[clang] [NFC][clang][AST] Add compile-time dispatch for random access iterators in append() (PR #162000)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Samarth Narang (snarang181)

<details>
<summary>Changes</summary>

This patch improves the `append()` implementation by introducing a compile-time dispatch between random access and non-random access iterators.

---
Full diff: https://github.com/llvm/llvm-project/pull/162000.diff


1 Files Affected:

- (modified) clang/include/clang/AST/ASTVector.h (+19-5) 


``````````diff
diff --git a/clang/include/clang/AST/ASTVector.h b/clang/include/clang/AST/ASTVector.h
index d5a04767ca133..ca30ec343492a 100644
--- a/clang/include/clang/AST/ASTVector.h
+++ b/clang/include/clang/AST/ASTVector.h
@@ -180,9 +180,21 @@ class ASTVector {
   size_t capacity() const { return this->capacity_ptr() - Begin; }
 
   /// append - Add the specified range to the end of the SmallVector.
-  template<typename in_iter>
+  template <typename in_iter>
   void append(const ASTContext &C, in_iter in_start, in_iter in_end) {
-    size_type NumInputs = std::distance(in_start, in_end);
+    using size_type =
+        typename std::remove_reference_t<decltype(*this)>::size_type;
+    using iterator_category =
+        typename std::iterator_traits<in_iter>::iterator_category;
+
+    size_t NumInputs = 0;
+    constexpr bool is_random_access =
+        std::is_base_of_v<std::random_access_iterator_tag, iterator_category>;
+
+    if constexpr (is_random_access)
+      NumInputs = static_cast<size_type>(in_end - in_start);
+    else
+      NumInputs = static_cast<size_type>(std::distance(in_start, in_end));
 
     if (NumInputs == 0)
       return;
@@ -192,9 +204,11 @@ class ASTVector {
       this->grow(C, this->size()+NumInputs);
 
     // Copy the new elements over.
-    // TODO: NEED To compile time dispatch on whether in_iter is a random access
-    // iterator to use the fast uninitialized_copy.
-    std::uninitialized_copy(in_start, in_end, this->end());
+    if constexpr (is_random_access)
+      std::uninitialized_copy_n(in_start, NumInputs, this->end());
+    else
+      std::uninitialized_copy(in_start, in_end, this->end());
+
     this->setEnd(this->end() + NumInputs);
   }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/162000