Re: [rfa/6.0] Better handle unspecified CFI values

Daniel Jacobowitz
2003-09-09T17:35:48Z

On Tue, Sep 09, 2003 at 12:23:27PM -0500, Jim Blandy wrote:
>
> Daniel Jacobowitz <drow@mvista.com> writes:
> > Yes - normally. On S/390, stdcall, et cetera (anywhere where the hack
> > would be wrong) it gets even worse. We can only compute expressions
> > describing a memory location where the register is saved, not computed
> > values. For stack pointers (and maybe frame pointers on some
> > architectures?) this isn't good enough.
>
> Not to pursue unimportant tangents, but why would the hack be wrong on
> the S/390? Its frames are normally FP-free, but aside from that,
> what's unusual about it?

I'm wrong about the S/390 being the problem - in fact it was the
_opposite_ of the problem. From:

http://gcc.gnu.org/ml/gcc-patches/2003-05/msg00904.html

From the previous discussions, it would appear to me that
the real difference between s390 and other platforms here
is that on s390, the stack pointer register is saved and
restored from the call stack frame, like any other call-
saved register. This means that uw_install_context can
install the target stack pointer simply by writing it
into the current stack frame, just like any other call-
saved register.

If this is possible, there is no need for the unwinder
routines to perform any special handling w.r.t. the
stack pointer register at all. This means that there
should be no need to define EH_RETURN_STACKADJ_RTX,
and everything related to this should be omitted by
the library. This also means that the special-cased
code for 'simulating' the stack pointer register in
uw_update_context (which is what caused the breakage
on s390) is just superfluous.

GCC has a local hack in it, essentially, which handles adjusting the
stack pointer for a frame back far enough. At least it seems that way
to me. This hack is equivalent to the problem that GDB has for
figuring out the saved SP.

--
Daniel Jacobowitz
MontaVista Software Debian GNU/Linux Developer