Re: loader breaks with -O2 optimizations

On Thu, Nov 01, 2007 at 08:27:26AM +0100, Stephan F. Yaraghchi wrote:
> On 10/31/07, Kris Kennaway <kris_at_freebsd.org> wrote:
> > Stephan F. Yaraghchi wrote:
> > > On 10/31/07, Kris Kennaway <kris_at_freebsd.org> wrote:
> > >> Stephan F. Yaraghchi wrote:
> > >>> After making world on a freshly installed 7.0-BETA1
> > >>> the system does not boot anymore due to a broken loader:
> > >>>
> > >>>
> > >>> FreeBSD/i386 bootstrap loader, Revision 1.1
> > >>> (root_at_bigblue, Tue Oct 30 11:26:32 CET 2007)
> > >>> Can't work out which disk we are booting from.
> > >>> Guessed BIOS device 0xffffffff not found by probes defaulting to disk0:
> > >>>
> > >>> panic: free: guard1 fail_at_ 0x6ded4 from
> > >>> /usr/src/sys/boot/i386/loader/../../common/module.c:959
> > >>>
> > >>> --> Press a key on the console to reboot <--
> > >>>
> > >>>
> > >>> I found out that the following line in my make.conf causes
> > >>> the problem:
> > >>>
> > >>> CFLAGS= -O2 -funroll-loops -pipe
> > >>>
> > >>>
> > >>> After changing down to -O1 and making /usr/src/sys/boot again
> > >>> the systems behaves properly at boot.
> > >>>
> > >>> Is this behavior intended?
> > >> Maybe, what happens if you use just -O2 -pipe? -funroll-loops is not an
> > >> appropriate thing to be using globally anyway, unless your intention is
> > >> to randomly make some code slower.
> > >>
> > >> Kris
> > >>
> > >
> > > Hi Kris,
> > >
> > > I tried all possible combinations of these switches -- only -O2 led to
> > > the described
> > > behaviour.
> >
> > Presumably you mean -O2 -funroll-loops, not -O2.  Or are you saying the
> > latter also breaks the loader?
> >
> > > Anyway, it's very interesting to hear that adding these optimizations
> > > to make.conf
> > > is not recommended, even that -funroll-loops is possibly slowing down
> > > certain code.
> > >
> > > I'm sure many people use it since it's a common tuning tip found on the web.
> > > I read about it in Dru Lavigne's "BSD Hacks" (O'Reilly)...
> >
> > Yes, unfortunately it's bogus advice.  What does she say this option is
> > good for?
> >
> > Kris
> >
> 
> Hi Kris,
> 
> you are right: It's the combination of both that causes loader to break.
> 
> Oliver suggested the defaults (-O2 -fno-strict-aliasing -pipe) and it works.
> 
> By the way: "BSD Hacks" is a collection of hacks compiled by Dru.
> Hack #69 deals with tuning and is authored by Avleen Vig.
> It says: "The -funroll-loops saves one CPU register that would otherwise
> be tied up in tracking the iteration of the loop, but at the expense of making
> a slightly larger binary."
> 
> -- 
> Stephan
> 

There's a good thread explaining why -funroll-loops is generally bad and
why it breaks the loader at
http://lists.freebsd.org/pipermail/freebsd-stable/2006-September/028340.html

--
Bruce