c# - 什么 VS 调试器使增量运算符执行得比什么都不做更快？

Question

这是不言自明的代码(执行十亿次操作):

int k = 0;

Stopwatch sw = new Stopwatch();
sw.Start();
for (int a = 0; a < 1000; a++)
    for (int b = 0; b < 1000; b++)
        for (int c = 0; c < 1000; c++)
            k++;

sw.Stop();
Console.WriteLine(sw.ElapsedMilliseconds);

sw = new Stopwatch();
sw.Start();

for (int a = 0; a < 1000; a++)
    for (int b = 0; b < 1000; b++)
        for (int c = 0; c < 1000; c++)
            ; // NO-OP

sw.Stop();
Console.WriteLine(sw.ElapsedMilliseconds);

结果(至少在我的电脑上)大约(以毫秒为单位)

2168
2564

第二个总是长半秒左右。

增加一个变量 10 亿次怎么可能比执行相同次数的空操作运行时间更长？

编辑:这只发生在 DEBUG 上。 Release 正确地做到了这一点，第一个持续时间更长，至少在我的电脑上是这样。正如评论中指出的那样，即使在 RELEASE 版本中，也有人遇到过这个问题。但是在 DEBUG 上发生了什么会产生这种效果？

Answer 1

问题如 Azodious 所述，您不能使用 Debug模式来测量时间，因为它不会准确。

开启 Release模式后，我得到以下数字:

递增 k:445

NOP:402

递增版本中还有 4 条 IL 指令:

IL_0001:  ldc.i4.0    
IL_0002:  stloc.0     
IL_0003:  ldc.i4.0    
IL_0004:  stloc.1     
IL_0005:  br.s        IL_003B
IL_0007:  ldc.i4.0    
IL_0008:  stloc.2     
IL_0009:  br.s        IL_0029
IL_000B:  ldc.i4.0    
IL_000C:  stloc.3     
IL_000D:  br.s        IL_0017
IL_000F:  ldloc.0     
IL_0010:  ldc.i4.1    
IL_0011:  add         
IL_0012:  stloc.0     
IL_0013:  ldloc.3     
IL_0014:  ldc.i4.1    
IL_0015:  add         
IL_0016:  stloc.3     
IL_0017:  ldloc.3     
IL_0018:  ldc.i4      E8 03 00 00 
IL_001D:  clt         
IL_001F:  stloc.s     04 
IL_0021:  ldloc.s     04 
IL_0023:  brtrue.s    IL_000F
IL_0025:  ldloc.2     
IL_0026:  ldc.i4.1    
IL_0027:  add         
IL_0028:  stloc.2     
IL_0029:  ldloc.2     
IL_002A:  ldc.i4      E8 03 00 00 
IL_002F:  clt         
IL_0031:  stloc.s     04 
IL_0033:  ldloc.s     04 
IL_0035:  brtrue.s    IL_000B
IL_0037:  ldloc.1     
IL_0038:  ldc.i4.1    
IL_0039:  add         
IL_003A:  stloc.1     
IL_003B:  ldloc.1     
IL_003C:  ldc.i4      E8 03 00 00 
IL_0041:  clt         
IL_0043:  stloc.s     04 
IL_0045:  ldloc.s     04 
IL_0047:  brtrue.s    IL_0007

NOP-verison 有相同数量的分支但少了一个add:

IL_0001:  ldc.i4.0    
IL_0002:  stloc.0     
IL_0003:  ldc.i4.0    
IL_0004:  stloc.1     
IL_0005:  br.s        IL_0037
IL_0007:  ldc.i4.0    
IL_0008:  stloc.2     
IL_0009:  br.s        IL_0025
IL_000B:  ldc.i4.0    
IL_000C:  stloc.3     
IL_000D:  br.s        IL_0013
IL_000F:  ldloc.3     
IL_0010:  ldc.i4.1    
IL_0011:  add         
IL_0012:  stloc.3     
IL_0013:  ldloc.3     
IL_0014:  ldc.i4      E8 03 00 00 
IL_0019:  clt         
IL_001B:  stloc.s     04 
IL_001D:  ldloc.s     04 
IL_001F:  brtrue.s    IL_000F
IL_0021:  ldloc.2     
IL_0022:  ldc.i4.1    
IL_0023:  add         
IL_0024:  stloc.2     
IL_0025:  ldloc.2     
IL_0026:  ldc.i4      E8 03 00 00 
IL_002B:  clt         
IL_002D:  stloc.s     04 
IL_002F:  ldloc.s     04 
IL_0031:  brtrue.s    IL_000B
IL_0033:  ldloc.1     
IL_0034:  ldc.i4.1    
IL_0035:  add         
IL_0036:  stloc.1     
IL_0037:  ldloc.1     
IL_0038:  ldc.i4      E8 03 00 00 
IL_003D:  clt         
IL_003F:  stloc.s     04 
IL_0041:  ldloc.s     04 
IL_0043:  brtrue.s    IL_0007

这些是在没有优化的情况下编译的，因为我想看看到底发生了什么。

它们在现实中的唯一区别是:

IL_0012:  stloc.0     
IL_0013:  ldloc.3     
IL_0014:  ldc.i4.1    
IL_0015:  add  

简单地说:您得到的数字很奇怪，因为您处于 Debug模式。