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Re: Command to break before exiting stack frame?


Tom> It seems to me that a tail call means there just isn't a return value
Tom> from the calling function, only from the callee, because the caller
Tom> doesn't even really have a separate return statement.

That makes sense actually. Although I would hope that the break on
exit command would break after any tail-recursive call exits.

Tom> On the whole I think it would be better to start with the compiler.  If
Tom> it emits epilogue markers, then the gdb work is not difficult.

Thanks for the tips. I've sent a message to the gcc mailing list.

On Tue, 19 Mar 2019 at 15:42, Tom Tromey <tom@tromey.com> wrote:
>
> Justin> Given its documentation, I would have expected the "finish" command to
> Justin> print the returned value at each tail call. It turns out that it
> Justin> prints the returned value only for the f(0) call. I would similarly
> Justin> expect a "break on exit" command to break on the exit of the frame in
> Justin> which it is called even if a tail-recursion occurs. Is there a reason
> Justin> that it doesn't?
>
> It seems to me that a tail call means there just isn't a return value
> from the calling function, only from the callee, because the caller
> doesn't even really have a separate return statement.
>
> I suppose, though, if it is like inlining, then this text from the
> manual also applies:
>
>    * GDB cannot locate the return value of inlined calls after using the
>      'finish' command.  This is a limitation of compiler-generated
>      debugging information; after 'finish', you can step to the next
>      line and print a variable where your program stored the return
>      value.
>
> Justin> On the actual implementation of this command: Is the implementation of
> Justin> such a feature feasible? If so, how much work would it take?
>
> For the compiler, I couldn't say.  For gdb, doing it without help from
> the compiler seems difficult, as you'd probably have to write an
> instruction decoder.  gdb already has these for some architectures
> (various kinds, actually, for different things), but probably not in a
> useful form.
>
> On the whole I think it would be better to start with the compiler.  If
> it emits epilogue markers, then the gdb work is not difficult.
>
> Tom


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