[libcxx-commits] [libcxx] [libc++] Optimize vector::__move_range for trivially copyable types (PR #207384)

[Louis Dionne via libcxx-commits]

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@@ -1120,14 +1122,26 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 void
 vector<_Tp, _Allocator>::__move_range(pointer __from_s, pointer __from_e, pointer __to) {
   pointer __old_last  = __layout_.__end_ptr();
   difference_type __n = __old_last - __to;
+#ifndef _LIBCPP_CXX03_LANG
+  if constexpr (__allocator_has_trivial_move_construct_v<allocator_type, value_type> &&
+                is_trivially_copyable<value_type>::value) {
+    if (__libcpp_is_constant_evaluated()) // construct the new trailing objects
+      std::uninitialized_fill_n(__old_last, (__from_e - __from_s) - __n, *__from_s);
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ldionne wrote:

I'm not a huge fan of this, it seems pretty unclean. I'd rather take the slow code path when we're constant evaluating.

https://github.com/llvm/llvm-project/pull/207384