为什么 Enumerable.Range 比直接的 Yield 循环更快？

Question

下面的代码检查执行相同解决方案的三种不同方法的性能。

    public static void Main(string[] args)
    {
        // for loop
        {
            Stopwatch sw = Stopwatch.StartNew();

            int accumulator = 0;
            for (int i = 1; i <= 100000000; ++i)
            {
                accumulator += i;
            }

            sw.Stop();

            Console.WriteLine("time = {0}; result = {1}", sw.ElapsedMilliseconds, accumulator);
        }

        //Enumerable.Range
        {
            Stopwatch sw = Stopwatch.StartNew();

            var ret = Enumerable.Range(1, 100000000).Aggregate(0, (accumulator, n) => accumulator + n);

            sw.Stop();
            Console.WriteLine("time = {0}; result = {1}", sw.ElapsedMilliseconds, ret);
        }

        //self-made IEnumerable<int>
        {
            Stopwatch sw = Stopwatch.StartNew();

            var ret = GetIntRange(1, 100000000).Aggregate(0, (accumulator, n) => accumulator + n);

            sw.Stop();
            Console.WriteLine("time = {0}; result = {1}", sw.ElapsedMilliseconds, ret);
        }
    }

    private static IEnumerable<int> GetIntRange(int start, int count)
    {
        int end = start + count;

        for (int i = start; i < end; ++i)
        {
            yield return i;
        }
    }
}

结果是：

time = 306; result = 987459712
time = 1301; result = 987459712
time = 2860; result = 987459712

“for 循环”比其他两个解决方案更快并不奇怪，因为 Enumerable.Aggregate 需要更多的方法调用。 然而，真正让我惊讶的是“Enumerable.Range”比“自制的IEnumerable”更快。 我认为 Enumerable.Range 会比简单的 GetIntRange 方法有更多的开销。

造成这种情况的可能原因有哪些？

Answer 1

为什么 Enumerable.Range 比您自制的 GetIntRange 慢？ 事实上，如果 Enumerable.Range 被定义为

public static class Enumerable {
    public static IEnumerable<int> Range(int start, int count) {
        var end = start + count;
        for(var current = start; current < end; ++current) {
            yield return current;
        }
    }
}

那么它应该与您自制的 GetIntRange 一样快。 这实际上是 Enumerable.Range 的参考实现，编译器或程序员没有任何技巧。

您可能希望将 GetIntRange 和 System.Linq.Enumerable.Range 与以下实现进行比较（当然，正如 Rob 指出的那样，在发布模式下编译）。 对于编译器从迭代器块生成的内容，此实现可能会稍微优化。

public static class Enumerable {
    public static IEnumerable<int> Range(int start, int count) {
        return new RangeEnumerable(start, count);
    }
    private class RangeEnumerable : IEnumerable<int> {
        private int _Start;
        private int _Count;
        public RangeEnumerable(int start, int count) {
            _Start = start;
            _Count = count;
        }
        public virtual IEnumerator<int> GetEnumerator() {
            return new RangeEnumerator(_Start, _Count);
        }
        IEnumerator IEnumerable.GetEnumerator() {
            return GetEnumerator();
        }
    }
    private class RangeEnumerator : IEnumerator<int> {
        private int _Current;
        private int _End;
        public RangeEnumerator(int start, int count) {
            _Current = start - 1;
            _End = start + count;
        }
        public virtual void Dispose() {
            _Current = _End;
        }
        public virtual void Reset() {
            throw new NotImplementedException();
        }
        public virtual bool MoveNext() {
            ++_Current;
            return _Current < _End;
        }
        public virtual int Current { get { return _Current; } }
        object IEnumerator.Current { get { return Current; } }
    }
}

Answer 2

我的猜测是您正在调试器中运行。 这是我的结果，从命令行使用“/o+/debug-”构建，

time = 142; result = 987459712
time = 1590; result = 987459712
time = 1792; result = 987459712

仍然有轻微的差异，但没有那么明显。 迭代器块实现的效率不如定制的解决方案，但它们非常好。

Answer 3

假设这是正在运行的发布版本，否则所有比较都会关闭，因为 JIT 将无法正常工作。

您可以使用 reflector 查看装配体，看看“yield”声明是什么也在扩大。 编译器将创建一个类来封装迭代器。 也许生成的代码中进行的内务处理比可能是手工编码的 Enumerable.Range 的实现要多

Answer 4

反射器输出略有不同（以及参数检查和额外的内化水平，这里绝对不相关）。 基本代码更像是：

public static IEnumerable<int> Range(int start, int count) {
    for(int current = 0; current < count; ++current) {
        yield return start + current;
    }
}

也就是说，它们不是另一个局部变量，而是为每个收益应用一个额外的加法。

我尝试对此进行基准测试，但我无法停止足够的外部进程来获得可理解的结果。 我还尝试了每个测试两次，以忽略 JIT 编译器的影响，但即使这样也有“有趣”的结果。

这是我的结果示例：

Run 0:
time = 4149; result = 405000000450000000
time = 25645; result = 405000000450000000
time = 39229; result = 405000000450000000
time = 29872; result = 405000000450000000

time = 4277; result = 405000000450000000
time = 26878; result = 405000000450000000
time = 26333; result = 405000000450000000
time = 26684; result = 405000000450000000

Run 1:
time = 4063; result = 405000000450000000
time = 22714; result = 405000000450000000
time = 34744; result = 405000000450000000
time = 26954; result = 405000000450000000

time = 4033; result = 405000000450000000
time = 26657; result = 405000000450000000
time = 25855; result = 405000000450000000
time = 25031; result = 405000000450000000

Run 2:
time = 4021; result = 405000000450000000
time = 21815; result = 405000000450000000
time = 34304; result = 405000000450000000
time = 32040; result = 405000000450000000

time = 3993; result = 405000000450000000
time = 24779; result = 405000000450000000
time = 29275; result = 405000000450000000
time = 32254; result = 405000000450000000

代码

using System;
using System.Linq;
using System.Collections.Generic;
using System.Diagnostics;

namespace RangeTests
{
  class TestRange
  {
    public static void Main(string[] args)
    {
      for(int l = 1; l <= 2; ++l)
      {
        const int N = 900000000;
        System.GC.Collect(2);
        // for loop
        {
            Stopwatch sw = Stopwatch.StartNew();

            long accumulator = 0;
            for (int i = 1; i <= N; ++i)
            {
                accumulator += i;
            }

            sw.Stop();

            Console.WriteLine("time = {0}; result = {1}", sw.ElapsedMilliseconds, accumulator);
        }
        System.GC.Collect(2);

        //Enumerable.Range
        {
            Stopwatch sw = Stopwatch.StartNew();

            var ret = Enumerable.Range(1, N).Aggregate(0, (long accumulator,int n) => accumulator + n);

            sw.Stop();
            Console.WriteLine("time = {0}; result = {1}", sw.ElapsedMilliseconds, ret);
        }
        System.GC.Collect(2);

        //self-made IEnumerable<int>
        {
            Stopwatch sw = Stopwatch.StartNew();

            var ret = GetIntRange(1, N).Aggregate(0, (long accumulator,int n) => accumulator + n);

            sw.Stop();
            Console.WriteLine("time = {0}; result = {1}", sw.ElapsedMilliseconds, ret);
        }
        System.GC.Collect(2);

        //self-made adjusted IEnumerable<int>
        {
            Stopwatch sw = Stopwatch.StartNew();

            var ret = GetRange(1, N).Aggregate(0, (long accumulator,int n) => accumulator + n);

            sw.Stop();
            Console.WriteLine("time = {0}; result = {1}", sw.ElapsedMilliseconds, ret);
        }
        System.GC.Collect(2);
        Console.WriteLine();
    } }

    private static IEnumerable<int> GetIntRange(int start, int count)
    {
        int end = start + count;

        for (int i = start; i < end; ++i)
        {
            yield return i;
        }
    }

    private static IEnumerable<int> GetRange(int start, int count)
    {
        for (int i = 0; i < count; ++i)
        {
            yield return start + i;
        }
    }
} }

以及编译的

csc.exe -optimize+ -debug- RangeTests.cs