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

John David Anglin
2004-05-17T17:54:00Z

> 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)