.
---
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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