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From: Jonathan Wakely <redi@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r14-9001] libstdc++: Use unsigned division in std::rotate [PR113811] Date: Thu, 15 Feb 2024 11:44:23 +0000 (GMT) [thread overview] Message-ID: <20240215114423.211A2384DB45@sourceware.org> (raw) https://gcc.gnu.org/g:4d819db7f229a23cb15ef68f310e0bb51d201c45 commit r14-9001-g4d819db7f229a23cb15ef68f310e0bb51d201c45 Author: Jonathan Wakely <jwakely@redhat.com> Date: Thu Feb 8 15:40:32 2024 +0000 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: --- libstdc++-v3/include/bits/stl_algo.h | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/libstdc++-v3/include/bits/stl_algo.h b/libstdc++-v3/include/bits/stl_algo.h index 9496b53f8871..7a0cf6b67370 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);
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