Optimize Enumerable.Range(...).Select(...) by stephentoub · Pull Request #37410 · dotnet/corefx (original) (raw)

Looking at some large code indexes, by far the most common uses of Enumerable.Range are when it's either directly iterated with a foreach, converted to an array with ToArray, or probably the most common, succeeded by Select (and then iterated or ToArray'd). The first two are already decently optimized, with a custom range iterator returned from Enumerable.Range and that iterator already having a ToArray override. But it's missing the Select case. This commit adds that, by copying the existing SelectArrayIterator and then just tweaking it to use _start/_end rather than accessing a stored _source array.

using BenchmarkDotNet.Attributes; using BenchmarkDotNet.Running; using System; using System.Collections.Generic; using System.Linq;

[InProcess] [MemoryDiagnoser] public class Test { public static void Main() => BenchmarkRunner.Run();

[Params(1, 10, 100, 1000)]
public int Size { get; set; }

[Benchmark] public void RangeSelectIterate() { foreach (int item in Enumerable.Range(0, Size).Select(i => i)) { } }
[Benchmark] public int[] RangeSelectToArray() => Enumerable.Range(0, Size).Select(i => i).ToArray();
[Benchmark] public List<int> RangeSelectToList() => Enumerable.Range(0, Size).Select(i => i).ToList();

}

cc: @maryamariyan, @cston, @danmosemsft