是否有一个编译器提示GCC强制分支预测总是以一定的方式？ [英] Is there a compiler hint for GCC to force branch prediction to always go a certain way?

问题描述

对于Intel架构，有没有办法指示GCC编译器生成始终在我的代码中强制分支预测的代码？英特尔硬件甚至支持这一点吗？其他编译器或硬件如何？

我将在C ++代码中使用它，我知道这种情况我希望运行快，不关心慢下来，

  for（;;）{
 if正常）{//如何告诉编译器总是分支预测真实值？ 
 doSomethingNormal（）; 
} else {
 exceptionCase（）; 
} 
} 
  

---

作为对Evdzhan Mustafa的一个后续问题，提示只是指定一个提示，第一次处理器遇到指令，所有后续的分支预测，正常运作？

解决方案

GCC支持函数 __ builtin_expect（long exp，long c）来提供这种功能。您可以此处查看文档。

其中 exp 是使用的条件， c 是预期值。例如，在你的情况下，你会需要

  if（__builtin_expect（normal，1））
  

由于尴尬的语法，这通常用于定义两个自定义宏，例如

  #define likely（x）__builtin_expect（!!（x），1）
 #define unlikely（x）__builtin_expect / code> 
 
 
 
。
 
 
 
  ：        strong>编译器/ cpu分支预测器可能比你在决定这样的事情更熟练，因此这可能是一个未成熟的微优化  
 
 
For the Intel architectures, is there a way to instruct the GCC compiler to generate code that always forces branch prediction a particular way in my code?  Does the Intel hardware even support this?  What about other compilers or hardwares?


I would use this in C++ code where I know the case I wish to run fast and do not care about the slow down when the other branch needs to be taken even when it has recently taken that branch.
for (;;) {
  if (normal) { // How to tell compiler to always branch predict true value?
    doSomethingNormal();
  } else {
    exceptionalCase();
  }
}




As a follow on question for Evdzhan Mustafa, can the hint just specify a hint for the first time the processor encounters the instruction, all subsequent branch prediction, functioning normally?
解决方案
GCC supports the function __builtin_expect(long exp, long c) to provide this kind of feature. You can check the documentation here.

Where exp is the condition used and c is the expected value. For example in you case you would want
if (__builtin_expect(normal, 1))
Because of the awkward syntax this is usually used by defining two custom macros like
#define likely(x)    __builtin_expect (!!(x), 1)
#define unlikely(x)  __builtin_expect (!!(x), 0)
just to ease the task.

Mind that:

this is non standard
a compiler/cpu branch predictor are likely more skilled than you in deciding such things so this could be a premature micro-optimization


                        
这篇关于是否有一个编译器提示GCC强制分支预测总是以一定的方式？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！