Re: [HACKERS] Do we need the gcc feature "__builtin_expect" topromote the branches prediction?

Hi all, 

       There is a lot of "if statement" in system, and GCC provides a feature,"__builtin_expect", which  let compilers know which branch is mostly run.

. as we known, miss-prediction will lead the performance lost(because the CPU will thrown away some instructions, and re-fetch some new instructions). so that we can tell GCC how produce more efficient code. for example as following. 

It will gain performance promotion i think. As i know, the in Linux kernel, this feature is also applied already. 

 #define likely(cond)     __builtin_expect(cond,true)

#define unlikely(cond)  __builtin_expect(cond,false)

if (likely(cond)) {

//most likely run.

   xxxx

} else //otherwise.

{

   xxxx

}

Best Regards.

Hom.