[Bug libstdc++/107466] [12 Regression] invalid -Wnarrowing error with std::subtract_with_carry_engine
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107466 Jonathan Wakely changed: What|Removed |Added Status|REOPENED|RESOLVED Resolution|--- |FIXED --- Comment #13 from Jonathan Wakely --- And fixed for real now, for 12.4, 13.3 and trunk.
[Bug libstdc++/107466] [12 Regression] invalid -Wnarrowing error with std::subtract_with_carry_engine
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107466 --- Comment #12 from GCC Commits --- The releases/gcc-12 branch has been updated by Jonathan Wakely : https://gcc.gnu.org/g:f2af87b9705d5a7e37b65bf342146ff25f025e49 commit r12-10143-gf2af87b9705d5a7e37b65bf342146ff25f025e49 Author: Jonathan Wakely Date: Thu Jan 11 15:09:12 2024 + 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. (cherry picked from commit c224dec0e7c88e7a95633023018cdcb6ee87c65f)
[Bug libstdc++/107466] [12 Regression] invalid -Wnarrowing error with std::subtract_with_carry_engine
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107466 --- Comment #11 from GCC Commits --- The releases/gcc-13 branch has been updated by Jonathan Wakely : https://gcc.gnu.org/g:3bdd80d56aa07d5975f551e0026f3cf9411124bf commit r13-8306-g3bdd80d56aa07d5975f551e0026f3cf9411124bf Author: Jonathan Wakely Date: Thu Jan 11 15:09:12 2024 + 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. (cherry picked from commit c224dec0e7c88e7a95633023018cdcb6ee87c65f)
[Bug libstdc++/107466] [12 Regression] invalid -Wnarrowing error with std::subtract_with_carry_engine
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107466 --- Comment #10 from GCC Commits --- The master branch has been updated by Jonathan Wakely : https://gcc.gnu.org/g:c224dec0e7c88e7a95633023018cdcb6ee87c65f commit r14-7216-gc224dec0e7c88e7a95633023018cdcb6ee87c65f Author: Jonathan Wakely Date: Thu Jan 11 15:09:12 2024 + 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.
[Bug libstdc++/107466] [12 Regression] invalid -Wnarrowing error with std::subtract_with_carry_engine
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107466
Jonathan Wakely changed:
What|Removed |Added
Status|RESOLVED|REOPENED
Resolution|FIXED |---
--- Comment #9 from Jonathan Wakely ---
I'm reopening this because the fix changes the behaviour of some code:
https://cplusplus.github.io/LWG/issue4014
As noted there, std::ranlux48_base(UINT_MAX+1LL)() changed behaviour.
Originally that yielded 22575453646312 but after the commits above it yields
23223501020940
This is what I proposed for 4014 and restores the original behaviour:
--- a/libstdc++-v3/include/bits/random.tcc
+++ b/libstdc++-v3/include/bits/random.tcc
@@ -541,8 +541,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
subtract_with_carry_engine<_UIntType, __w, __s, __r>::
seed(result_type __value)
{
+ // _GLIBCXX_RESOLVE_LIB_DEFECTS
+ // 3809. Is std::subtract_with_carry_engine supposed to work?
+ // 4014. LWG 3809 changes behavior of some existing code
+ if (__value == 0u)
+ __value = default_seed;
+ else
+ __value %= 2147483563u;
+
std::linear_congruential_engine
- __lcg(__value == 0u ? default_seed : __value);
+ __lcg(__value);
const size_t __n = (__w + 31) / 32;
[Bug libstdc++/107466] [12 Regression] invalid -Wnarrowing error with std::subtract_with_carry_engine
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107466 Jonathan Wakely changed: What|Removed |Added Resolution|--- |FIXED Status|ASSIGNED|RESOLVED --- Comment #8 from Jonathan Wakely --- Fixed for 12.3
[Bug libstdc++/107466] [12 Regression] invalid -Wnarrowing error with std::subtract_with_carry_engine
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107466 --- Comment #7 from CVS Commits --- The releases/gcc-12 branch has been updated by Jonathan Wakely : https://gcc.gnu.org/g:d8def0b55572fc5bc02a0fee6a98787cd50ee885 commit r12-9485-gd8def0b55572fc5bc02a0fee6a98787cd50ee885 Author: Jonathan Wakely Date: Mon Nov 28 09:44:52 2022 + libstdc++: Make 16-bit std::subtract_with_carry_engine work [PR107466] This implements the proposed resolution of LWG 3809, so that std::subtract_with_carry_engine can be used with a 16-bit result_type. Currently this produces a narrowing error when instantiating the std::linear_congruential_engine to create the initial state. It also truncates the default_seed constant when passing it as a result_type argument. Change the type of the constant to uint_least32_t and pass 0u when the default_seed should be used. libstdc++-v3/ChangeLog: PR libstdc++/107466 * include/bits/random.h (subtract_with_carry_engine): Use 32-bit type for default seed. Use 0u as default argument for subtract_with_carry_engine(result_type) constructor and seed(result_type) member function. * include/bits/random.tcc (subtract_with_carry_engine): Use 32-bit type for default seed and engine used for initial state. * testsuite/26_numerics/random/subtract_with_carry_engine/cons/lwg3809.cc: New test. (cherry picked from commit a64775a0edd46980036b757041f0c065ed9f8d22)
[Bug libstdc++/107466] [12 Regression] invalid -Wnarrowing error with std::subtract_with_carry_engine
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107466 Richard Biener changed: What|Removed |Added Priority|P3 |P2
[Bug libstdc++/107466] [12 Regression] invalid -Wnarrowing error with std::subtract_with_carry_engine
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107466 Richard Biener changed: What|Removed |Added Assignee|unassigned at gcc dot gnu.org |redi at gcc dot gnu.org Status|NEW |ASSIGNED
