Messages in this thread | | | Date | Sat, 10 Jan 2009 02:08:51 +0100 | From | Andi Kleen <> | Subject | Re: [patch] measurements, numbers about CONFIG_OPTIMIZE_INLINING=y impact |
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> I thought -Os actually disabled the basic-block reordering, doesn't it?
Not in current gcc head no (just verified by stepping through)
> > And I thought it did that exactly because it generates bigger code and > much worse I$ patterns (ie you have a lot of "conditional branch to other > place and then unconditional branch back" instead of "conditional branch > over the non-taken code". > > Also, I think we've had about as much good luck with guessing > "likely/unlikely" as we've had with "inline" ;)
That's true.
But if you look at the default heuristics that gcc has (gcc/predict.def in the gcc sources) like == NULL, < 0, branch guarding etc. I would expect a lot of them to DTRT for the kernel.
Honza at some point even fixed goto to be unlikely after I complained :)
> Sadly, apart from some of the "never happens" error cases, the kernel > doesn't tend to have lots of nice patterns. We have almost no loops (well, > there are loops all over, but most of them we hopefully just loop over > once or twice in any good situation), and few really predictable things.
That actually makes us well suited to gcc, it has a relatively poor loop optimizer compared to other compilers ;-)
> Or rather, they can easily be very predictable under one particular load, > and the totally the other way around under another ..
Yes that is why we got good branch predictors in CPUs I guess.
-Andi -- ak@linux.intel.com
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