[{"id":3677252,"web_url":"http://patchwork.ozlabs.org/comment/3677252/","msgid":"","list_archive_url":null,"date":"2026-04-14T15:10:03","subject":"Re: [PATCH] libstdc++: Fix philox_engine counter increment carry","submitter":{"id":48004,"url":"http://patchwork.ozlabs.org/api/people/48004/","name":"Jonathan Wakely","email":"jwakely@redhat.com"},"content":"On Tue, 14 Apr 2026 at 15:04 +0000, Tyuleneva, Elena wrote:\n>Hello!\n>\n>The philox counter increment that happens in philox_engine::_M_transition(),\n>applies +1 to _M_x[0] before the bitwise OR with (_M_x[1] << __w), so the carry\n>from _M_x[0] is merged into the _M_x[1] portion rather\n>than being added to it. This causes the counter to cycle prematurely\n>and never advance past {0, 0, 1, 2^w - 1}.\n>\n>Fix by changing operations order, which results in forming the full-width value\n>from _M_x[1]:_M_x[0] and then adding 1 on both the n == 4 and n == 2 paths.\n\nThanks for the patch. I'll take a look at it tomorrow, but for now\nI've CC'd the libstdc++ mailing list which should be included for\nlibstdc++ patches.\n\n\n>libstdc++-v3/ChangeLog:\n>\n> * include/bits/random.tcc (philox_engine::operator()): Fix counter\n> increment to propagate carry correctly.\n> * testsuite/26_numerics/random/philox_engine/operators/counter_carry.cc:\n> New test.\n>\n>Signed-off-by: ElenaTyuleneva elena.tyuleneva@intel.com\n>---\n>libstdc++-v3/include/bits/random.tcc | 8 +-\n>.../philox_engine/operators/counter_carry.cc | 83 +++++++++++++++++++\n>2 files changed, 87 insertions(+), 4 deletions(-)\n>create mode 100644 libstdc++-v3/testsuite/26_numerics/random/philox_engine/operators/counter_carry.cc\n>\n>diff --git a/libstdc++-v3/include/bits/random.tcc b/libstdc++-v3/include/bits/random.tcc\n>index 7a63e868a89..3380b63f355 100644\n>--- a/libstdc++-v3/include/bits/random.tcc\n>+++ b/libstdc++-v3/include/bits/random.tcc\n>@@ -944,8 +944,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n> if constexpr (__n == 4)\n> {\n> __type __uh\n>- = (static_cast<__type>(_M_x[1]) << __w)\n>- | (static_cast<__type>(_M_x[0]) + 1);\n>+ = ((static_cast<__type>(_M_x[1]) << __w)\n>+ | static_cast<__type>(_M_x[0])) + 1;\n> __type __lh\n> = (static_cast<__type>(_M_x[3]) << __w)\n> | static_cast<__type>(_M_x[2]);\n>@@ -964,8 +964,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n> else\n> {\n> __type __num =\n>- (static_cast<__type>(_M_x[1]) << __w)\n>- | (static_cast<__type>(_M_x[0]) + 1);\n>+ ((static_cast<__type>(_M_x[1]) << __w)\n>+ | static_cast<__type>(_M_x[0])) + 1;\n> _M_x[0] = __num & max();\n> _M_x[1] = (__num >> __w) & max();\n> }\n>diff --git a/libstdc++-v3/testsuite/26_numerics/random/philox_engine/operators/counter_carry.cc b/libstdc++-v3/testsuite/26_numerics/random/philox_engine/operators/counter_carry.cc\n>new file mode 100644\n>index 00000000000..b1527f6a67b\n>--- /dev/null\n>+++ b/libstdc++-v3/testsuite/26_numerics/random/philox_engine/operators/counter_carry.cc\n>@@ -0,0 +1,83 @@\n>+// { dg-do run { target c++26 } }\n>+\n>+// N5014 29.5.4.5 Class Template philox_engine\n>+\n>+#include \n>+#include \n>+\n>+#include \n>+\n>+// Helper: set counter, skip one block (n outputs), then compare\n>+// the next block against a reference engine at the expected counter.\n>+template\n>+void\n>+verify_carry(const std::array& current_counter,\n>+ const std::array& expected_incremented_counter)\n>+{\n>+ Engine eng_a;\n>+ eng_a.set_counter(current_counter);\n>+ // Skip the block produced from current_counter\n>+ eng_a.discard(N);\n>+\n>+ Engine eng_b;\n>+ eng_b.set_counter(expected_incremented_counter);\n>+\n>+ // Both engines should now produce identical output\n>+ for (std::size_t i = 0; i < N; ++i)\n>+ VERIFY( eng_a() == eng_b() );\n>+}\n>+\n>+\n>+int main()\n>+{\n>+ // philox4x32 (n=4, w=32)\n>+ verify_carry(std::array{0, 0, 0, 5},\n>+ std::array{0, 0, 0, 6});\n>+ verify_carry(std::array{0, 0, 0, 0xFFFFFFFF},\n>+ std::array{0, 0, 1, 0});\n>+ verify_carry(std::array{0, 0, 1, 0xFFFFFFFF},\n>+ std::array{0, 0, 2, 0});\n>+ verify_carry(std::array{0, 0, 0xFFFFFFFF, 0xFFFFFFFF},\n>+ std::array{0, 1, 0, 0});\n>+ verify_carry(std::array{0, 1, 0xFFFFFFFF, 0xFFFFFFFF},\n>+ std::array{0, 2, 0, 0});\n>+ verify_carry(std::array{0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF},\n>+ std::array{0, 0, 0, 0});\n>+\n>+ // philox4x64 (n=4, w=64)\n>+ constexpr auto philox4x64_max = std::philox4x64::max();\n>+ verify_carry(std::array{0, 0, 0, philox4x64_max},\n>+ std::array{0, 0, 1, 0});\n>+ verify_carry(std::array{0, 0, 1, philox4x64_max},\n>+ std::array{0, 0, 2, 0});\n>+ verify_carry(std::array{0, 0, philox4x64_max, philox4x64_max},\n>+ std::array{0, 1, 0, 0});\n>+ verify_carry(std::array{0, 1, philox4x64_max, philox4x64_max},\n>+ std::array{0, 2, 0, 0});\n>+ verify_carry(std::array{philox4x64_max, philox4x64_max, philox4x64_max, philox4x64_max},\n>+ std::array{0, 0, 0, 0});\n>+\n>+ // philox2x32 (n=2, w=32)\n>+ using philox2x32 = std::philox_engine+ 32, 2, 10, 0xCD9E8D57, 0x9E3779B9>;\n>+ verify_carry(std::array{0, 5},\n>+ std::array{0, 6});\n>+ verify_carry(std::array{0, 0xFFFFFFFF},\n>+ std::array{1, 0});\n>+ verify_carry(std::array{1, 0xFFFFFFFF},\n>+ std::array{2, 0});\n>+ verify_carry(std::array{0xFFFFFFFF, 0xFFFFFFFF},\n>+ std::array{0, 0});\n>+ // philox2x64 (n=2, w=64)\n>+ using philox2x64 = std::philox_engine+ 64, 2, 10, 0xCD9E8D57, 0x9E3779B9>;\n>+ constexpr auto philox2x64_max = philox2x64::max();\n>+ verify_carry(std::array{0, 5},\n>+ std::array{0, 6});\n>+ verify_carry(std::array{0, philox2x64_max},\n>+ std::array{1, 0});\n>+ verify_carry(std::array{1, philox2x64_max},\n>+ std::array{2, 0});\n>+ verify_carry(std::array{philox2x64_max, philox2x64_max},\n>+ std::array{0, 0});\n>+}\n>--\n>2.40.1\n>\n>The changes were tested on x86_64-pc-linux-gnu.\n>\n>Best regards,\n>Elena\n>Intel Deutschland GmbH\n>Registered Address: Dornacher Strasse 1, 85622 Feldkirchen, Germany\n>Tel: +49 89 991 430, www.intel.de\n>Managing Directors: Harry Demas, Jeffrey Schneiderman, Yin Chong Sorrell\n>Chairperson of the Supervisory Board: Nicole Lau\n>Registered Seat: Munich\n>Commercial Register: Amtsgericht Muenchen HRB 186928","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\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=iMIJxDGW;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=2620:52:6:3111::32; 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