[lldb-dev] Problem unwinding from inside of a CRT function

jingham at apple.com jingham at apple.com
Fri Jan 16 15:15:52 PST 2015

> On Jan 16, 2015, at 2:21 PM, Zachary Turner <zturner at google.com> wrote:
> On Fri Jan 16 2015 at 1:53:00 PM Greg Clayton <gclayton at apple.com> wrote:
> Please trust that we know what we are doing when it comes to single stepping. I am glad you are thinking about how things are done, but just be sure think about the problem in a wider scope than "the code I am thinking about is linear" and think about all sorts of single stepping and what you would expect to happen.
> I'm not not-trusting that you know what you're doing.  I'm trying to understand it for myself, so that I can make the necessary changes to get it working on Windows.
> On the other hand, Windows does have plenty of different rules and assumptions when it comes to how it generates code.  So while I'm confident that you guys have thought about it and made something that works well within the context of non-Windows, it's a given that some of the functionality is going to be broken when the assumptions change.  Not because you didn't know what you were doing, but because the code wasn't designed with these assumptions in mind.   Otherwise it would have just already worked and this thread would have never been created :)
> So it's not enough for me to just say "I have to trust that it works, therefore I must be doing something wrong", but I also need to understand the architecture and the details well enough to be able figure out if it doesn't work because of some fundamental differences in Windows vs. non-Windows, or if it doesn't work because I have a bug somewhere in my process plugin, or something else.
> One of the questions I keep asking myself is: Why, when using one of the native Windows debuggers such as MSVC or WinDbg, if I step over a call, does it always work even if the called function has no debug info, no unwind info, and uses FPO?  
> Maybe it's related to them having better function bounds in their COFF parser, as you suggested.  I'm going to look into that, but I just want to re-emphasize that none of my posts, including this one, are intended to second guess anyone's decisions.  But at the same time it's only natural  to expect that since many of the assumptions were made without Windows in mind, they may prove to be slightly wrong.  
> There's not very many detailed design documents about how things work, and in certain areas code documentation is sparse.  So the purpose of me asking is simply to understand how it works.
> Hope this makes sense.

Sure.  I think we take a lot of these complexities for granted at this point.  It's fun to be reminded from time to time that this stuff is a little above flipping burgers...

None of us here have worked on MSVC, so I don't know if they use fundamentally different algorithms for stepping.  But there really aren't that many different ways you could do it, at least that I can think of.  If I had to guess, I would bet they make their unwinder really smart, because there are so many clients of that, and if that works well, then stepping is pretty straight-forward.  And then given the age of the tools they probably have a bunch of special cases layered on top to make things work beautifully.


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