Re: Kernel GCC Optimizations

[Arnaldo Carvalho de Melo <acme@conectiva.com.br>]

Em Sat, Dec 21, 2002 at 07:20:28PM -0600, scott thomason escreveu:
> On Saturday 21 December 2002 04:10 pm, folkert@vanheusden.com wrote:
> > > > Is there any risk using -O3 instead of -O2 to compile the
> > > > kernel, and why?
> > >
> > > * It might uncover subtle bugs that would otherwise not occur.
> >
> > I wonder: for the sake of performance and good use of the precious
> > clock- cycles, shouldn't there be made a start of fixing those
> > bugs? Assuming that the bugs you're talking about are not
> > compiler-bugs, they *are* bugs in the code that should be fixed,
> > shouldn't they?
> >
> > > * Compiling with unusual options means that less people will know
> > > about any problems it causes you.
> >
> > So, let's make it -O6 per default for 2.7.x/3.1.x?
> 
> Let's not. I'd rather have the best kernel developers concentrating on 
> finishing important kernel features rather than digging their way out 
> of esoteric optimizer debugging sessions only to find it was a flaw 
> in gcc. The difference in performance boost between -O2 and greater 
> levels isn't usually enough to make a significant impact, not as 
> significant as the introduction of important new features, for 
> example.

Sometimes even _reducing_ the optimization for performance level makes it
faster, try with -Os. And this was already discussed here and elsewhere,
reading the archives would help a lot avoiding adding more noise to the list.

- Arnaldo