[patch] Fix oddity in personality routine

Tue Nov 17 16:22:00 GMT 2009

Jack Howarth wrote:
> On Tue, Nov 17, 2009 at 02:54:53PM +0000, Andrew Haley wrote:
>> Jack Howarth wrote:
>>>>> In the unwinder walk, are there any particular places where I could
>>> get additional debug information in gdb that would be helpful to diagnose this
>>> issue?
>> Sure. The trace you provided is very incomplete, and in particular
>> I can't see any stepping into _Unwind_RaiseException.
>>>> The main loop looks like this:
>>>> while (1)
>> {
>> _Unwind_FrameState fs;
>>>> /* Set up fs to describe the FDE for the caller of cur_context. The
>> 	 first time through the loop, that means __cxa_throw. */
>> code = uw_frame_state_for (&cur_context, &fs);
>>>> if (code == _URC_END_OF_STACK)
>> 	/* Hit end of stack with no handler found. */
>> 	return _URC_END_OF_STACK;
>>>> if (code != _URC_NO_REASON)
>> 	/* Some error encountered. Usually the unwinder doesn't
>> 	 diagnose these and merely crashes. */
>> 	return _URC_FATAL_PHASE1_ERROR;
>>>> /* Unwind successful. Run the personality routine, if any. */
>> if (fs.personality)
>> 	{
>> 	 code = (*fs.personality) (1, _UA_SEARCH_PHASE, exc->exception_class,
>> 				 exc, &cur_context);
>> 	 if (code == _URC_HANDLER_FOUND)
>> 	 break;
>> 	 else if (code != _URC_CONTINUE_UNWIND)
>> 	 return _URC_FATAL_PHASE1_ERROR;
>> 	}
>>>> /* Update cur_context to describe the same frame as fs. */
>> uw_update_context (&cur_context, &fs);
>> }
>>>> So, for each stack frame, we read the unwinder data and then call the
>> appropriate personality routine (in this case, in libgcj.)
>>>> cur_context.ra is the program counter for the current stack frame.
>> So,
>>>> (gdb) x cur_context.ra
>> 0x7ffff659ca7f <_ZN4java3net14URLClassLoader9findClassEJPNS_4lang5ClassEPNS2_6StringE+255>:	0x41c58949
>>>> tells you which frame is being inspected. So, you can see where the
>> exception handler should be, and you can step into the personality
>> routine to see why it's not recognized.

> At which points should I be sampling with 'x cur_context.ra'? Is there a particular
> breakpoint that would be useful to set in order to do this?

Before and sfter uw_frame_state_for. Then step into the personality routine.
> Also, would it help if I
> recompiled libgcj with -O0 as well?

Probably.
Andrew.