Re: [patch/rfa/hppa] Use frame pointer for unwinding

["John David Anglin" <dave@hiauly1.hia.nrc.ca>]

> ok, feature request? ;-) as noted below this is not really required for
> gdb to work correctly; i'm just wondering if the same Save_SP scheme
> works for both hp compiler and gcc. In some other email you had
> indicated that the hp compiler doesn't set the Save_SP flag (or did 
> i misunderstand you?)....

It's my understanding that none of the HP compilers set the Save_SP
flag.  This is why the flag was selected.  Originally, it indicated
that a frame pointer was needed for the frame.  Now, we also save
the entry value of SP in the current frame marker (current_SP - 4)
when a frame pointer is needed and TARGET_HPUX_UNWIND_LIBRARY is
true.

> > Note that the the previous SP (frame pointer) is saved in the frame
> > marker of frame 1.  This value is accessible from frame 2 (i.e.,
> > effectively the frame pointer is always saved under hpux when Save_SP
> > is true -- it's just done by the caller).  However, I think gdb
> > should avoid using the saved SP value in the frame marker as not
> > all versions of GCC support this.  It's also not supported under
> > linux.
> 
> right now gdb uses the value of the frame pointer that is stored at the
> start of the frame; that is, for "normal frames", it looks for a
> specific code sequence:
> 
> stw,ma rN, xxx(sp)
> 
> in the code, and if it sees this, it notes that a frame pointer has 
> been stored at offset 0 of the stack. During unwinding, it finds out 
> if the current frame should have saved the fp (by looking at the Save_SP
> flag) and if so it retrieves it from the stack.

A stw,ma r3, xxx(sp) instruction in the entry sequence is unique to
GCC (32-bit runtime).  If you find this, Save_SP should also be set
as it is part of the sequence to save %r3 and copy the stack pointer
to %r3 in a frame which needs a frame pointer..

Dave
-- 
J. David Anglin                                  dave.anglin@nrc-cnrc.gc.ca
National Research Council of Canada              (613) 990-0752 (FAX: 952-6602)