> On Wed, Sep 13, 2006 at 12:17:15PM +0300, Stefan Lambrev wrote:
> > Hello,
> >
> > Oliver Fromme wrote:
> > >Marc G. Fournier <[EMAIL PROTECTED]> wrote:
> > > > What are ppl currently using for CFLAGS/COPTFLAGS in /etc/make.conf for
>
> > > > building kernel/world? I know awhile back it wasn't recommended to go
> > > > above -O2, for instance, but suspect that has changed ... ?
> > >
> > >The best optimization is probably to not override the
> > >defaults at all, because they're already pretty optimal.
> > >In fact, by overriding the defaults there's a good chance
> > >to make things worse. :-)
> > >
> > >The default CFLAGS are "-O2 -pipe -fno-strict-aliasing".
> > >Anything above -O2 isn't supported, and using -O2 without
> > >-fno-strict-aliasing also isn't supported (and will create
> > >broken code for some programs). A common mistake is to
> > >specify CFLAGS="-O2 -pipe" and omit -fno-strict-aliasing.
> > >That'll shot you in the foot sooner or later.
> > >
> > >Best regards
> > > Oliver
> > >
> > >
> > May be default flags have to be set here:
> > /usr/src/share/examples/etc/make.conf ?
> > I'm asking because in this file I read:
> >
> > # CFLAGS controls the compiler settings used when compiling C code.
> > # Note that optimization settings other than -O and -O2 are not recommended
> > # or supported for compiling the world or the kernel - please revert any
> > # nonstandard optimization settings to "-O" or -O2 before submitting bug
> > # reports without patches to the developers.
> > #
> > #CFLAGS= -O -pipe
> >
> > May be "-fno-strict-aliasing" have to be added here then ?
> >
>
>
> A couple of things. Will having gcc unroll loops have any
> negative consequences? (I can't imagine how:: but better
> informed than to have something crash inexplicability.)
> With 6.X safe at -O2 and with -funroll-loops, that should be
> a slight gain, right? (It also will make an upgrade from 5.5
> to 6.[12] that much more rational.)
>
> [Dumb] questions:: first, what does the compiler do with
> "-fno-strict-aliasing"? And is there any guess, any SWAG even,
> on when FreeBSD will safe with -O3??
Lots of code is not strict-aliasing safe. Gcc itself can't
determine all the cases which a construct is not strict-aliasing
safe so even after getting rid of all the warnings gcc
produces you can't be sure your code is strict-aliasing
safe. Think of -fstrict-aliasing as optimisation without
a saftey net. If your code doesn't cast pointers you should
be safe otherwise you need to be really, really, really
careful when you turn this on.
> thanks, people,
>
> gary
>
> > --
> > Best Wishes,
> > Stefan Lambrev
> > ICQ# 24134177
> >
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>
> --
> Gary Kline [EMAIL PROTECTED] www.thought.org Public service Unix
>
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