[OpenBIOS] cross-pcc compilation issue with GCC 4.5.0

[Stefan Reinauer]

On 4/26/10 10:29 PM, Blue Swirl wrote:
> On 4/26/10, Mark Cave-Ayland <mark.cave-ayland at siriusit.co.uk> wrote:
>   
>> Segher Boessenkool wrote:
>>
>>
>>     
>>> That patch [which doesn't display in the mail archive btw] seems to remove
>>> -print-libgcc-file-name, and uses -lgcc instead -- not such a hot idea?
>>>
>>> I don't even know how to build OpenBIOS, so I'm not going to fix it,
>>>       
>> sorry.
>>     
>>>       
>>  If you have a PPC cross-compiler installed, the build is easy:
>>
>>  ./config/scripts/switch-arch cross-ppc
>>  make
>>
>>  Then copy the resulting binary from obj-ppc/ into your share/qemu
>> directory.
>>
>>
>>     
>>> I'll do my best with directed questions of course :-)
>>>
>>>       
>>  Even an idea as to what to look at first would be a great help...
>>     
> I think I can now reproduce the crash, previously I had no problems
> which made debugging a bit difficult. I changed gcc's memory model
> flag to -mcmodel=medlow and now Milax crashes in __umodti3 like Igor
> reported. Perhaps this flag may be different when gcc builds libgcc.
>
> The problem seems to be that gcc's libgcc divides by zero.
> Breakpoint 10, __umodti3 (u=<value optimized out>,
>     v=0x0000000000000000fffffffffffffffe) at ../../gcc/libgcc2.c:911
> 911     in ../../gcc/libgcc2.c
> 11: $i4 = 0x0
> 10: $i0 = 0x1
> 1: x/i $pc
> 0xffd1f980 <__umodti3+952>:     udivx  %i0, %i4, %g1
>
> Why this doesn't happen with our libgcc? Why it doesn't happen with
> -mcmodel=medany? What a puzzle.
>
> One solution could be to check for division by zero in mudivmod(),
> which is where __umodti3 is called from.
>   

Changing the code model  most likely changes the ABI, so it's no wonder
it does not work anymore.
The same thing happens if your gcc is compiled with a different
"regparm" setting than your code.

This was generally reported to the gcc folks last year but I think
nothing has happened since then:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41055

In coreboot we fixed the regparm issue by using a wrapper that converts
between the ABIs:

http://tracker.coreboot.org/trac/coreboot/browser/trunk/src/arch/i386/Makefile.inc#L91
http://tracker.coreboot.org/trac/coreboot/browser/trunk/src/lib/gcc.c

Not sure whether something similar is possible for the code model, too,
but it sounds less trivial.
So until then the GCC folks' answer is: If your code uses compile time
options that change the ABI of your compiler, you need to compile a
compiler (well, libgcc) with those options, too. So the simplest way is
just not assuming any ABI details if possible and not using CFLAGS that
change the default ABI.

Stefan