Unwinding dummy frames on the i386

[Mark Kettenis <kettenis at chello dot nl>]

Hi Andrew,

After you committed the unwind_dummy_id stuff, I'm having some
problems getting the i386 target working again.

I tried using generic_save_dummy_frame on this target.  This works
fine if we're unwinding from a normal frame into a dummy frame.
However, if we're unwinding from a sentinel frame, things start to get
a bit difficult.  This basically happens if we've hit the special
breakpoint upon return from the called function.  On the i386
returning from a function pops the return address from the stack.
Since the return address was pushed by GDB an accounted for in the
stack pointer passed to generic_save_dummy_frame_tos(), things don't
match up.  This fixable in i386_unwind_dummy_id(), however things are
a bit more complicated.

If a function returns a struct, the caller is supposed to push as a
"hidden" parameter the address of a bit of memory where the called
function is supposed to store the struct.  This extra push is again
accounted for in the stack pointer passed to
i386_save_dummy_frame_tos().  However, since the called function is
supposed to remove this address from the stack, again things don't
match up for the unwind-from-sentinel case.  I don't see a clean way
to fixup this.

For now I've worked around this by having my own do-nothing
i386_save_dummy_frame_tos(), and having i386_unwind_dummy_id() return
contents of the (saved) frame pointer register for the frame's base.
Is there a better way to solve this?

Mark