Re: How to compose kernel makeoptions

[charles <charles.westerman%gmail.com@localhost>]

Thanks Chuck,

so i guess i can ignore/not worry about the makeoptions line in the config
or/and add "-march=pentium3"

and if that's so ... well it was strange to see the new option as a
makeoption instead of what is was before, so i'm making something out of
nothing perhaps.

where do i find the march definitions? is there a master list?

thanks,
charles

On Fri, Feb 25, 2011 at 7:58 PM, Chuck Swiger <cswiger%mac.com@localhost> wrote:

> On Feb 25, 2011, at 3:01 PM, charles wrote:
> > the last time i compiled a kernel i just plugged in an i686.
> >
> > Where do i look for information about makeoptions settings for pentium3,
> or
> > processor, optimization? I understand what the config option is asking
> for
> > but so far can't figure out where to find that information.
>
> The default compiler optimizations are likely to be "-O2
> -fno-strict-aliasing", which were chosen because they are almost certain to
> result in beneficial improvements to performance and code size, most of the
> time.  Potential optimizations which are not helpful on average are not
> enabled by default, until the situations where they are known to be useful
> can be identified by the compiler at compile-time.
>
> Using non-default optimization options isn't like discovering buried
> treasure that nobody else was aware of.  The options aren't enabled by
> default for good reason(s), usually because the tradeoffs they make aren't
> helpful in general (yet), or because their usage has known bugs which result
> in faulty executables being produced.
>
> You can try adding "-march=pentium3", but it is unlikely to result in a
> noticeable difference for macro performance, although it's possible that
> some microbenchmarks might show a more significant difference.
>
> However, one issue that might make some difference is whether there are
> some specialized assembly routines which are chosen based upon the CPU type.
>  I'm more familiar with this in FreeBSD than in NetBSD, ie:
>
>
> http://www.freebsd.org/cgi/cvsweb.cgi/~checkout~/src/sys/i386/i386/support.s
>
> ...where things like bzero, bcopy, copyin/copyout, etc have optimized
> variants will be used if the right CPU type is set.  Perhaps someone more
> familiar with the NetBSD equivalent can offer some thoughts.
>
> Regards,
> --
> -Chuck