Re: [patch] measurements, numbers about CONFIG_OPTIMIZE_INLINING=y impact

Andi Kleen
2009-01-10T00:54:52Z

> 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