我在这里碰壁了,希望你们中的一些人能够教育我。我正在使用BenchmarkDotNet进行性能基准测试,但是遇到了这种奇怪的情况:声明成员为const
会显著降低性能。
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System;
namespace PerfTest
{
[DisassemblyDiagnoser(printAsm: true, printSource: true)]
public class Test
{
private int[] data;
private int Threshold = 90;
private const int ConstThreshold = 90;
[GlobalSetup]
public void GlobalSetup()
{
data = new int[1000];
var random = new Random(42);
for (var i = 0; i < data.Length; i++)
{
data[i] = random.Next(100);
}
}
static void Main(string[] args)
{
var summary = BenchmarkRunner.Run<Test>();
}
[Benchmark(Baseline = true)]
public void ClampToMemberValue()
{
for (var i = 0; i < data.Length; i++)
{
if (data[i] > Threshold) data[i] = Threshold;
}
}
[Benchmark]
public void ClampToConstValue()
{
for (var i = 0; i < data.Length; i++)
{
if (data[i] > ConstThreshold) data[i] = ConstThreshold;
}
}
}
}
请注意,这两种测试方法之间唯一的区别在于它们是针对普通成员变量还是常量成员进行比较。
根据BenchmarkDotNet的数据,使用常量值会明显降低性能,我不明白为什么会这样。
BenchmarkDotNet=v0.11.5, OS=Windows 10.0.18362
Intel Core i7-5820K CPU 3.30GHz (Broadwell), 1 CPU, 12 logical and 6 physical cores
.NET Core SDK=3.0.100
[Host] : .NET Core 3.0.0 (CoreCLR 4.700.19.46205, CoreFX 4.700.19.46214), 64bit RyuJIT
DefaultJob : .NET Core 3.0.0 (CoreCLR 4.700.19.46205, CoreFX 4.700.19.46214), 64bit RyuJIT
| Method | Mean | Error | StdDev | Ratio |
|------------------- |---------:|---------:|---------:|------:|
| ClampToMemberValue | 590.4 ns | 1.980 ns | 1.852 ns | 1.00 |
| ClampToConstValue | 724.6 ns | 4.184 ns | 3.709 ns | 1.23 |
仅从JIT编译的代码来看,我无法解释它。这里是两种方法的代码。唯一的区别在于比较是针对寄存器还是字面量。
00007ff9`7f1b8500 PerfTest.Test.ClampToMemberValue()
for (var i = 0; i < data.Length; i++)
^^^^^^^^^
00007ff9`7f1b8504 33c0 xor eax,eax
for (var i = 0; i < data.Length; i++)
^^^^^^^^^^^^^^^
00007ff9`7f1b8506 488b5108 mov rdx,qword ptr [rcx+8]
00007ff9`7f1b850a 837a0800 cmp dword ptr [rdx+8],0
00007ff9`7f1b850e 7e2e jle 00007ff9`7f1b853e
00007ff9`7f1b8510 8b4910 mov ecx,dword ptr [rcx+10h]
if (data[i] > Threshold) data[i] = Threshold;
^^^^^^^^^^^^^^^^^^^^^^^^
00007ff9`7f1b8513 4c8bc2 mov r8,rdx
00007ff9`7f1b8516 458b4808 mov r9d,dword ptr [r8+8]
00007ff9`7f1b851a 413bc1 cmp eax,r9d
00007ff9`7f1b851d 7324 jae 00007ff9`7f1b8543
00007ff9`7f1b851f 4c63c8 movsxd r9,eax
00007ff9`7f1b8522 43394c8810 cmp dword ptr [r8+r9*4+10h],ecx
00007ff9`7f1b8527 7e0e jle 00007ff9`7f1b8537
if (data[i] > Threshold) data[i] = Threshold;
^^^^^^^^^^^^^^^^^^^^
00007ff9`7f1b8529 4c8bc2 mov r8,rdx
00007ff9`7f1b852c 448bc9 mov r9d,ecx
00007ff9`7f1b852f 4c63d0 movsxd r10,eax
00007ff9`7f1b8532 47894c9010 mov dword ptr [r8+r10*4+10h],r9d
for (var i = 0; i < data.Length; i++)
^^^
00007ff9`7f1b8537 ffc0 inc eax
00007ff9`7f1b8539 394208 cmp dword ptr [rdx+8],eax
00007ff9`7f1b853c 7fd5 jg 00007ff9`7f1b8513
}
^
00007ff9`7f1b853e 4883c428 add rsp,28h
并且
00007ff9`7f1a8500 PerfTest.Test.ClampToConstValue()
for (var i = 0; i < data.Length; i++)
^^^^^^^^^
00007ff9`7f1a8504 33c0 xor eax,eax
for (var i = 0; i < data.Length; i++)
^^^^^^^^^^^^^^^
00007ff9`7f1a8506 488b5108 mov rdx,qword ptr [rcx+8]
00007ff9`7f1a850a 837a0800 cmp dword ptr [rdx+8],0
00007ff9`7f1a850e 7e2d jle 00007ff9`7f1a853d
if (data[i] > ConstThreshold) data[i] = ConstThreshold;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
00007ff9`7f1a8510 488bca mov rcx,rdx
00007ff9`7f1a8513 448b4108 mov r8d,dword ptr [rcx+8]
00007ff9`7f1a8517 413bc0 cmp eax,r8d
00007ff9`7f1a851a 7326 jae 00007ff9`7f1a8542
00007ff9`7f1a851c 4c63c0 movsxd r8,eax
00007ff9`7f1a851f 42837c81105a cmp dword ptr [rcx+r8*4+10h],5Ah
00007ff9`7f1a8525 7e0f jle 00007ff9`7f1a8536
if (data[i] > ConstThreshold) data[i] = ConstThreshold;
^^^^^^^^^^^^^^^^^^^^^^^^^
00007ff9`7f1a8527 488bca mov rcx,rdx
00007ff9`7f1a852a 4c63c0 movsxd r8,eax
00007ff9`7f1a852d 42c74481105a000000 mov dword ptr [rcx+r8*4+10h],5Ah
for (var i = 0; i < data.Length; i++)
^^^
00007ff9`7f1a8536 ffc0 inc eax
00007ff9`7f1a8538 394208 cmp dword ptr [rdx+8],eax
00007ff9`7f1a853b 7fd3 jg 00007ff9`7f1a8510
}
^
00007ff9`7f1a853d 4883c428 add rsp,28h
我相信我肯定忽略了什么,但我目前无法理解它,因此我正在寻求关于如何解释这个问题的建议。
cmp
和mov
指令用于常量路径,占用的内存比基于寄存器的指令多,因为编码数字需要额外的字节,并且总共需要更多的CPU周期来执行(对于mov
是9个字节对5个字节,对于cmp
是6个字节对5个字节)。即使对于非常量版本有额外的mov ecx,dword ptr [rcx+10h]
指令,但在发布版本中,它很可能被JIT编译器优化为循环之外。 - Dmytro Mukalovvolatile
)明确指定。但在这种特殊情况下,即使这样做也没有关系,因为已经完成了 - 我第一次忽略了这一点(ecx
只初始化一次)。因此,总的来说,constcmp
和mov
迭代指令比它们的非const对应指令更重要。 - Dmytro Mukalov