Re: RFA: tolerate unavailable struct return values

[Michael Snyder <msnyder at cygnus dot com>]

Daniel Jacobowitz wrote:
> 
> On Fri, Nov 30, 2001 at 03:49:52PM -0500, Jim Blandy wrote:
> >
> > Daniel Jacobowitz <drow@mvista.com> writes:
> > > On Thu, Nov 29, 2001 at 05:09:13PM -0500, Jim Blandy wrote:
> > > >
> > > > On some architectures, it's impossible for GDB to find structs
> > > > returned by value.  These shouldn't be failures.  Should they be
> > > > passes?
> > >
> > > Out of curiousity, which architectures?  And to be pedantic, I suspect
> > > that it might be "not always possible" rather than actually
> > > impossible.
> >
> > The one I have in mind is the S/390, although I'm pretty sure there
> > are others.  I've included the bug report I sent to the S/390 GCC
> > maintainers below.
> >
> > One approach would be to hope that the return buffer's address was
> > still there in the register it was passed in.  But there's no way to
> > tell when you're wrong.  GDB will just print garbage, and the user
> > will think their program is wrong.  Better to simply say, "I can't
> > find this information reliably", and let the user, who knows their
> > program, find another way to get the info --- setting a breakpoint on
> > the return statement, or looking at where the caller put the
> > structure.
> 
> Hmmmm.  I wonder if MIPS could ever be affected by this?  I don't think
> the MIPS ABI specifies that $a0 remains live.  It looks as if the value
> of $a0 is always returned in $v0 in such functions, though.

It's not an uncommon problem, and I imagine we get it wrong a lot of the time.