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

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

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24 changes: 19 additions & 5 deletions clang/include/clang/AST/ASTVector.h
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand All @@ -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.
Comment on lines -195 to -196
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@zwuis zwuis Oct 5, 2025 •
edited
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Is the comment correct? Can you confirm that std::distance and std::uninitialized_copy do NOT perform compile time dispatch? How is std::uninitialized_copy_n faster here?

Update: I can confirm that the implementation of std::uninitialized_copy doesn't perform compile time dispatch, but I don't see how is std::uninitialized_copy_n faster.

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@zwuis,thanks for your review.

std::distance and std::uninitialized_copy both rely on iterator tag dispatch, which determines the implementation at overload resolution time rather than through explicit compile-time branching. While std::distance will use an O(1) path for random access iterators, that dispatch happens inside the library and isn’t guaranteed to inline cleanly into templated call sites like append(). Moreover, std::uninitialized_copy always performs an element-by-element loop regardless of iterator category, offering no benefit when the range size (NumInputs) is already known. Using std::uninitialized_copy_n with a precomputed element count avoids the per-iteration sentinel checks and enables the compiler to generate tighter loops.

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);
}

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