[libcxx-commits] [libcxx] [libc++] Simplify most of the segmented iterator optimizations (PR #164797)

[Louis Dionne via libcxx-commits]

================
@@ -27,33 +28,28 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _ForwardIterator, class _Sentinel, class _Tp>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _ForwardIterator
 __fill(_ForwardIterator __first, _Sentinel __last, const _Tp& __value) {
+#ifndef _LIBCPP_CXX03_LANG
+  if constexpr (is_same<_ForwardIterator, _Sentinel>::value && __is_segmented_iterator_v<_ForwardIterator>) {
+    using __local_iterator_t = typename __segmented_iterator_traits<_ForwardIterator>::__local_iterator;
+    std::__for_each_segment(__first, __last, [&](__local_iterator_t __lfirst, __local_iterator_t __llast) {
+      std::__fill(__lfirst, __llast, __value);
+    });
+    return __last;
+  }
+#endif
   for (; __first != __last; ++__first)
     *__first = __value;
   return __first;
 }
 
 template <class _RandomAccessIterator,
           class _Tp,
-          __enable_if_t<__has_random_access_iterator_category<_RandomAccessIterator>::value &&
-                            !__is_segmented_iterator_v<_RandomAccessIterator>,
-                        int> = 0>
+          __enable_if_t<__has_random_access_iterator_category<_RandomAccessIterator>::value, int> = 0>
----------------
ldionne wrote:

```suggestion
          __enable_if_t<__has_random_access_iterator_category<_RandomAccessIterator>::value && !__segmented_iterator<_RandomAccessIterator>::value, int> = 0>
```

This preserves previous "functionality" or at least code flow for segmented-and-random-access iterators. We can discuss removing that in a future patch, especially if we generalize `for_each_n_segment`.

https://github.com/llvm/llvm-project/pull/164797