[issue920] Incomplete backtrace in gdb

[Simon 'corecode' Schubert]

Simon 'corecode' Schubert wrote:
With current HEAD and gdb 6.7.1 I can't get complete backtraces any 
more. Example from Xorg.core:
could you supply the core and binary for debugging?
I received both from hasso privately.

I have to say that gdb is not really at fault here.  Thorough 
investigation revealed:

#7  0x0807b766 in ?? ()

This function really doesn't exist.  Well, it does, but it doesn't have a 
visible symbol.  Because of this, gdb can't analyze the function prologue 
and thus assumes that there is nothing on the stack - of course this is 
wrong.  We could change gdb to assume that there is always a stack frame 
and this way it could work better for this case.

Previous gdb versions mistakenly assumed a different calling function and 
happily unwound the prologue of the wrong function.  This led to better 
backtraces, but possibly wrong ones.

cheers
  simon
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