Skip to content

[WIP][ADT] Avoid slow size queries on append #136365

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Draft
wants to merge 2 commits into
base: main
Choose a base branch
from
Draft

[WIP][ADT] Avoid slow size queries on append #136365

Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
078be5e
[WIP][ADT] Avoid slow size queries on append
kuhar Apr 18, 2025
3d891d2
Address comments
kuhar Apr 18, 2025
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
64 changes: 50 additions & 14 deletions llvm/include/llvm/ADT/SmallVector.h
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -36,10 +36,18 @@ template <typename T> class ArrayRef;

template <typename IteratorT> class iterator_range;

template <class Iterator>
using EnableIfConvertibleToInputIterator = std::enable_if_t<std::is_convertible<
namespace detail {
template <typename Iterator, typename IteratorCategory>
inline constexpr bool IsOfIteratorCategory = std::is_convertible_v<
typename std::iterator_traits<Iterator>::iterator_category,
std::input_iterator_tag>::value>;
IteratorCategory>;

template <typename Iterator>
using EnableIfConvertibleToInputIterator =
std::enable_if_t<std::is_convertible_v<
typename std::iterator_traits<Iterator>::iterator_category,
std::input_iterator_tag>>;
Comment on lines +47 to +49
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Could we use the newly introduced IsOfIteratorCategory here?

Copy link
Member Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Actually no because of SFINAE weirdness...

} // namespace detail

/// This is all the stuff common to all SmallVectors.
///
Expand Down Expand Up @@ -679,13 +687,37 @@ class SmallVectorImpl : public SmallVectorTemplateBase<T> {
void swap(SmallVectorImpl &RHS);

/// Add the specified range to the end of the SmallVector.
template <typename ItTy, typename = EnableIfConvertibleToInputIterator<ItTy>>
template <typename ItTy,
typename = detail::EnableIfConvertibleToInputIterator<ItTy>>
void append(ItTy in_start, ItTy in_end) {
this->assertSafeToAddRange(in_start, in_end);
size_type NumInputs = std::distance(in_start, in_end);
this->reserve(this->size() + NumInputs);
this->uninitialized_copy(in_start, in_end, this->end());
this->set_size(this->size() + NumInputs);
if constexpr (detail::IsOfIteratorCategory<
ItTy, std::random_access_iterator_tag>) {
// Only reserve the required extra size upfront when the size calculation
// is guaranteed to be O(1).
size_type NumInputs = std::distance(in_start, in_end);
this->reserve(this->size() + NumInputs);
this->uninitialized_copy(in_start, in_end, this->end());
this->set_size(this->size() + NumInputs);
} else {
// Otherwise, append using `in_end` as the sentinel and reserve more space
// as necessary.
for (ItTy It = in_start; It != in_end;) {
iterator Dest = this->end();
size_type InitialSize = this->size();
size_type ExtraCap = this->capacity() - InitialSize;
size_type NumCopied = 0;
for (; NumCopied != ExtraCap && It != in_end; ++It, ++NumCopied)
::new ((void *)(Dest + NumCopied)) T(*It);

size_type NewSize = InitialSize + NumCopied;
this->set_size(NewSize);

if (It != in_end) {
this->reserve(NewSize + 1);
}
}
}
}

/// Append \p NumInputs copies of \p Elt to the end.
Expand Down Expand Up @@ -720,7 +752,8 @@ class SmallVectorImpl : public SmallVectorTemplateBase<T> {
// FIXME: Consider assigning over existing elements, rather than clearing &
// re-initializing them - for all assign(...) variants.

template <typename ItTy, typename = EnableIfConvertibleToInputIterator<ItTy>>
template <typename ItTy,
typename = detail::EnableIfConvertibleToInputIterator<ItTy>>
void assign(ItTy in_start, ItTy in_end) {
this->assertSafeToReferenceAfterClear(in_start, in_end);
clear();
Expand Down Expand Up @@ -871,12 +904,14 @@ class SmallVectorImpl : public SmallVectorTemplateBase<T> {
return I;
}

template <typename ItTy, typename = EnableIfConvertibleToInputIterator<ItTy>>
template <typename ItTy,
typename = detail::EnableIfConvertibleToInputIterator<ItTy>>
iterator insert(iterator I, ItTy From, ItTy To) {
// Convert iterator to elt# to avoid invalidating iterator when we reserve()
size_t InsertElt = I - this->begin();

if (I == this->end()) { // Important special case for empty vector.
// Important special case for appending to a vector, including empty vector.
if (I == this->end()) {
append(From, To);
return this->begin()+InsertElt;
}
Expand All @@ -887,8 +922,8 @@ class SmallVectorImpl : public SmallVectorTemplateBase<T> {
this->assertSafeToAddRange(From, To);

size_t NumToInsert = std::distance(From, To);
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Are you going to address this call to std::distance? If we have an input iterator that is not a forward iterator, then we would be iterating the range twice.

Copy link
Member Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

This is only invoked when we insert in the middle of the vector -- I'm not sure how much we care about this. We could potentially first append and then when we know size do a rotate, but I'm not sure it's worth it

Copy link
Member Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Unless I misread the code...

Copy link
Member Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I think this is what the if statement above already handles -- it's not just the empty case, unlike what the comment claims.


// Ensure there is enough space.
// Ensure there is enough space so that we do not have to re-allocate mid
// insertion.
reserve(this->size() + NumToInsert);

// Uninvalidate the iterator.
Expand Down Expand Up @@ -1212,7 +1247,8 @@ class LLVM_GSL_OWNER SmallVector : public SmallVectorImpl<T>,
this->assign(Size, Value);
}

template <typename ItTy, typename = EnableIfConvertibleToInputIterator<ItTy>>
template <typename ItTy,
typename = detail::EnableIfConvertibleToInputIterator<ItTy>>
SmallVector(ItTy S, ItTy E) : SmallVectorImpl<T>(N) {
this->append(S, E);
}
Expand Down
Loading