[Bug libstdc++/113811] std::rotate does 64-bit signed division

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113811

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
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<unsigned> to use a smaller type than
make_unsigned<_Distance> if max(n,k) < UINT_MAX.