On Tuesday 19 February 2008 16:58, Willy Tarreau wrote: > On Tue, Feb 19, 2008 at 01:33:53PM +1100, Nick Piggin wrote: > > > Note in particular the last predictors; assuming branch ending > > > with goto, including call, causing early function return or > > > returning negative constant are not taken. Just these alone > > > are likely 95+% of the unlikelies in the kernel. > > > > Yes, gcc should be able to do pretty good heuristics, considering > > the quite good numbers that cold CPU predictors can attain. However > > for really performance critical code (or really "never" executed > > code), then I think it is OK to have the hints and not have to rely > > on gcc heuristics. > > in my experience, the real problem is that gcc does what *it* wants and not > what *you* want. I've been annoyed a lot by the way it coded some loops > that could really be blazingly fast, but which resulted in a ton of > branches due to its predictors. And using unlikely() there was a real mess, > because instead of just hinting the compiler with probabilities to write > some linear code for the *most* common case, it ended up with awful > branches everywhere with code sent far away and even duplicated for some > branches. > > Sometimes, for performance critical paths, I would like gcc to be dumb and > follow *my* code and not its hard-coded probabilities. For instance, in a > tree traversal, you really know how you want to build your loop. And these > days, it seems like the single method of getting it your way is doing asm, > which obviously is not portable :-(
Probably all true. > Maybe one thing we would need would be the ability to assign probabilities > to each branch based on what we expect, so that gcc could build a better > tree keeping most frequently used code tight. I don't know if that would *directly* lead to gcc being smarter. I think perhaps they probably don't benchmark on code bases that have much explicit annotation (I'm sure they wouldn't seriously benchmark any parts of Linux as part of daily development). I think the key is to continue to use annotations _properly_, and eventually gcc should go in the right direction if enough code uses it. And if you have really good examples like it sounds like above, then I guess that should be reported to gcc? _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@ozlabs.org https://ozlabs.org/mailman/listinfo/linuxppc-dev
