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

and another things come into my mind, in some scenario, as we know that the
native library is not the most effective way to do that, such as,
allocation a large amount of memories by using "alloc()"... and
"memmove()", so on. As the SIMD instruction became the standard in CPU,
therefore, we can that to do something more effectively. for example, using
SIMD to impl 'alloc',"memmove" and so on, or the other database operations.
and it seems that there some one have done this job.

https://pdfs.semanticscholar.org/eff8/5ca6394eb2ed1e48fe48aa73d34d5a14760c.pdf

On Fri, Jun 2, 2017 at 11:32 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:

> 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
>