[lldb-dev] Make a StackFrame return immediately

[Filipe Cabecinhas]

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
> >
> >
>
>
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