是否可以告诉分支预测器跟随分支的可能性有多大? [英] Is it possible to tell the branch predictor how likely it is to follow the branch?

问题描述

为了清楚起见，我不会在这里寻求任何形式的可移植性，因此任何将我绑定到某个盒子的解决方案都可以.

Just to make it clear, I'm not going for any sort of portability here, so any solutions that will tie me to a certain box is fine.

基本上，我有一个 if 语句，该语句在 99% 的时间评估为真，并且我试图勉强维持每一个性能时钟，我可以发出某种编译器命令吗(使用 GCC 4.1.2 和 x86ISA，如果重要的话)告诉分支预测器它应该为那个分支缓存?

Basically, I have an if statement that will 99% of the time evaluate to true, and am trying to eke out every last clock of performance, can I issue some sort of compiler command (using GCC 4.1.2 and the x86 ISA, if it matters) to tell the branch predictor that it should cache for that branch?

推荐答案

是的.http://kerneltrap.org/node/4705

__builtin_expect 是一种方法gcc(版本 >= 2.96)提供程序员指示分支预测信息编译器.的返回值__builtin_expect 是第一个参数(只能是整数)传递给它.

The __builtin_expect is a method that gcc (versions >= 2.96) offer for programmers to indicate branch prediction information to the compiler. The return value of __builtin_expect is the first argument (which could only be an integer) passed to it.

if (__builtin_expect (x, 0))
                foo ();

     [This] would indicate that we do not expect to call `foo', since we
     expect `x' to be zero. 

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