On Tue, Feb 19, 2008 at 10:28:46AM +0100, Andi Kleen wrote: > > Sometimes, for performance critical paths, I would like gcc to be dumb and > > follow *my* code and not its hard-coded probabilities. > > If you really want that, simple: just disable optimization @)
already tried. It fixed some difficulties, but create new expected issues with data being reloaded often from memory instead of being passed along a few registers. Don't forget that optimizing for x86 requires a lot of smartness from the compiler because of the very small number of registers! > > 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. > > Just use profile feedback then for user space. I don't think it's a good > idea for kernel code though because it leads to unreproducible binaries > which would wreck the development model. I never found this to be practically usable in fact, because you have to use it on the *exact* same source. End of game for cross-compilation. It would be good to be able to use a few pragmas in the code to say "hey, I want this block optimized like this". This is what I understood the __builtin_expect() was for, except that it tends to throw unpredicted branches too far away. > > Hmm I've just noticed -fno-guess-branch-probability in the man, I never > > tried > > it. > > Or -fno-reorder-blocks Thanks for the hint, I will try it. Willy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
