[libcxx-commits] [libcxx] [libc++] Optimize rotate (PR #120890)

[Louis Dionne via libcxx-commits]

================
@@ -89,46 +86,39 @@ __rotate_forward(_ForwardIterator __first, _ForwardIterator __middle, _ForwardIt
   return __r;
 }
 
-template <typename _Integral>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 _Integral __algo_gcd(_Integral __x, _Integral __y) {
-  do {
-    _Integral __t = __x % __y;
-    __x           = __y;
-    __y           = __t;
-  } while (__y);
-  return __x;
-}
+template <class _AlgPolicy, class _Iter, class _Sent>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 _Iter __rotate_random(_Iter __first, _Iter __middle, _Sent __last) {
+  auto __left  = _IterOps<_AlgPolicy>::distance(__first, __middle);
+  auto __right = _IterOps<_AlgPolicy>::distance(__middle, __last);
+  auto __end   = __first + __right;
 
-template <class _AlgPolicy, typename _RandomAccessIterator>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 _RandomAccessIterator
-__rotate_gcd(_RandomAccessIterator __first, _RandomAccessIterator __middle, _RandomAccessIterator __last) {
-  typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
-  typedef typename iterator_traits<_RandomAccessIterator>::value_type value_type;
-  using _Ops = _IterOps<_AlgPolicy>;
+  if (__left == 0 || __first == __last)
+    return __first;
 
-  const difference_type __m1 = __middle - __first;
-  const difference_type __m2 = _Ops::distance(__middle, __last);
-  if (__m1 == __m2) {
-    std::__swap_ranges<_AlgPolicy>(__first, __middle, __middle, __last);
+  if (__left == __right) {
+    std::__swap_ranges<_AlgPolicy>(__first, __middle, __middle);
----------------
ldionne wrote:

Can you file an issue to use `for_each` in `swap_ranges` for deque? That would greatly speed it up. It would also benefit the tree containers.

https://github.com/llvm/llvm-project/pull/120890