Optimize Enumerable.Range(...).Select(...)

[stephentoub]

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<Test>();

    [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();
}

Method	Size	Mean Before	Mean After	Memory Before	Memory After
RangeSelectIterate	1	83.65 ns	57.00 ns	96 B	88 B
RangeSelectToArray	1	72.15 ns	50.64 ns	128 B	120 B
RangeSelectToList	1	77.64 ns	58.61 ns	160 B	152 B
RangeSelectIterate	10	190.89 ns	123.68 ns	96 B	88 B
RangeSelectToArray	10	138.11 ns	71.17 ns	160 B	152 B
RangeSelectToList	10	151.71 ns	84.80 ns	192 B	184 B
RangeSelectIterate	100	1,136.12 ns	689.90 ns	96 B	88 B
RangeSelectToArray	100	741.59 ns	279.15 ns	520 B	512 B
RangeSelectToList	100	727.97 ns	336.18 ns	552 B	544 B
RangeSelectIterate	1000	10,603.56 ns	6,362.80 ns	96 B	88 B
RangeSelectToArray	1000	6,729.38 ns	2,371.04 ns	4120 B	4112 B
RangeSelectToList	1000	6,665.97 ns	2,799.10 ns	4152 B	4144 B

cc: @maryamariyan, @cston, @danmosemsft

[ahsonkhan]

[Benchmark] public void RangeSelectIterate()

Add these to dotnet/performance?

cc @adamsitnik, @billwert

[ahsonkhan]

Some nits/questions, but looks like you are following the existing code patterns used in other iterators.

[adamsitnik]

LGTM, but I left some questions to get a better understanding.

[stephentoub]

Thanks for reviewing, @ahsonkhan and @adamsitnik.

[timandy]

Does 'repeat.select' should be optimized also?

[stephentoub]

I found very little usage of Repeat in general outside of test code, and very little of that used Select. What's the scenario where it's important that be a fast as possible?

[timandy]

This expression is always true,even overflow.

Does it should be?

Debug.Assert(unchecked((uint)(end - start) <= (uint)int.MaxValue));

[stephentoub]

Yup, it's missing the casts. We'd welcome a PR if you'd like to fix it. Thanks.