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Re: RFA: tolerate unavailable struct return values
- From: Elena Zannoni <ezannoni at cygnus dot com>
- To: Michael Snyder <msnyder at cygnus dot com>
- Cc: Daniel Jacobowitz <drow at mvista dot com>, Jim Blandy <jimb at cygnus dot com>, gdb-patches at sources dot redhat dot com
- Date: Wed, 12 Dec 2001 13:40:06 -0500
- Subject: Re: RFA: tolerate unavailable struct return values
- References: <20011129220913.2D72A5E9D8@zwingli.cygnus.com><20011129173644.A15429@nevyn.them.org><npwv08c7bj.fsf@zwingli.cygnus.com><20011130163218.A29232@nevyn.them.org><3C07FF91.239D7794@cygnus.com>
Michael Snyder writes:
> 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.
Have you looked at the macro VALUE_RETURNED_FROM_STACK ? I defined that
long time ago for hppa. It looks like the rs6000-tdep.c tries to deal
with the same problem as well.
Maybe we should clean up that code, which came in as part of the HP
merge :-(.
Elena