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Re: RFA: tolerate unavailable struct return values
- From: Jim Blandy <jimb at zwingli dot cygnus dot com>
- To: Elena Zannoni <ezannoni at cygnus dot com>
- Cc: Michael Snyder <msnyder at cygnus dot com>,Daniel Jacobowitz <drow at mvista dot com>, Jim Blandy <jimb at cygnus dot com>,gdb-patches at sources dot redhat dot com
- Date: 17 Dec 2001 18:10:46 -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><15383.42118.203889.389960@localhost.localdomain>
Elena Zannoni <ezannoni@cygnus.com> writes:
> 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 :-(.
Looking at the following code in infcmd.c:
/* We cannot determine the contents of the structure because
it is on the stack, and we don't know where, since we did not
initiate the call, as opposed to the call_function_by_hand case */
#ifdef VALUE_RETURNED_FROM_STACK
value = 0;
#ifdef UI_OUT
ui_out_text (uiout, "Value returned has type: ");
ui_out_field_string (uiout, "return-type", TYPE_NAME (value_type));
ui_out_text (uiout, ".");
ui_out_text (uiout, " Cannot determine contents\n");
#else /* UI_OUT */
printf_filtered ("Value returned has type: %s.", TYPE_NAME (value_type));
printf_filtered (" Cannot determine contents\n");
#endif /* UI_OUT */
#else
value = value_being_returned (value_type, stop_registers, structure_return);
and then at the following code in valops.c:
#ifdef VALUE_RETURNED_FROM_STACK
if (struct_return)
return (value_ptr) VALUE_RETURNED_FROM_STACK (value_type, struct_addr);
#endif
the stuff in infcmd.c looks backwards to me. Wasn't the intention to
fall back to printing the type when VALUE_RETURNED_FROM_STACK is *not*
defined?
TYPE_NAME is just the structure tag; don't we want to print the
`struct' or `union' there? The `type_print' function takes care of
that.
Frankly, I don't even think it's that useful to print the type. The
user knows what function was called, and can use `ptype' or `whatis'
if they want to know the type.