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Improve List<T>.AddRange performance for enumerables #76043

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Merged
stephentoub merged 4 commits into from
Sep 25, 2022
Merged

Improve List<T>.AddRange performance for enumerables #76043

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a0ed565
Improve List<T>.AddRange performance for enumerables
stephentoub Sep 22, 2022
65553da
Merge branch 'dotnet:main' into listaddrange
stephentoub Sep 24, 2022
ab65cf2
Merge branch 'dotnet:main' into listaddrange
stephentoub Sep 24, 2022
1f5445d
Merge branch 'dotnet:main' into listaddrange
stephentoub Sep 25, 2022
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35 changes: 33 additions & 2 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 @@ -241,7 +241,38 @@ int IList.Add(object? item)
// capacity or the new size, whichever is larger.
//
public void AddRange(IEnumerable<T> collection)
=> InsertRange(_size, collection);
{
if (collection == null)
{
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.collection);
}

if (collection is ICollection<T> c)
{
int count = c.Count;
if (count > 0)
{
if (_items.Length - _size < count)
{
Grow(_size + count);
}

c.CopyTo(_items, _size);
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@eiriktsarpalis eiriktsarpalis Sep 23, 2022

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I'm guessing this could regress cases where we're appending the list to itself?

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@stephentoub stephentoub Sep 23, 2022

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Regress functionally? We have this test:

runtime/src/libraries/System.Collections/tests/Generic/List/List.Generic.Tests.AddRange.cs

Line 50 in 9b9b0c9

public void AddRange_AddSelfAsEnumerable_ThrowsExceptionWhenNotEmpty()

Is there a case that's missing?

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@eiriktsarpalis eiriktsarpalis Sep 23, 2022

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Functionality looks good, I meant to say regress performance.

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@stephentoub stephentoub Sep 23, 2022

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I don't believe so. I just tried with:

[Benchmark]
public List<int> AddToSelf()
{
    var list = new List<int>();
    list.Add(1);
    list.Add(2);
    list.AddRange(list);
    list.AddRange(list);
    list.AddRange(list);
    list.AddRange(list);
    list.AddRange(list);
    list.AddRange(list);
    return list;
}

and depending on the run there was either no difference or a small difference in favor in of the new version.

Is there a specific aspect of the change you think would regress that? I can try to address it if so. Though I'm not particularly concerned about the performance of that case, anyway.

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@eiriktsarpalis eiriktsarpalis Sep 23, 2022

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Assuming we cared about optimizing that particular case, hardcoding the operation in the style of InsertRange might be marginally faster compared to using ICollection<T>.CopyTo, at the expense of added branching for every other case. But like you said, it doesn't seem important enough to justify.

_size += count;
_version++;
}
}
else
{
using (IEnumerator<T> en = collection.GetEnumerator())
{
while (en.MoveNext())
{
Add(en.Current);
}
}
}
}

public ReadOnlyCollection<T> AsReadOnly()
=> new ReadOnlyCollection<T>(this);
Expand Down Expand Up @@ -776,6 +807,7 @@ public void InsertRange(int index, IEnumerable<T> collection)
c.CopyTo(_items, index);
}
_size += count;
_version++;
}
}
else
Expand All @@ -788,7 +820,6 @@ public void InsertRange(int index, IEnumerable<T> collection)
}
}
}
_version++;
}

// Returns the index of the last occurrence of a given value in a range of
Expand Down