{"id":2226669,"url":"http://patchwork.ozlabs.org/api/patches/2226669/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/patch/bmm.hhuo2dtkd4.gcc.gcc-TEST.peppe.44.1.2@forge-stage.sourceware.org/","project":{"id":17,"url":"http://patchwork.ozlabs.org/api/projects/17/?format=json","name":"GNU Compiler Collection","link_name":"gcc","list_id":"gcc-patches.gcc.gnu.org","list_email":"gcc-patches@gcc.gnu.org","web_url":null,"scm_url":null,"webscm_url":null,"list_archive_url":"","list_archive_url_format":"","commit_url_format":""},"msgid":"","list_archive_url":null,"date":"2026-04-22T18:10:42","name":"[v1,2/2] libstdc++: add constexpr stable_partition","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"7994b774cdcc3eba868905b59968fa613440abf9","submitter":{"id":93222,"url":"http://patchwork.ozlabs.org/api/people/93222/?format=json","name":"peppe via Sourceware Forge","email":"forge-bot+peppe@forge-stage.sourceware.org"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/gcc/patch/bmm.hhuo2dtkd4.gcc.gcc-TEST.peppe.44.1.2@forge-stage.sourceware.org/mbox/","series":[{"id":501068,"url":"http://patchwork.ozlabs.org/api/series/501068/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/list/?series=501068","date":"2026-04-22T18:10:42","name":"WIP: more_constexpr_stable_algorithms","version":1,"mbox":"http://patchwork.ozlabs.org/series/501068/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2226669/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2226669/checks/","tags":{},"related":[],"headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=38.145.34.32; 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server2.sourceware.org","From":"peppe via Sourceware Forge ","Date":"Wed, 22 Apr 2026 18:10:42 +0000","Subject":"[PATCH v1 2/2] libstdc++: add constexpr stable_partition","To":"gcc-patches mailing list ","Message-ID":"\n ","X-Mailer":"batrachomyomachia","X-Requested-Reviewer":"redi","X-Pull-Request-Organization":"gcc","X-Pull-Request-Repository":"gcc-TEST","X-Pull-Request":"https://forge.sourceware.org/gcc/gcc-TEST/pulls/44","References":"\n ","In-Reply-To":"\n ","X-Patch-URL":"\n https://forge.sourceware.org/peppe/gcc-TEST/commit/bfb9835a6472095f7e80eb19c7074686efd4bc96","X-BeenThere":"gcc-patches@gcc.gnu.org","X-Mailman-Version":"2.1.30","Precedence":"list","List-Id":"Gcc-patches mailing list ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Reply-To":"gcc-patches mailing list ,\n peppe@noreply.localhost","Errors-To":"gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org"},"content":"From: Giuseppe D'Angelo \n\nThis completes the implementation of P2562R1 for C++26.\n\nUnlike the other constexpr algorithms of the same family,\nstable_partition does not have a constexpr-friendly version \"ready to\nuse\" during constant evaluation. In fact, it is not even available on\nfreestanding, because it always allocates a temporary memory buffer.\n\nThis commit implements the simplest possible strategy: during constant\nevaluation allocate a buffer of length 1 on the stack, and use that as\na working area.\n\nlibstdc++-v3/ChangeLog:\n\n\t* include/bits/algorithmfwd.h (stable_partition): Mark it\n\tas constexpr for C++26.\n\t* include/bits/ranges_algo.h (__stable_partition_fn): Likewise.\n\t* include/bits/stl_algo.h (stable_partition): Mark it as\n\tconstexpr for C++26; during constant evaluation use a new\n\tcodepath where a temporary buffer of 1 element is used.\n\t* testsuite/25_algorithms/headers/algorithm/synopsis.cc\n\t(stable_partition): Add constexpr.\n\t* testsuite/25_algorithms/stable_partition/constexpr.cc: New test.\n---\n libstdc++-v3/include/bits/algorithmfwd.h | 1 +\n libstdc++-v3/include/bits/ranges_algo.h | 2 +\n libstdc++-v3/include/bits/stl_algo.h | 21 ++++++++-\n .../headers/algorithm/synopsis.cc | 1 +\n .../stable_partition/constexpr.cc | 44 +++++++++++++++++++\n 5 files changed, 67 insertions(+), 2 deletions(-)\n create mode 100644 libstdc++-v3/testsuite/25_algorithms/stable_partition/constexpr.cc","diff":"diff --git a/libstdc++-v3/include/bits/algorithmfwd.h b/libstdc++-v3/include/bits/algorithmfwd.h\nindex 05894b580028..a727b2783814 100644\n--- a/libstdc++-v3/include/bits/algorithmfwd.h\n+++ b/libstdc++-v3/include/bits/algorithmfwd.h\n@@ -649,6 +649,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)\n \n #if _GLIBCXX_HOSTED\n template\n+ _GLIBCXX26_CONSTEXPR\n _BIter\n stable_partition(_BIter, _BIter, _Predicate);\n #endif\ndiff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h\nindex 2814d90061cc..f36e7dd59911 100644\n--- a/libstdc++-v3/include/bits/ranges_algo.h\n+++ b/libstdc++-v3/include/bits/ranges_algo.h\n@@ -2389,6 +2389,7 @@ namespace ranges\n \t typename _Proj = identity,\n \t indirect_unary_predicate> _Pred>\n requires permutable<_Iter>\n+ _GLIBCXX26_CONSTEXPR\n subrange<_Iter>\n operator()(_Iter __first, _Sent __last,\n \t\t _Pred __pred, _Proj __proj = {}) const\n@@ -2404,6 +2405,7 @@ namespace ranges\n \t indirect_unary_predicate, _Proj>>\n \t _Pred>\n requires permutable>\n+ _GLIBCXX26_CONSTEXPR\n borrowed_subrange_t<_Range>\n operator()(_Range&& __r, _Pred __pred, _Proj __proj = {}) const\n {\ndiff --git a/libstdc++-v3/include/bits/stl_algo.h b/libstdc++-v3/include/bits/stl_algo.h\nindex bb7dbfbd8e04..71ead103d2bf 100644\n--- a/libstdc++-v3/include/bits/stl_algo.h\n+++ b/libstdc++-v3/include/bits/stl_algo.h\n@@ -1447,6 +1447,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)\n /// move-assign an element onto itself.\n template\n+ _GLIBCXX26_CONSTEXPR\n _ForwardIterator\n __stable_partition_adaptive(_ForwardIterator __first,\n \t\t\t\t_ForwardIterator __last,\n@@ -1507,6 +1508,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)\n }\n \n template\n+ _GLIBCXX26_CONSTEXPR\n _ForwardIterator\n __stable_partition(_ForwardIterator __first, _ForwardIterator __last,\n \t\t _Predicate __pred)\n@@ -1521,11 +1523,25 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)\n typedef typename iterator_traits<_ForwardIterator>::difference_type\n \t_DistanceType;\n \n+ const _DistanceType __len = std::distance(__first, __last);\n+\n+#if __glibcxx_constexpr_algorithms >= 202306L // >= C++26\n+ if consteval {\n+\t// Simulate a _Temporary_buffer of length 1:\n+\t_ValueType __buf = std::move(*__first);\n+\t*__first = std::move(__buf);\n+\treturn std::__stable_partition_adaptive(__first, __last, __pred,\n+\t\t\t\t\t\t__len,\n+\t\t\t\t\t\t&__buf,\n+\t\t\t\t\t\t_DistanceType(1));\n+ }\n+#endif\n+\n _Temporary_buffer<_ForwardIterator, _ValueType>\n-\t__buf(__first, std::distance(__first, __last));\n+\t__buf(__first, __len);\n return\n \tstd::__stable_partition_adaptive(__first, __last, __pred,\n-\t\t\t\t\t _DistanceType(__buf.requested_size()),\n+\t\t\t\t\t __len,\n \t\t\t\t\t __buf.begin(),\n \t\t\t\t\t _DistanceType(__buf.size()));\n }\n@@ -1548,6 +1564,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)\n * relative ordering after calling @p stable_partition().\n */\n template\n+ _GLIBCXX26_CONSTEXPR\n inline _ForwardIterator\n stable_partition(_ForwardIterator __first, _ForwardIterator __last,\n \t\t _Predicate __pred)\ndiff --git a/libstdc++-v3/testsuite/25_algorithms/headers/algorithm/synopsis.cc b/libstdc++-v3/testsuite/25_algorithms/headers/algorithm/synopsis.cc\nindex 8d5c1fb7ac73..072dd0781046 100644\n--- a/libstdc++-v3/testsuite/25_algorithms/headers/algorithm/synopsis.cc\n+++ b/libstdc++-v3/testsuite/25_algorithms/headers/algorithm/synopsis.cc\n@@ -349,6 +349,7 @@ namespace std\n partition(_BIter, _BIter, _Predicate);\n \n template\n+ _GLIBCXX26_CONSTEXPR\n _BIter\n stable_partition(_BIter, _BIter, _Predicate);\n \ndiff --git a/libstdc++-v3/testsuite/25_algorithms/stable_partition/constexpr.cc b/libstdc++-v3/testsuite/25_algorithms/stable_partition/constexpr.cc\nnew file mode 100644\nindex 000000000000..8decc93eb827\n--- /dev/null\n+++ b/libstdc++-v3/testsuite/25_algorithms/stable_partition/constexpr.cc\n@@ -0,0 +1,44 @@\n+// { dg-do compile { target c++26 } }\n+\n+#include \n+#include \n+\n+constexpr auto\n+create_array()\n+{\n+ return std::to_array({0, 10, 1, 2, 3, 3, 4, -1, -2, -4, 5, 6});\n+}\n+\n+constexpr bool\n+test01()\n+{\n+ auto ar = create_array();\n+ auto pred = [](int i) { return i % 2 == 0; };\n+ std::stable_partition(ar.begin(), ar.end(), pred);\n+ return std::is_partitioned(ar.begin(), ar.end(), pred);\n+}\n+\n+static_assert(test01());\n+\n+constexpr bool\n+test02()\n+{\n+ auto ar = create_array();\n+ auto pred = [](int i) { return i % 2 == 0; };\n+ std::ranges::stable_partition(ar, pred);\n+ return std::ranges::is_partitioned(ar, pred);\n+}\n+\n+static_assert(test02());\n+\n+constexpr bool\n+test03()\n+{\n+ auto ar = create_array();\n+ auto pred = [](int i) { return i % 2 == 0; };\n+ auto proj = [](int i) { return i + 1; };\n+ std::ranges::stable_partition(ar, pred, proj);\n+ return std::ranges::is_partitioned(ar, pred, proj);\n+}\n+\n+static_assert(test03());\n","prefixes":["v1","2/2"]}