[lldb-dev] Make a StackFrame return immediately

[Greg Clayton]

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.

> 
> (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