Re: Does GDB 6.7.1 for PowerPC require the framepointer register for backtracing?

[Stefan Bylund <steby@enea.se>]

Hi Daniel,

Thanks for your information. We use GCC 3.4.4, and when debugging we use 
the compiler options -g -O0. I tried to add the compiler option 
-fomit-frame-pointer (which seems to be included by -O1 and higher but 
not by -O0) and then it works!!! So, my conclusion is that GDB 6.7.1 for 
PowerPC tries to take advantage of frame pointer information in the 
DWARF-2 debug information while GDB 6.3 does not. Is that correct? Is it 
always safe to use -fomit-frame-pointer on PowerPC, i.e. will it not 
make some type of C/C++ code undebuggable?

Regards,
Stefan


Daniel Jacobowitz wrote:

>On Mon, Feb 11, 2008 at 12:03:25PM +0100, Stefan Bylund wrote:
>  
>
>>Hi,
>>
>>We recently upgraded from GDB 6.3 to GDB 6.7.1 and noticed that for  
>>PowerPC we can longer get a backtrace in certain cases. The problem is  
>>that when we initially attach to an existing process on a remote target,  
>>only the PC and SP registers are available. This worked fine with GDB 6.3 
>>when doing a backtrace. However, GDB 6.7.1 fails when doing a backtrace in 
>>this case, and it seems that it tries to use the framepointer register 
>>(R31) for doing the backtrace and since this register is not available 
>>(i.e. it has value 0 in GDB's perspective), the backtrace fails.
>>
>>Does GDB 6.7.1 requires the framepoint register (R31) for doing a backtrace?
>>    
>>
>
>It depends on your program and the available symbols and unwind
>information.  For instance, it will use DWARF unwind tables if
>they are provided.
>
>  
>
>>I have also noticed that if I don't install the dwarf2_frame_sniffer()  
>>and rs6000_adjust_frame_regnum() functions in rs6000_gdbarch_init() in  
>>rs6000-tdep.c, the backtrace works again even if the framepointer  
>>register is not available. However, I'm not sure exactly why... Is it  
>>safe to omit these two functions?
>>    
>>
>
>It sounds to me like you have DWARF information that says to use the
>frame pointer.  Do you know if that's true?
>
>  
>


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Stefan Bylund
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