[committed] libstdc++: Use unsigned division in std::rotate [PR113811]

[committed] libstdc++: Use unsigned division in std::rotate [PR113811]

[Jonathan Wakely]

Tested aarch64-linux and x86_64-linux. Pushed to trunk.

-- >8 --

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.
---
 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 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);
-- 
2.43.0