loader breaks with -O2 optimizations

[Stephan F. Yaraghchi]

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@ 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


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