我可以给编译器/ JIT提供哪些优化提示？

Question

我已经介绍过了,现在我正在寻找可以从我的热点中挤出一切可能的性能.

我知道[MethodImplOptions.AggressiveInlining]和ProfileOptimization类.还有其他人吗？

---

[编辑] 我刚刚发现了[TargetedPatchingOptOut].没关系,显然不需要一个.

Answer 1

您已经用尽了.NET 4.5中添加的选项以直接影响jitted代码.下一步是查看生成的机器代码,以发现任何明显的低效率.使用调试器执行此操作,首先阻止它禁用优化程序.工具+选项,调试,常规,取消选中"抑制模块加载时的JIT优化"选项.在热代码上设置一个断点,Debug + Disassembly来查看它.

没有那么多要考虑,抖动优化器通常做得很好.要查找的一件事是尝试消除数组边界检查失败,固定关键字是一种不安全的解决方法.一个极端情况是内联方法的失败尝试和不能有效使用cpu寄存器的抖动,这是x86抖动的一个问题,并使用MethodImplOptions.NoInlining修复.优化器在将不变代码从循环中提升时并不是非常有效,但在寻找优化它的方法时,这几乎总是首先要考虑的是C#代码.

想要知道的最重要的事情就是当你完成并且不能希望更快地完成它.您只能通过比较苹果和橙子以及使用C++/CLI在本机代码中编写热代码来实现目标.确保此代码是使用#pragma unmanaged生成的,因此它获得了完整的优化器爱.从托管代码切换到本机代码执行会产生相关成本,因此请确保本机代码的执行时间足够长.否则这不一定容易做到,你肯定无法保证成功.虽然知道你已经完成了可以节省你很多时间绊倒死胡同.

Answer 2

是的还有更多技巧:-)

我实际上已经做了很多关于优化C#代码的研究.到目前为止,这些是最重要的结果:

1. 直接传递的Func和Action通常由JIT'ter内联.请注意,您不应将它们存储为变量,因为它们将被称为委托.有关更多详细信息,请参阅此帖子.
2. 注意过载.不使用调用Equals IEquatable<T>通常是一个糟糕的计划 - 所以如果你使用f.ex. 哈希,一定要实现正确的重载和接口,因为它可以保证你的性能.
3. 从其他类调用的泛型从不内联.这样做的原因是这里概述的"魔术" .
4. 如果你使用数据结构,请确保尝试使用数组:-)真的,这些东西很快就像......好吧,几乎我想的任何东西.我通过使用自己的哈希表并使用数组而不是列表来优化了很多东西.
5. 在很多情况下,表查找比计算事物或使用vtable查找,开关,多个if语句甚至计算等构造更快.如果你有分支机构,这也是一个好方法; 失败的分支预测往往会成为一个巨大的痛苦.另请参阅这篇文章 - 这是我在C#中使用的很多技巧,它在很多情况下都很有效.哦,查找表当然是数组.
6. 尝试制作(小)类结构.由于值类型的性质,结构的一些优化不同于类的优化.例如,方法调用更简单,因为编译器确切地知道要调用的方法.结构数组通常比类数组更快,因为它们每个数组操作需要少1个内存操作.
7. 不要使用多维数组.虽然我更喜欢Foo[],但Foo[][]通常比...更快Foo[,].
8. 如果您正在复制数据,则在一周中的任何一天都优先使用Array.Copy上的Buffer.BlockCopy.对字符串也要谨慎:字符串操作可能是一个性能耗尽者.

还有一个名为"针对英特尔奔腾处理器的优化"的指南,其中包含大量技巧(如移位或乘法而非分割).虽然编译器现在做了很多努力,但这有时也会有所帮助.

当然这些只是优化; 最大的性能提升通常是改变算法和/或数据结构的结果.请务必查看哪些选项可供您使用,并且不要过多地限制.NET框架......在我自己检查反编译代码之前,我自然倾向于不信任.NET实现. ..有很多东西可以更快地实现(大多数情况下有充分理由).

HTH

亚历克斯向我指出,Array.Copy根据一些人的说法,这实际上更快.而且由于我真的不知道这些年来发生了什么变化,我认为唯一正确的做法是创造一个全新的基准并进行测试.

如果你只是对结果感兴趣,那就去吧.在大多数情况下,呼叫Buffer.BlockCopy明显优于大家Array.Copy.在.NET 4.5.2上使用16 GB内存(> 10 GB可用)的Intel Skylake进行测试.

码:

static void TestNonOverlapped1(int K)
{
    long total = 1000000000;
    long iter = total / K;
    byte[] tmp = new byte[K];
    byte[] tmp2 = new byte[K];
    for (long i = 0; i < iter; ++i)
    {
        Array.Copy(tmp, tmp2, K);
    }
}

static void TestNonOverlapped2(int K)
{
    long total = 1000000000;
    long iter = total / K;
    byte[] tmp = new byte[K];
    byte[] tmp2 = new byte[K];
    for (long i = 0; i < iter; ++i)
    {
        Buffer.BlockCopy(tmp, 0, tmp2, 0, K);
    }
}

static void TestOverlapped1(int K)
{
    long total = 1000000000;
    long iter = total / K;
    byte[] tmp = new byte[K + 16];
    for (long i = 0; i < iter; ++i)
    {
        Array.Copy(tmp, 0, tmp, 16, K);
    }
}

static void TestOverlapped2(int K)
{
    long total = 1000000000;
    long iter = total / K;
    byte[] tmp = new byte[K + 16];
    for (long i = 0; i < iter; ++i)
    {
        Buffer.BlockCopy(tmp, 0, tmp, 16, K);
    }
}

static void Main(string[] args)
{
    for (int i = 0; i < 10; ++i)
    {
        int N = 16 << i;

        Console.WriteLine("Block size: {0} bytes", N);

        Stopwatch sw = Stopwatch.StartNew();

        {
            sw.Restart();
            TestNonOverlapped1(N);

            Console.WriteLine("Non-overlapped Array.Copy: {0:0.00} ms", sw.Elapsed.TotalMilliseconds);
            GC.Collect(GC.MaxGeneration);
            GC.WaitForFullGCComplete();
        }

        {
            sw.Restart();
            TestNonOverlapped2(N);

            Console.WriteLine("Non-overlapped Buffer.BlockCopy: {0:0.00} ms", sw.Elapsed.TotalMilliseconds);
            GC.Collect(GC.MaxGeneration);
            GC.WaitForFullGCComplete();
        }

        {
            sw.Restart();
            TestOverlapped1(N);

            Console.WriteLine("Overlapped Array.Copy: {0:0.00} ms", sw.Elapsed.TotalMilliseconds);
            GC.Collect(GC.MaxGeneration);
            GC.WaitForFullGCComplete();
        }

        {
            sw.Restart();
            TestOverlapped2(N);

            Console.WriteLine("Overlapped Buffer.BlockCopy: {0:0.00} ms", sw.Elapsed.TotalMilliseconds);
            GC.Collect(GC.MaxGeneration);
            GC.WaitForFullGCComplete();
        }

        Console.WriteLine("-------------------------");
    }

    Console.ReadLine();
}

x86 JIT上的结果:

Block size: 16 bytes
Non-overlapped Array.Copy: 4267.52 ms
Non-overlapped Buffer.BlockCopy: 2887.05 ms
Overlapped Array.Copy: 3305.01 ms
Overlapped Buffer.BlockCopy: 2670.18 ms
-------------------------
Block size: 32 bytes
Non-overlapped Array.Copy: 1327.55 ms
Non-overlapped Buffer.BlockCopy: 763.89 ms
Overlapped Array.Copy: 2334.91 ms
Overlapped Buffer.BlockCopy: 2158.49 ms
-------------------------
Block size: 64 bytes
Non-overlapped Array.Copy: 705.76 ms
Non-overlapped Buffer.BlockCopy: 390.63 ms
Overlapped Array.Copy: 1303.00 ms
Overlapped Buffer.BlockCopy: 1103.89 ms
-------------------------
Block size: 128 bytes
Non-overlapped Array.Copy: 361.18 ms
Non-overlapped Buffer.BlockCopy: 219.77 ms
Overlapped Array.Copy: 620.21 ms
Overlapped Buffer.BlockCopy: 577.20 ms
-------------------------
Block size: 256 bytes
Non-overlapped Array.Copy: 192.92 ms
Non-overlapped Buffer.BlockCopy: 108.71 ms
Overlapped Array.Copy: 347.63 ms
Overlapped Buffer.BlockCopy: 353.40 ms
-------------------------
Block size: 512 bytes
Non-overlapped Array.Copy: 104.69 ms
Non-overlapped Buffer.BlockCopy: 65.65 ms
Overlapped Array.Copy: 211.77 ms
Overlapped Buffer.BlockCopy: 202.94 ms
-------------------------
Block size: 1024 bytes
Non-overlapped Array.Copy: 52.93 ms
Non-overlapped Buffer.BlockCopy: 38.84 ms
Overlapped Array.Copy: 144.39 ms
Overlapped Buffer.BlockCopy: 154.09 ms
-------------------------
Block size: 2048 bytes
Non-overlapped Array.Copy: 45.64 ms
Non-overlapped Buffer.BlockCopy: 30.11 ms
Overlapped Array.Copy: 118.33 ms
Overlapped Buffer.BlockCopy: 109.16 ms
-------------------------
Block size: 4096 bytes
Non-overlapped Array.Copy: 30.93 ms
Non-overlapped Buffer.BlockCopy: 30.72 ms
Overlapped Array.Copy: 119.73 ms
Overlapped Buffer.BlockCopy: 104.66 ms
-------------------------
Block size: 8192 bytes
Non-overlapped Array.Copy: 30.37 ms
Non-overlapped Buffer.BlockCopy: 26.63 ms
Overlapped Array.Copy: 90.46 ms
Overlapped Buffer.BlockCopy: 87.40 ms
-------------------------

x64 JIT上的结果:

Block size: 16 bytes
Non-overlapped Array.Copy: 1252.71 ms
Non-overlapped Buffer.BlockCopy: 694.34 ms
Overlapped Array.Copy: 701.27 ms
Overlapped Buffer.BlockCopy: 573.34 ms
-------------------------
Block size: 32 bytes
Non-overlapped Array.Copy: 995.47 ms
Non-overlapped Buffer.BlockCopy: 654.70 ms
Overlapped Array.Copy: 398.48 ms
Overlapped Buffer.BlockCopy: 336.86 ms
-------------------------
Block size: 64 bytes
Non-overlapped Array.Copy: 498.86 ms
Non-overlapped Buffer.BlockCopy: 329.15 ms
Overlapped Array.Copy: 218.43 ms
Overlapped Buffer.BlockCopy: 179.95 ms
-------------------------
Block size: 128 bytes
Non-overlapped Array.Copy: 263.00 ms
Non-overlapped Buffer.BlockCopy: 196.71 ms
Overlapped Array.Copy: 137.21 ms
Overlapped Buffer.BlockCopy: 107.02 ms
-------------------------
Block size: 256 bytes
Non-overlapped Array.Copy: 144.31 ms
Non-overlapped Buffer.BlockCopy: 101.23 ms
Overlapped Array.Copy: 85.49 ms
Overlapped Buffer.BlockCopy: 69.30 ms
-------------------------
Block size: 512 bytes
Non-overlapped Array.Copy: 76.76 ms
Non-overlapped Buffer.BlockCopy: 55.31 ms
Overlapped Array.Copy: 61.99 ms
Overlapped Buffer.BlockCopy: 54.06 ms
-------------------------
Block size: 1024 bytes
Non-overlapped Array.Copy: 44.01 ms
Non-overlapped Buffer.BlockCopy: 33.30 ms
Overlapped Array.Copy: 53.13 ms
Overlapped Buffer.BlockCopy: 51.36 ms
-------------------------
Block size: 2048 bytes
Non-overlapped Array.Copy: 27.05 ms
Non-overlapped Buffer.BlockCopy: 25.57 ms
Overlapped Array.Copy: 46.86 ms
Overlapped Buffer.BlockCopy: 47.83 ms
-------------------------
Block size: 4096 bytes
Non-overlapped Array.Copy: 29.11 ms
Non-overlapped Buffer.BlockCopy: 25.12 ms
Overlapped Array.Copy: 45.05 ms
Overlapped Buffer.BlockCopy: 47.84 ms
-------------------------
Block size: 8192 bytes
Non-overlapped Array.Copy: 24.95 ms
Non-overlapped Buffer.BlockCopy: 21.52 ms
Overlapped Array.Copy: 43.81 ms
Overlapped Buffer.BlockCopy: 43.22 ms
-------------------------