Re: Unwinding dummy frames on the i386

[Andrew Cagney <ac131313@redhat.com>]

>    Date: Fri, 07 Mar 2003 17:13:24 -0500
>    From: Andrew Cagney <ac131313@redhat.com>
> 
>    > 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?
> 
>    Can i386_save_dummy_frame_tos() save the `frame pointer register' that 
>    the unwind_dummy_id() method later returns?
> 
> If reading the current value of FP_REGNUM from current_regcache is OK
> the answer is yes.

Yes.  Just add a comment to gdbarch.sh saying that tos's successor needs 
to be explicitly parameterized with the new regcache (along with 
anything else that might prove useful - old regcache, struct_addr, bp 
addr :-).

 > Hmm, in that case it's probably an even better
> idea to actually set the frame pointer register to the top-of-stack in
> i386_save_dummy_frame_tos().  That way, we won't have a problem when
> there are several frameless function calls in a row.  Is writing into
> current_regcache acceptable?

Shouldn't the push arguments code have already adjusted the FP?

Anyway, regardless, you want to add frame_align() to the i386 
architecture vector.  It forces a non-empty correctly aligned dummy 
frame (thus eliminating the frameless dummy frame problem) (but read the 
comments that go with the call).

> 
>    While the TOS in save_dummy_frame_tos() might have once stood for 
>    top-of-stack, there isn't anything saying that it has to be the stack 
>    top.  A better name for tha functioin's successor would be 
>    save_dummy_frame_id()
> 
> Agreed.

Andrew