Okay. Thanks On Fri, Jun 2, 2017 at 7:27 PM, Julien Rouhaud <julien.rouh...@dalibo.com> wrote:
> On 02/06/2017 12:50, Craig Ringer wrote: > > > > > > On 2 Jun. 2017 16:42, "Hao Lee" <mixt...@gmail.com > > <mailto:mixt...@gmail.com>> 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. > > > > > > Compilers and CPUs are really good at guessing this. > > > > Humans are wrong about it more than we'd like too. > > +1 > > > > > It's surprisingly rarely s good idea to use branch prediction hints. > > > > See the vsrious Linux kernel discussions about this. > > > > If you find concrete sites of frequent or costly branch mis-prediction > > please point them out, with benchmarks. > > And also see this thread: > https://www.postgresql.org/message-id/CAKJS1f8yqRW3qx2CO9r4bqqvA2Vx6 > 8=3awbh8cjwtp9zxeo...@mail.gmail.com > > BTW Andres added support for likely/unlikely in aa3ca5e3dd6, but its > usage is still really limited. > > -- > Julien Rouhaud > http://dalibo.com - http://dalibo.org >
