================
@@ -428,50 +653,60 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 void __split_buffer<_Tp,
_Allocator>::shrink_to_fi
}
}
-template <class _Tp, class _Allocator>
+template <class _Tp, class _Allocator, template <class, class, class> class
_Layout>
template <class... _Args>
-_LIBCPP_CONSTEXPR_SINCE_CXX20 void __split_buffer<_Tp,
_Allocator>::emplace_front(_Args&&... __args) {
- if (__begin_ == __first_) {
- if (__end_ < __cap_) {
- difference_type __d = __cap_ - __end_;
+_LIBCPP_CONSTEXPR_SINCE_CXX20 void __split_buffer<_Tp, _Allocator,
_Layout>::emplace_front(_Args&&... __args) {
+ if (__front_spare() == 0) {
+ pointer __end = end();
+ if (__back_spare() > 0) {
+ // The elements are pressed up against the front of the buffer: we need
to move them back a
+ // little bit to make `emplace_front` have amortised O(1) complexity.
+ difference_type __d = __back_spare();
__d = (__d + 1) / 2;
- __begin_ = std::move_backward(__begin_, __end_, __end_ + __d);
- __end_ += __d;
+ __set_begin(std::move_backward(begin(), __end, __end + __d));
+
+ // `begin()` was moved further into the buffer, so we need to update the
pointer-based
+ // layout's end with it. We don't need to do anything for the size-based
layout here, because
+ // the number of elements hasn't changed yet.
+ __set_size_if_pointer_based(__d);
----------------
ldionne wrote:
The fact that we special-case on the layout here is a smell. The split buffer's
layout should be generic and `__split_buffer` should work regardless of the
layout passed to it (even if we had e.g. a 3rd layout that is somehow different
from the existing two).
I believe we should always call `__set_sentinel(__end + __d)`. In the
size-based layout, this will redundantly set `__size_`, but at least it will
always be correct (if we e.g. had a different layout), and it will remove the
need for the arguably ad-hoc `__set_size_if_pointer_based` function. I suspect
that the redundant store will be entirely in the noise compared to all the work
we're doing in `emplace_front`.
https://github.com/llvm/llvm-project/pull/139632
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