loader breaks with -O2 optimizations

Kris Kennaway kris at FreeBSD.org
Thu Nov 1 02:23:39 PDT 2007

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@ 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."

OK, so it's factually correct but just not good advice :)


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