[libc++] Simplify vector's emplace_back, __move_range and __construct_at_end functions

[winner245]

This PR slightly simplifies the implementation of std::vector's emplace_back and __construct_at_end functions to make the code more concise while maintaining the same functionality.

• emplace_back: The simplified version removes the unnecessary temporary pointer variable __end and directly manipulates member variable this->__end_.
• __construct_at_end and __move_range: the simplified version removes unnecessary local variables __pos and __new_end, and makes the loop more straightforward.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Peng Liu (winner245)

Changes

This PR slightly simplifies the implementation of std::vector's emplace_back and __construct_at_end functions to make the code more concise while maintaining the same functionality.

• emplace_back: The simplified version removes the unnecessary temporary pointer variable __end and directly manipulates member variable this->__end_.
• __construct_at_end: the simplified version removes unnecessary local variables __pos and __new_end, and makes the loop more straightforward.

---

Full diff: https://github.com/llvm/llvm-project/pull/120521.diff

1 Files Affected:

• (modified) libcxx/include/__vector/vector.h (+8-14)

diff --git a/libcxx/include/__vector/vector.h b/libcxx/include/__vector/vector.h
index 6ba7ba7bcf724b..7ad209be02d4aa 100644
--- a/libcxx/include/__vector/vector.h
+++ b/libcxx/include/__vector/vector.h
@@ -879,9 +879,8 @@ vector<_Tp, _Allocator>::__recommend(size_type __new_size) const {
 template <class _Tp, class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 void vector<_Tp, _Allocator>::__construct_at_end(size_type __n) {
   _ConstructTransaction __tx(*this, __n);
-  const_pointer __new_end = __tx.__new_end_;
-  for (pointer __pos = __tx.__pos_; __pos != __new_end; __tx.__pos_ = ++__pos) {
-    __alloc_traits::construct(this->__alloc_, std::__to_address(__pos));
+  for (; __tx.__pos_ != __tx.__new_end_; ++__tx.__pos_) {
+    __alloc_traits::construct(this->__alloc_, std::__to_address(__tx.__pos_));
   }
 }
 
@@ -895,9 +894,8 @@ template <class _Tp, class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 inline void
 vector<_Tp, _Allocator>::__construct_at_end(size_type __n, const_reference __x) {
   _ConstructTransaction __tx(*this, __n);
-  const_pointer __new_end = __tx.__new_end_;
-  for (pointer __pos = __tx.__pos_; __pos != __new_end; __tx.__pos_ = ++__pos) {
-    __alloc_traits::construct(this->__alloc_, std::__to_address(__pos), __x);
+  for (; __tx.__pos_ != __tx.__new_end_; ++__tx.__pos_) {
+    __alloc_traits::construct(this->__alloc_, std::__to_address(__tx.__pos_), __x);
   }
 }
 
@@ -1101,16 +1099,12 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 inline
     void
 #endif
     vector<_Tp, _Allocator>::emplace_back(_Args&&... __args) {
-  pointer __end = this->__end_;
-  if (__end < this->__cap_) {
+  if (this->__end_ < this->__cap_)
     __construct_one_at_end(std::forward<_Args>(__args)...);
-    ++__end;
-  } else {
-    __end = __emplace_back_slow_path(std::forward<_Args>(__args)...);
-  }
-  this->__end_ = __end;
+  else
+    (void)__emplace_back_slow_path(std::forward<_Args>(__args)...);
 #if _LIBCPP_STD_VER >= 17
-  return *(__end - 1);
+  return *(this->__end_ - 1);
 #endif
 }
 

[philnik777]

This basically reverts https://reviews.llvm.org/D82111 and https://reviews.llvm.org/D80588, which we shouldn't do unless we can show that the compiler got smarter.

[winner245]

Thank you for providing the context and related PRs. After reviewing those PRs, I understand the objectives of the current code, and they indeed improve the performance. Therefore, I am closing this PR.