為什么Enumerable.Range比直接yield循環更快？

Question

下面的代碼是檢查執行相同解決方案的三種不同方式的性能。

    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;
        }
    }
}

結果是：

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

因為Enumerable.Aggregate需要更多的方法調用，所以“for循環”比其他兩個解決方案更快也就不足為奇了。 然而，“Enumerable.Range”比“自制IEnumerable”更快，這讓我感到驚訝。 我認為Enumerable.Range比簡單的GetIntRange方法有更多的開銷。

這有什么可能的原因？

Answer 1

為什么Enumerable.Range要比你自制的GetIntRange ？ 實際上，如果將Enumerable.Range定義為

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;
        }
    }
}

然后它應該與你自制的GetIntRange一樣快。 這實際上是Enumerable.Range的參考實現，沒有編譯器或程序員的任何技巧。

您可能希望將GetIntRange和System.Linq.Enumerable.Range與以下實現進行比較（當然，正如Rob指出的那樣，在發布模式下進行編譯）。 對於編譯器將從迭代器塊生成的內容，可以稍微優化該實現。

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

我的猜測是你在調試器中運行。 以下是我的結果，從命令行使用“/ o + / debug-”構建

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

仍有一點點差異，但並不是那么明顯。 迭代器塊實現不如定制解決方案那么高效，但它們非常好。

Answer 3

假設這是一個正在運行的發布版本，否則所有比較都將關閉，因為JIT將無法正常工作。

您可以使用反射器查看裝配體，並查看“yield”語句的擴展情況。 編譯器將創建一個類來封裝迭代器。 也許生成的代碼中的內務處理比Enumerable.Range的實現更多，這可能是手工編碼的

Answer 4

反射器輸出的細微差別（以及參數檢查和內部化的額外水平在這里絕對不相關）。 基本代碼更像是：

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

也就是說，它們不是另一個局部變量，而是為每個產量應用額外的添加。

我試圖對此進行基准測試，但我無法阻止足夠的外部進程來獲得可理解的結果。 我還嘗試了兩次每次測試以忽略JIT編譯器的效果，但即使這樣也有“有趣”的結果。

以下是我的結果示例：

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

和代碼

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;
        }
    }
} }

用。編譯

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