Re: Preserving the argument spills for GDB

[Jean Christophe Beyler <jean dot christophe dot beyler at gmail dot com>]

I will try to add more details about my issue. Another way of putting this is :

For my architecture there is:

- A certain number of input registers which allow calls without going
on the stack
- There is not automatically a frame pointer on the stack either

What is needed, from the GCC side, to provide GDB with enough
information to preserve the arguments that are passed and allow to see
a backtrace and walk the stack.

Do we need to change the ABI and store certain indispensable things on
the stack for GDB ?

I see very little documentation about this or questions, so I don't
know exactly where to look to.

Note: in O0 levels, the compiler does actually store the arguments on
the stack and has a frame pointer, this gives GDB the possibility to
do show the arguments passed to a function at a break point but not
the backtrace.

In O3, however, I don't even get the arguments correct.

Thanks for any input,
Jc

On Tue, Nov 3, 2009 at 2:01 PM, Jean Christophe Beyler
<jean.christophe.beyler@gmail.com> wrote:
> Dear all,
>
> I've been working on handling the Debugging information for the use of
> GDB on my port. Though I definitely know that, when compiling in -O3,
> some information is lost and the debugger can't always have all the
> information, I'd like to at least keep the values of the arguments
> when doing a backtrace. Since my architecture uses register passing
> for arguments, relatively quickly this is lost since it is not stored
> on the stack.
>
> Therefore, I would like to, when doing a backtrace, have the argument
> information. I asked a few clarifications on the GDB mailing list and,
> it seems, that I need to copy the arguments on the stack (like what is
> done by default when using -O0). However, when compiling in -O3, the
> compiler of course removes these stores on the stack and relies solely
> (and justly so) on the input registers.
>
> I've reread the GCC internals and have been looking at anything
> regarding the stack but can't seem to figure this one out. How exactly
> do I explain to the compiler that I want to keep those spills to the
> stack for debugging purposes ?
>
> Thanks for your help,
> Jc
>