[Bug libstdc++/113811] std::rotate does 64-bit signed division
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113811 Jonathan Wakely changed: What|Removed |Added Resolution|--- |FIXED Status|NEW |RESOLVED Target Milestone|--- |14.0 --- Comment #5 from Jonathan Wakely --- Fixed for gcc-14, thanks for the suggestion.
[Bug libstdc++/113811] std::rotate does 64-bit signed division
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113811 --- Comment #4 from GCC Commits --- The master branch has been updated by Jonathan Wakely : https://gcc.gnu.org/g:4d819db7f229a23cb15ef68f310e0bb51d201c45 commit r14-9001-g4d819db7f229a23cb15ef68f310e0bb51d201c45 Author: Jonathan Wakely Date: Thu Feb 8 15:40:32 2024 + libstdc++: Use unsigned division in std::rotate [PR113811] Signed 64-bit division is much slower than unsigned, so cast the n and k values to unsigned before doing n %= k. We know this is safe because neither value can be negative. libstdc++-v3/ChangeLog: PR libstdc++/113811 * include/bits/stl_algo.h (__rotate): Use unsigned values for division.
[Bug libstdc++/113811] std::rotate does 64-bit signed division
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113811 --- Comment #3 from Jonathan Wakely --- It seems fairly easy to do: commit 12a028d76bbdf26d34d4d90a2ecdc39c6c0a4bd4 (HEAD -> master) Author: Jonathan Wakely Date: Thu Feb 8 15:40:32 2024 libstdc++: Use unsigned division in std::rotate [PR113811] Signed 64-bit division is much slower than unsigned, so cast the n and k values to unsigned before doing n %= k. We know this is safe because neither value can be negative. libstdc++-v3/ChangeLog: PR libstdc++/113811 * include/bits/stl_algo.h (__rotate): Use unsigned values for division. diff --git a/libstdc++-v3/include/bits/stl_algo.h b/libstdc++-v3/include/bits/stl_algo.h index 9496b53f887..7a0cf6b6737 100644 --- a/libstdc++-v3/include/bits/stl_algo.h +++ b/libstdc++-v3/include/bits/stl_algo.h @@ -1251,6 +1251,12 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2) typedef typename iterator_traits<_RandomAccessIterator>::value_type _ValueType; +#if __cplusplus >= 201103L + typedef typename make_unsigned<_Distance>::type _UDistance; +#else + typedef _Distance _UDistance; +#endif + _Distance __n = __last - __first; _Distance __k = __middle - __first; @@ -1281,7 +1287,7 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2) ++__p; ++__q; } - __n %= __k; + __n = static_cast<_UDistance>(__n) % static_cast<_UDistance>(__k); if (__n == 0) return __ret; std::swap(__n, __k); @@ -1305,7 +1311,7 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2) --__q; std::iter_swap(__p, __q); } - __n %= __k; + __n = static_cast<_UDistance>(__n) % static_cast<_UDistance>(__k); if (__n == 0) return __ret; std::swap(__n, __k); Conditionally using 32-bit types would be a bit trickier, as it needs runtime branches, or making the type of __n and __k a template parameter, so we can call __rotate_with to use a smaller type than make_unsigned<_Distance> if max(n,k) < UINT_MAX.
[Bug libstdc++/113811] std::rotate does 64-bit signed division
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113811 Jonathan Wakely changed: What|Removed |Added Status|UNCONFIRMED |NEW Last reconfirmed||2024-02-08 Ever confirmed|0 |1 --- Comment #2 from Jonathan Wakely --- They're never negative unless the arguments to the algorithm are incorrect, in which case maybe trapping is better than trashing the data! I was going to suggest that we could just add an assertion and/or hint that they're never negative, but I guess PR 102580 means that wouldn't help.
[Bug libstdc++/113811] std::rotate does 64-bit signed division
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113811 --- Comment #1 from Richard Biener --- In case __n is the minimum signed integer and __k is -1 the division would also trap ;) So yes, they should be unsigned.
