为什么86 JIT比64更聪明？ [英] Why x86 JIT is smarter than x64?

问题描述

我运行一个非常简单的程序

I'm running a very simple program

    static void Main(string[] args)
    {
        Console.WriteLine(Get4S());
        Console.WriteLine(Get4());
    }

    private static int Get4S()
    {
        return 4;
    }

    private static int Get4()
    {
        int res = 0;
        for (int i = 0; i < 4; i++)
        {
            res++;
        }
        return res;
    }

当它工作在 86 它内联 Get4S 方法和 Get4 ASM code是：

when it works under x86 it inlines Get4S method and Get4 asm code is:

00000000  push        ebp 
00000001  mov         ebp,esp 
00000003  xor         eax,eax 
00000005  inc         eax 
00000006  inc         eax 
00000007  inc         eax 
00000008  inc         eax 
00000009  pop         ebp 
0000000a  ret 

但在x64上运行时，我们得到 Get4S 方法，但 Get4 ASM完全不优化相同的ASM

BUT when running under x64 we get same asm for Get4S method, but Get4 asm is not optimized at all:

00000000  xor         eax,eax 
00000002  xor         edx,edx 
00000004  inc         eax 
00000006  inc         edx 
00000008  cmp         edx,4 
0000000b  jl          0000000000000004 
0000000d  ret 

我假定 64 JIT展开循环，然后看到结果可以编译时进行计算，并与编译时的结果功能将被内联。但它没有happend。

I supposed that x64 JIT unroll the loop, then see that result can be computed in compile-time, and function with compile-time result will be inlined. But nothing from it happend.

为什么 64 在这种情况下如此愚蠢？..

Why x64 is so stupid in this case?..

推荐答案

我得到了这一点。这是因为当选择 64 创建被用于RyuJIT，即使的.Net 4.5.2 目标平台选择。所以我通过添加app.config文件本节固定它：

I got the point. It's because RyuJIT is used when x64 build is selected, even if .Net 4.5.2 target platform is selected. So i fixed it by adding this section in App.Config file:

<configuration>
  <runtime>
    <useLegacyJit enabled="1" />
  </runtime>
</configuration>

此标记使传统的x64 JIT（加引号，是因为我觉得他比闪亮RyuJIT好得多），并在main方法的结果ASM是：

This markup enables "legacy" x64 JIT (in quotes, because I think he's much better than "shiny" RyuJIT), and result ASM in main method is:

00000000  sub         rsp,28h 
00000004  mov         ecx,4 
00000009  call        000000005EE75870 
0000000e  mov         ecx,4 
00000013  call        000000005EE75870 
00000018  nop 
00000019  add         rsp,28h 
0000001d  ret 

这两种方法是在编译时计算和他们的价值观内联。

both methods was calculated in compile time and inlined by theirs values.

结论：当净4.6 安装，旧的x64抖动是由 RyuJIT 替换下的所有解决方案 CLR4.0 。因此，只有将其关闭的方法是 useLegacyJit 开关或 COMPLUS_AltJit 环境变量

Conclusion: When .Net 4.6 is installed, old x64 jitter is replaced by RyuJIT for all solutions under CLR4.0. So only way to turn it off is useLegacyJit switch or COMPLUS_AltJit environment variable

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