loader breaks with -O2 optimizations

[Kris Kennaway]

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