rwessel (rwessel.delete@this.yahoo.com) on June 6, 2022 5:26 am wrote:
> Peter Lewis (peter.delete@this.notyahoo.com) on June 6, 2022 3:55 am wrote:
> > > There's PGO of course, but that really only works for interpreted or jitted languages.
> >
> > The Intel C/C++ Compiler has Profile-Guided Optimization (PGO).
> > Have you had some bad experience with PGO for compiled languages?
>
>
> The fundamental problem with PGO is that it's (probably fundamentally) too hard to use the
> vast majority of the time, at least with languages compiled in the traditional way.
>
> Unless you have simple to generate (and maintain!*) training datasets, have a very small area
> where you can separately apply PGO (and can make the training sets small enough to make them
> maintainable), or you can afford to invest in a quite large infrastructure to use and maintain
> PGO (because, say, you have an absurd number of machine on which you're going to run this code
> - consider Google), PGO is just too hard to use, and so is useless 99% of the time.
>
> As Peter pointed out, JIT'd languages can take advantage of PGO as well.
>
> *Code with limited lifespan, or code you somehow know isn't going to
> change in the future, can reduce the maintenance requirements here.


Has anyone ever tried having the CPU's branch predictor collecting info the OS can use to 'update' binaries with branch prediction info?

Having data that is generated by the end user tweak their binaries' branch prediction seems like a better solution than having the developer try to come up with training data for the PGO phase. I know PGO does more than just predict branches, but in the case just limiting it to the topic at hand.

I'm not sure how it would be implemented - you probably wouldn't actually update the binary itself (it may be on read only storage or shared by others) so there would need to be some sort of auxiliary data file (maybe stored somewhere like /var/lib or in the user's home directory / profile?) that would be used to tweak things when the executable is loaded.

Just an idle thought here, I haven't really considered it more than the few minutes than it took to write this post so I could be missing some really big gotchas with this idea!