https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107466
--- Comment #10 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jonathan Wakely <r...@gcc.gnu.org>: https://gcc.gnu.org/g:c224dec0e7c88e7a95633023018cdcb6ee87c65f commit r14-7216-gc224dec0e7c88e7a95633023018cdcb6ee87c65f Author: Jonathan Wakely <jwak...@redhat.com> Date: Thu Jan 11 15:09:12 2024 +0000 libstdc++: Fix non-portable results from 64-bit std::subtract_with_carry_engine [PR107466] I implemented the resolution of LWG 3809 in r13-4364-ga64775a0edd469 but it was recently noted in the MSVC STL github repo that the change causes possible truncation for 64-bit seeds. Whether the truncation occurs (and to what value) depends on the width of uint_least32_t which is not portable, so the output of the PRNG for 64-bit seed values is no longer the same as in C++20, and no longer portable across platforms. That new issue was filed as LWG 4014. I proposed a new change which reduces the seed by the LCG's modulus before the conversion to uint_least32_t. This ensures that 64-bit seed values are consistently reduced by the modulus before any truncation. This removes the platform-dependent behaviour and restores the old behaviour for std::subtract_with_carry_engine specializations using a 64-bit result type (such as std::ranlux48_base). libstdc++-v3/ChangeLog: PR libstdc++/107466 * include/bits/random.tcc (subtract_with_carry_engine::seed): Implement proposed resolution of LWG 4014. * testsuite/26_numerics/random/pr60037-neg.cc: Adjust dg-error line number. * testsuite/26_numerics/random/subtract_with_carry_engine/cons/lwg3809.cc: Check for expected result of 64-bit engine with seed that doesn't fit in 32-bits.
