[lldb-dev] Make a StackFrame return immediately

Tue Jul 26 18:08:07 PDT 2011

Yes, I tried that. I still get the same problem.

Another weird thing is the "thread step-over" problem. It seems, somehow,
the thread (the only one) is just not selected anymore. But the next time I
try to run the same command, it is selected (And I can't find out where it
is getting unselected…). I will try to continue the debugging process.

If I run the command "thread select 1" right after the "frame return"
command, I don't get the error, since I manually selected the thread.

Any clues?

Regards,

  Filipe

On Tue, Jul 26, 2011 at 17:42, Jim Ingham <jingham at apple.com> wrote:

> Did you try calling ClearStackFrames() in Thread::ReturnFromSelectedFrame
> (after you call the ReturnFrom call, of course)?  That should flush all the
> cached frames, and the next time somebody tries to access the frames for
> that stack, they will be automatically recomputed from the live register
> state you've written.  That should also make the stepping after the return
> work correctly.
>
> Jim
>
> On Jul 26, 2011, at 5:29 PM, Filipe Cabecinhas wrote:
>
> > Hi all,
> >
> > Multi-level returns were not working as intended, as I would write to the
> frame's RegisterContext (yielding an error), instead of writing to the live
> RegisterContext. I have fixed that and created another test-case for it.
> >
> > I still have the problem of not rebuilding the StackFrameList, and the
> first "thread step-over" command right after a "frame return" is yielding an
> error. Subsequent commands work fine.
> >
> > Regards,
> >
> >   Filipe
> >
> > On Mon, Jul 25, 2011 at 15:08, Filipe Cabecinhas <
> filcab+lldb-dev at gmail.com> wrote:
> > Hi all,
> >
> > I've refactored the code to put most of the work on the StackFrameList
> class, but I still have one problem:
> > The state of execution isn't being updated.
> >
> > - If I try to print the pc register after returning, it will just give me
> the old value (still in the function).
> > - The first thread step-over command after returning from a function
> isn't working, too. But that may have to do with:
> > - I can't update the stackframe list. thread backtrace always puts me
> inside the function.
> >
> > These three problems may be interconnected. I'll try to debug further.
> >
> > I also can't do what Jim suggested. I got the Block reference, from the
> frame. I can see if it's inlined and its size, but can't tell where it
> starts/ends.
> >
> > I'll send two patches: They're the same except for the implemented
> command.
> >
> > In one, the command is "thread return", in the other it is "frame
> return".
> >
> > My problem is… Except for "frame select", the frame commands only work on
> the current frame (not on other frames), so I would see "frame return" as
> return only from the current frame. While "thread return" could return from
> any frame (defaulting for the current thread). What do you think?
> >
> > I'm also sending a test directory, using the "thread return" variation.
> >
> > Regards,
> >
> >   Filipe Cabecinhas
> >
> >
> > On Fri, Jul 22, 2011 at 16:25, Filipe Cabecinhas <
> filcab+lldb-dev at gmail.com> wrote:
> > Thanks for the help and advice. I will ping back the list with more stuff
> when I'm done.
> >
> > Regards,
> >
> >   Filipe
> >
> >
> > On Fri, Jul 22, 2011 at 16:23, Jim Ingham <jingham at apple.com> wrote:
> >
> > On Jul 22, 2011, at 3:46 PM, Filipe Cabecinhas wrote:
> >
> > >
> > >
> > > On Fri, Jul 22, 2011 at 15:18, Jim Ingham <jingham at apple.com> wrote:
> > > Filipe,
> > >
> > > Thanks for working on this.
> > >
> > > Note, gdb's "return" command doesn't do anything to attempt to clean up
> local objects.  It just pops the frames off the stack & returns the value.
>  This, like moving the PC by hand, is one of those commands that you just
> shouldn't use if you don't know what could go wrong...  Anyway, people
> mostly use this to hack system routines to do something else, in which case
> you stop at the very beginning and return directly - you haven't gotten far
> enough to need to do any cleanup.
> > >
> > > Seems to me that the sensible thing to do about the return expression
> is take whatever the user gives you and cast it to the return type of the
> function you are returning from, if you know it.  If you don't then just
> write it as whatever they have given you, and the user will have to get it
> right when they write the expression.  That's the best you can do in this
> case.
> > >
> > > OK.
> > >
> > > It also seems to me that much more of this could be generic.  The
> algorithm gdb uses is to get the register context one frame up on the stack,
> and write that to the real registers, and then throw away the cached stack
> and remake it from the changed registers.  That can all be done generically.
>  Mind there are target specific bits but they are all hidden under the
> register code.  It is the unwinder's job to know what the register state of
> the functions up on the stack are, you should rely on that.
> > >
> > > The only ABI specific bit is that you need to ask the ABI where to
> store the return value.  For things smaller than a register that is easy.
>  For struct returns it can be hard, because the compiler usually passes in a
> pointer to the save location, but you would have to track the history of
> that value to know where it is stored at the point you are trying to return
> from, and that's not trivial.  Again, if you are at the beginning of the
> function you're returning from, this is easy to do.
> > >
> > > Ok. You're suggesting taking the code out of the ABI and putting it on
> the command (or maybe the StackFrame)? I initially wrote the code thinking
> calling conventions could vary a lot but, for the returns, they don't vary
> that much (and lldb allows us to abstract most of what varies). Or would it
> be preferred to just contain the code in the command object?
> >
> > Yes. This functionality seems generally useful, so the code to do this
> should go somewhere in the core.  Then the command would just be a thin
> wrapper.  StackFrame doesn't seem right to me, since you are operating on
> the list of frames more than the individual frame.  So either
> StackFrameList, or its owner Thread, seem good places.
> >
> > >
> > > I suppose the command should also be changed to the "frame" command, to
> mimic gdb (it returns from the current frame, not the bottom-most).
> > >
> >
> > Yes, that makes sense, you would either return from the currently
> selected frame, or from a frame given explicitly in the command.  Note in
> general in lldb we try to avoid positional arguments, so I would do:
> >
> > frame return --frame <FRAME NUMBER> <expression>
> >
> > BTW, you can also make LLDB commands "RAW" commands, which means that
> everything after the options is passed unparsed to the command.  That's very
> convenient for commands that take an expression as their argument, since
> then you don't end up having to backslash yourself to death.  See the
> "expression" command for an example of this.
> >
> > Oh, yeah, another thing, though very trivial, since you defined your
> command with:
> >
> >                       eFlagProcessMustBeLaunched |
> eFlagProcessMustBePaused
> >
> > you don't have to check whether the process & target are valid, the
> command interpreter shouldn't call you if they are not.
> >
> > Again, thanks for working on this!
> >
> > Jim
> >
> >
> > > You can also check whether the block you are in is inlined, and if it
> is, then all you do is move the PC to the end of the block.  You can't
> really do anything about the return value then, because you can't really
> tell where it is going, but at least you can return properly from the
> inlined function.
> > >
> > > Hope that helps...
> > > I will try that one too, thanks.
> > >
> > > Regards,
> > >
> > >   Filipe
> > >
> > >
> > > Jim
> > >
> > > On Jul 22, 2011, at 12:35 PM, Filipe Cabecinhas wrote:
> > >
> > > >
> > > >
> > > > On Mon, Jul 18, 2011 at 18:05, Greg Clayton <gclayton at apple.com>
> wrote:
> > > > On Jul 18, 2011, at 5:23 PM, Filipe Cabecinhas wrote:
> > > >
> > > > > Hi,
> > > > >
> > > > > Is there a way to mimic gdb's "return <expr>" command?
> > > >
> > > > Not if you want to change the return value.
> > > >
> > > > If you don't want to change the return value you can use "thread
> step-out". "thread step-out" (which is aliased to "finish"), is context
> sensitive to the frame you are currently in, so if you run and stop in a
> frame and then do a "frame select 12", and then to a "thread step-out", you
> will return to frame 13.
> > > >
> > > > Yes, that command will run until the frame returns (like gdb's finish
> command), but what if we want to return immediately?
> > > >
> > > > > (gdb) help return
> > > > > Make selected stack frame return to its caller.
> > > > > Control remains in the debugger, but when you continue
> > > > > execution will resume in the frame above the one now selected.
> > > > > If an argument is given, it is an expression for the value to
> return.
> > > > >
> > > > >
> > > > > I've been looking at the StackFrame class, but it doesn't look like
> it can do that.
> > > > > If I wanted to implement it, where should I look first? I can get
> the return address
> (StackFrame.GetRegisterContext().get().GetReturnAddress(), I think) write it
> to the PC (StackFrame.ChangePC()), but I have no idea how to get the
> expression's result into the return registers/memory.
> > > >
> > > > LLDB currently doesn't have any real idea of where the return address
> goes, we currently let the compiler handle all ABI issues by the way we make
> expressions.
> > > >
> > > > There is another issue where if a function isn't external, the
> compiler can make a call to this function and how the function returns the
> value, violate the ABI. In most cases you won't get affected by this, but it
> would be nice if we knew for sure from the compiler or debug info where the
> return value is. The old ARM compiler used to inject artificial
> DW_TAG_variable debug information entries into the DWARF that would tell you
> the result of functions which has a location that describes the returned
> value and where it is.
> > > >
> > > >
> > > > > ClangExpression isn't a big help there, since the result comes to
> debugger-land.
> > > >
> > > > Yep, and even so there is the issue that internal functions can
> violate the ABI...
> > > >
> > > > FYI: anything ABI related is currently in the ABI plug-ins:
> > > >
> > > > lldb/source/Plugins/ABI/*
> > > >
> > > > The ABI function:
> > > >
> > > >    virtual bool
> > > >    ABI::GetReturnValue (Thread &thread,
> > > >                         Value &value) const = 0;
> > > >
> > > > Take a look a the ABIMacOSX_i386 and ABIMacOSX_x86_64 versions of
> this function and see if this does close to what you want. You can also fill
> in more functionality inside these for types you want it to support.
> Currently we fill the "value" argument with the result, but we don't fill in
> the context (See the "void Value::SetContext (ContextType context_type, void
> *p)" function for details, but the  ABI::GetReturnValue functions can be
> modified to fill in the register context for return values that are returned
> in registers, and the address (See the "Value::SetValueType (...)" function)
> if needed.
> > > >
> > > > These functions currently will attempt to extract the return value
> for a function according to the ABI rules for simple pointer size or less
> types only (no structs, floats, complex etc). So this might help you for the
> simple cases. If you were to implement this command you would want to add a
> new "return" subcommand in the "thread" multi-word command. In the "Execute"
> function of the new "return" command you would want to evaluate an
> expression an store the result, set a breakpoint at the return address,
> install a breakpoint callback and run and hit the breakpoint, then try and
> instert the expression result into the appropriate location (you would need
> to modify the "ABI::GetReturnValue(...)" to fill in the "value" param more
> completely with the location of the return type.
> > > >
> > > > Greg Clayton
> > > >
> > > >
> > > > I implemented a thread return command, which works (right now) for
> integers and pointers, for i386 and x86-64. The command has some caveats
> (like the ones discussed here), but it works (at least) for a few simple
> examples in C.
> > > >
> > > > "thread return <expr>" returns the result of evaluating that
> expression. I'm not casting the expression, so there may be some problems
> (especially when returning floating point types is implemented).
> > > >
> > > > But…
> > > > We have no idea if the compiler generated a stackframe for this
> function or not. We may be returning from two functions, and not one.
> > > > We have no ideia if the compiler changed the calling conventions for
> this code.
> > > > We don't know about the callee-saved registers (I suppose there isn't
> a way to find out the epilogue of the function… Especially since it may be
> mixed with "regular code")
> > > >
> > > > There are a lot more issues for C++ code (e.g: calling dtors).
> > > >
> > > > I also have some problems updating the state lldb thinks the thread
> is in.
> > > >
> > > > "register read pc" won't read the real value
> > > > "thread backtrace" doesn't get updated.
> > > >
> > > > How can I fix that? I am also looking for comments on stuff to
> fix/improve.
> > > >
> > > > I also have some tests (a directory to put in lldb/test/).
> > > >
> > > > Regards,
> > > >
> > > >   Filipe Cabecinhas
> > > >
> > > > P.S: Converting between a ValueObject object and
> > > >
> <thread-return.patch><thread_return.zip>_______________________________________________
> > > > lldb-dev mailing list
> > > > lldb-dev at cs.uiuc.edu
> > > > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
> > >
> > >
> >
> >
> >
> >
> > <frame_return.zip><frame-return.patch>
>
>
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