[libcxx-commits] [libcxx] [libc++] Use relocation in vector::emplace_back (PR #159365)

[Nikolas Klauser via libcxx-commits]

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@@ -469,15 +487,60 @@ class vector {
   _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void push_back(value_type&& __x) { emplace_back(std::move(__x)); }
 
   template <class... _Args>
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference __emplace_back(_Args&&... __args) {
+    if constexpr (__libcpp_is_trivially_relocatable<value_type>::value &&
+                  __allocator_has_trivial_move_construct_v<allocator_type, value_type> &&
+                  __allocator_has_trivial_destroy_v<allocator_type, value_type>) {
+      // This path is written in a way to have the fast path as compact as possible. Specifically,
+      // there is a branch in case there isn't enough capacity left over, which will return to the same location within
+      // the function after growing the vector. This ensures that the relocation code exists only once and the
+      // reallocation path is common across all `vector` instantiations of trivially relocatable types.
+      union _Tmp {
+        _LIBCPP_DIAGNOSTIC_PUSH
+        // Clang complains about __exclude_from_explicit_instantiation__ on a local class member.
+        _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wignored-attributes")
+        _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Tmp() {}
+        _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 ~_Tmp() {}
+        _LIBCPP_DIAGNOSTIC_POP
+        value_type __val_;
+      };
+      _Tmp __tmp;
+
+      __alloc_traits::construct(__alloc_, std::addressof(__tmp.__val_), std::forward<_Args>(__args)...);
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philnik777 wrote:

If the construction throws we have to roll back to the old allocation, since iterators have to stay valid. This is essentially what the other branch does.

https://github.com/llvm/llvm-project/pull/159365