[libcxx-commits] [libcxx] [libc++] Optimize ranges::{for_each, for_each_n} for segmented iterators (PR #132896)

[Louis Dionne via libcxx-commits]

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@@ -41,9 +42,16 @@ struct __for_each {
   template <class _Iter, class _Sent, class _Proj, class _Func>
   _LIBCPP_HIDE_FROM_ABI constexpr static for_each_result<_Iter, _Func>
   __for_each_impl(_Iter __first, _Sent __last, _Func& __func, _Proj& __proj) {
-    for (; __first != __last; ++__first)
-      std::invoke(__func, std::invoke(__proj, *__first));
-    return {std::move(__first), std::move(__func)};
+    if constexpr (random_access_iterator<_Iter> && sized_sentinel_for<_Sent, _Iter>) {
+      auto __n   = __last - __first;
+      auto __end = __first + __n;
+      std::for_each(__first, __end, [&](auto&& __val) { std::invoke(__func, std::invoke(__proj, __val)); });
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ldionne wrote:

I think we should define a single `std::__for_each` function and call that from the classic and from the ranges algorithms. That way we implement the actual logic in a single place. We have precedent for that in a bunch of places, you can look for places where we define `std::__foo` that takes an `_AlgPolicy`.

https://github.com/llvm/llvm-project/pull/132896