Re: Do we need the gcc feature "__builtin_expect" to promote the branches prediction?

Hi,

On 2017-06-02 16:40:56 +0800, Hao Lee wrote:
> 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
> }

We already do this in a few cases, that are performance critical enough
to matter. But in most cases the CPUs branch predictor does a good
enough job on its own.

- Andres