Re: [MinGW] RFC/RFA: Get Partial Stack Traces on Windows

[Bryce McKinlay <mckinlay at redhat dot com>]

Ranjit Mathew wrote:
> That function has just three independent sections (with my patch):
>
>   1. If the frame is from the interpreter.
>   2. If the target has a working dladdr().
>   3. If the target is Windows.
>
> It's neatly separated in my opinion but I can change it if you
> want me to. The problem is that I will have to anyway copy over
> the code for #1 above, so I can't escape the #ifdef-ing. Moreover,
> since this is stack tracing code, it belongs (again, in my opinion)
> to stacktrace.cc more than it does elsewhere.
>   

I was thinking you'd just move out the windows-specific part to a 
"platform_getLineNumberForFrame()" (or whatever), which would be called 
from getLineNumberForFrame() after the interpreter-specific code. I 
agree, we don't want to duplicate the interpreter part of the code.
> By the way Bryce, in fallback_backtrace() why do you set the IP of
> the faulting instruction to be the operand part of a "CALL <XYZ>"
> instruction instead of the actual beginning of the instruction? You
> then compensate for it in getLineNumberForFrame() by decrementing
> the offset by one. Is it because the DWARF-2 unwinder has the
> same curious behaviour?
>   

Actually I didn't write fallback_backtrace(), that code was already 
there - but this sounds about right. The Dwarf-2 unwinder normally gives 
us the return address, which needs to be rolled back in order to get the 
correct line number. But, in the case of a frame where a fault occurred, 
it gives the address of the faulting instruction. I think there is 
actually a slight bug with this on Dwarf-2 since the "decrement the 
offset by one" isn't correct for a segfault/NullPointerException frame, 
so sometimes we get the wrong line number.

Bryce