Messages in this thread | | | Date | Wed, 20 Feb 2008 08:32:45 +0100 | From | Willy Tarreau <> | Subject | Re: [PATCH 1/3] Fix Unlikely(x) == y |
| |
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
| |