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Avoid extra Array.Copy in List<T>.Insert(Range) when the list is full #90089

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Merged
merged 10 commits into from
Oct 25, 2023
Merged

Avoid extra Array.Copy in List<T>.Insert(Range) when the list is full #90089

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b21da46
Avoid extra copy when insertting on `List` at capacity
karakasa Aug 7, 2023
cbda48b
typo
karakasa Aug 7, 2023
9e82661
fix typo
karakasa Aug 7, 2023
875dff6
Remove impossible branches
karakasa Aug 7, 2023
4ac69c8
fixed semicolon typo
karakasa Aug 7, 2023
c04b3cc
improve InsertRange perf
karakasa Aug 7, 2023
02e1aee
Remove redundant capacity argument
karakasa Aug 7, 2023
174c7bf
Remove unnecessary assertion
karakasa Aug 7, 2023
a50b0d6
Apply suggestions from code review
karakasa Oct 19, 2023
ce465db
Put shared logic into a standalone method
karakasa Oct 25, 2023
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50 changes: 45 additions & 5 deletions src/libraries/System.Private.CoreLib/src/System/Collections/Generic/List.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -449,6 +449,43 @@ public int EnsureCapacity(int capacity)
/// </summary>
/// <param name="capacity">The minimum capacity to ensure.</param>
internal void Grow(int capacity)
{
Capacity = GetNewCapacity(capacity);
}

/// <summary>
/// Enlarge this list so it may contain at least <paramref name="insertionCount"/> more elements
/// And copy data to their after-insertion positions.
/// This method is specifically for insertion, as it avoids 1 extra array copy.
/// You should only call this method when Count + insertionCount > Capacity.
/// </summary>
/// <param name="indexToInsert">Index of the first insertion.</param>
/// <param name="insertionCount">How many elements will be inserted.</param>
private void GrowForInsertion(int indexToInsert, int insertionCount = 1)
{
Debug.Assert(insertionCount > 0);

int requiredCapacity = checked(_size + insertionCount);
int newCapacity = GetNewCapacity(requiredCapacity);

// Inline and adapt logic from set_Capacity

T[] newItems = new T[newCapacity];
if (indexToInsert != 0)
{
Array.Copy(_items, newItems, length: indexToInsert);
}

if (_size != indexToInsert)
{
Array.Copy(_items, indexToInsert, newItems, indexToInsert + insertionCount, _size - indexToInsert);
}

_items = newItems;
}

[MethodImpl(MethodImplOptions.AggressiveInlining)]
private int GetNewCapacity(int capacity)
{
Debug.Assert(_items.Length < capacity);

Expand All @@ -462,7 +499,7 @@ internal void Grow(int capacity)
// Capacities exceeding Array.MaxLength will be surfaced as OutOfMemoryException by Array.Resize.
if (newCapacity < capacity) newCapacity = capacity;

Capacity = newCapacity;
return newCapacity;
}

public bool Exists(Predicate<T> match)
Expand Down Expand Up @@ -737,8 +774,11 @@ public void Insert(int index, T item)
{
ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_ListInsert);
}
if (_size == _items.Length) Grow(_size + 1);
if (index < _size)
if (_size == _items.Length)
{
GrowForInsertion(index, 1);
}
else if (index < _size)
{
Array.Copy(_items, index, _items, index + 1, _size - index);
}
Expand Down Expand Up @@ -785,9 +825,9 @@ public void InsertRange(int index, IEnumerable<T> collection)
{
if (_items.Length - _size < count)
{
Grow(checked(_size + count));
GrowForInsertion(index, count);
}
if (index < _size)
else if (index < _size)
{
Array.Copy(_items, index, _items, index + count, _size - index);
}
Expand Down