[libcxx-commits] [libcxx] [libc++] Optimize std::min_element (PR #100616)

[Nikolas Klauser via libcxx-commits]

================
@@ -27,20 +28,115 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+template <class _Iter, class _Sent, class = void>
+struct __ConstTimeDistance : false_type {};
+
+#if _LIBCPP_STD_VER >= 20
+
+template <class _Iter, class _Sent>
+struct __ConstTimeDistance< _Iter, _Sent, __enable_if_t< sized_sentinel_for<_Sent, _Iter> >> : true_type {};
+
+#else
+
+template <class _Iter>
+struct __ConstTimeDistance<
+    _Iter,
+    _Iter,
+    __enable_if_t< is_same<typename iterator_traits<_Iter>::iterator_category, random_access_iterator_tag>::value> >
+    : true_type {};
+
+#endif // _LIBCPP_STD_VER >= 20
+
 template <class _Comp, class _Iter, class _Sent, class _Proj>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Iter
-__min_element(_Iter __first, _Sent __last, _Comp& __comp, _Proj& __proj) {
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Iter __min_element(
+    _Iter __first,
+    _Sent __last,
+    _Comp __comp,
+    _Proj& __proj,
+    /*_ConstTimeDistance*/ false_type) {
   if (__first == __last)
     return __first;
 
   _Iter __i = __first;
   while (++__i != __last)
     if (std::__invoke(__comp, std::__invoke(__proj, *__i), std::__invoke(__proj, *__first)))
       __first = __i;
-
   return __first;
 }
 
+template <class _Comp, class _Iter, class _Sent, class _Proj>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Iter __min_element(
+    _Iter __first,
+    _Sent __last,
+    _Comp __comp,
+    _Proj& __proj,
+    /*ConstTimeDistance*/ true_type) {
+  if (__first == __last)
+    return __first;
+
+  typedef typename std::iterator_traits<_Iter>::difference_type diff_type;
+  diff_type __n = std::distance(__first, __last);
+
+  if (__n <= 64) {
+    _Iter __i = __first;
+    while (++__i != __last)
+      if (std::__invoke(__comp, std::__invoke(__proj, *__i), std::__invoke(__proj, *__first)))
+        __first = __i;
+    return __first;
+  }
+
+  diff_type __block_size = 256;
+
+  diff_type __n_blocked = __n - (__n % __block_size);
+  _Iter __block_start = __first, __block_end = __first;
----------------
philnik777 wrote:

```suggestion
  diff_type __n_blocked = __n - (__n % __block_size);
  _Iter __block_start = __first;
  _Iter __block_end = __first;
```

https://github.com/llvm/llvm-project/pull/100616