HashSet性能添加vs包含现有元素

Question

出于某种原因,似乎Adda HashSet上的Contains操作比元素已经存在时的操作要慢HashSet.

这是证明:

    Stopwatch watch = new Stopwatch();
    int size = 10000;
    int iterations = 10000;


    var s = new HashSet<int>();
    for (int i = 0; i < size; i++) {
        s.Add(i);
    }

    Console.WriteLine(watch.Time(() =>
    {
        for (int i = 0; i < size; i++) {
            s.Add(i);
        }
    }, iterations));

    s = new HashSet<int>();
    for (int i = 0; i < size; i++) {
        s.Add(i);
    }

    // outputs: 47,074,764

    Console.WriteLine(watch.Time(() =>
    {
        for (int i = 0; i < size; i++) {
            if (!s.Contains(i))
                s.Add(i);
        }
    }, iterations));

    // outputs: 41,125,219

为什么Contains比Add现有元素更快？

注意:我正在使用Stopwatch另一个SO问题的扩展名.

    public static long Time(this Stopwatch sw, Action action, int iterations) {
        sw.Reset();
        sw.Start();
        for (int i = 0; i < iterations; i++) {
            action();
        }
        sw.Stop();

        return sw.ElapsedTicks;
    }

更新:内部测试表明,大型性能差异只发生在.NET框架的x64版本上.使用32位版本的框架Contains似乎以相同的速度运行添加(实际上看起来带有包含的版本在某些测试运行中运行速度慢了一些)在X64版本的框架上,带有包含的版本似乎运行速度提高约15%.

Answer 1

AddIfNotPresent执行Contains不执行的额外划分.看一下IL for Contains:

IL_000a:  call       instance int32 class System.Collections.Generic.HashSet`1<!T>::InternalGetHashCode(!0)
  IL_000f:  stloc.0
  IL_0010:  ldarg.0
  IL_0011:  ldfld      int32[] class System.Collections.Generic.HashSet`1<!T>::m_buckets
  IL_0016:  ldloc.0
  IL_0017:  ldarg.0
  IL_0018:  ldfld      int32[] class System.Collections.Generic.HashSet`1<!T>::m_buckets
  IL_001d:  ldlen
  IL_001e:  conv.i4
  IL_001f:  rem
  IL_0020:  ldelem.i4
  IL_0021:  ldc.i4.1
  IL_0022:  sub
  IL_0023:  stloc.1

这是计算哈希码的存储区位置.结果保存在本地内存位置1.

AddIfNotPresent执行类似的操作,但它还将计算值保存在位置2,以便在项目不存在时它可以将项目插入到该位置的哈希表中.这样做可以节省,因为其中一个位置稍后会在寻找项目的循环中进行修改.无论如何,这是AddIfNotPresent的相关代码:

IL_0011:  call       instance int32 class System.Collections.Generic.HashSet`1<!T>::InternalGetHashCode(!0)
  IL_0016:  stloc.0
  IL_0017:  ldloc.0
  IL_0018:  ldarg.0
  IL_0019:  ldfld      int32[] class System.Collections.Generic.HashSet`1<!T>::m_buckets
  IL_001e:  ldlen
  IL_001f:  conv.i4
  IL_0020:  rem
  IL_0021:  stloc.1
  IL_0022:  ldarg.0
  IL_0023:  ldfld      int32[] class System.Collections.Generic.HashSet`1<!T>::m_buckets
  IL_0028:  ldloc.0
  IL_0029:  ldarg.0
  IL_002a:  ldfld      int32[] class System.Collections.Generic.HashSet`1<!T>::m_buckets
  IL_002f:  ldlen
  IL_0030:  conv.i4
  IL_0031:  rem
  IL_0032:  ldelem.i4
  IL_0033:  ldc.i4.1
  IL_0034:  sub
  IL_0035:  stloc.2

无论如何,我认为额外的分歧是导致Add比Case包含更多时间的原因.乍一看,似乎可以考虑额外的分歧,但我不能肯定地说,不花一点时间来破译IL.