在C ...的If-Else和Ternary运算符之间的速度差异？ [英] Speed difference between If-Else and Ternary operator in C...?

问题描述

所以在一个同事的建议下，我只是测试了三元运算符和等价的If-Else块之间的速度差异...并且似乎三元运算符产生的代码在1x和2x之间比If-其他。我的代码是：

  gettimeofday（& tv3，0）; 
 for（i = 0; i  {
 a = i& 1; 
 if（a）a = b; else a = c; 
} 
 gettimeofday（& tv4，0）; 
 
 
 gettimeofday（& tv1，0）; 
 for（i = 0; i  {
 a = i& 1; 
 a = a？ b：c; 
} 
 gettimeofday（& tv2，0）; 
  

（对不起，使用gettimeofday而不是clock_gettime ...我会努力改善自己。 p>

我尝试更改块的时间顺序，但结果似乎持续。是什么赋予了？此外，If-Else在执行速度方面显示出更多的可变性。我应该检查gcc生成的程序集吗？

顺便说一句，这是在优化级别零（-O0）。 p>

我是想象这个，还是有一些我没有考虑到的，或者这是一个机器相关的东西，还是什么？

解决方案

很有可能三元运算符被编译成 cmov ，而if / else结果在 cmp + jmp 。只需看看程序集（使用-S）就可以了。启用优化后，无论如何，任何更好的编译器都应该在两种情况下产生相同的代码。

So at the suggestion of a colleague, I just tested the speed difference between the ternary operator and the equivalent If-Else block... and it seems that the ternary operator yields code that is between 1x and 2x faster than If-Else. My code is:

  gettimeofday(&tv3, 0);
  for(i = 0; i < N; i++)
  {
     a = i & 1;
     if(a) a = b; else a = c;
  }
  gettimeofday(&tv4, 0);


  gettimeofday(&tv1, 0);
  for(i = 0; i < N; i++)
  {
     a = i & 1;
     a = a ? b : c;
  }
  gettimeofday(&tv2, 0);

(Sorry for using gettimeofday and not clock_gettime... I will endeavor to better myself.)

I tried changing the order in which I timed the blocks, but the results seem to persist. What gives? Also, the If-Else shows much more variability in terms of execution speed. Should I be examining the assembly that gcc generates?

By the way, this is all at optimization level zero (-O0).

Am I imagining this, or is there something I'm not taking into account, or is this a machine-dependent thing, or what? Any help is appreciated.

解决方案

There's a good chance that the ternary operator gets compiled into a cmov while the if/else results in a cmp+jmp. Just take a look at the assembly (using -S) to be sure. With optimizations enabled, it won't matter any more anyway, as any good compiler should produce the same code in both cases.

这篇关于在C ...的If-Else和Ternary运算符之间的速度差异？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！