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Re: [RFC] fixing extract_struct_value_address
- From: Elena Zannoni <ezannoni at redhat dot com>
- To: Andrew Cagney <ac131313 at ges dot redhat dot com>
- Cc: Michael Snyder <msnyder at redhat dot com>,gdb-patches at sources dot redhat dot com, kevinb at redhat dot com, cagney at redhat dot com,Elena Zannoni <ezannoni at redhat dot com>
- Date: Thu, 22 Aug 2002 12:33:23 -0400
- Subject: Re: [RFC] fixing extract_struct_value_address
- References: <3D6418C5.FBF117D@redhat.com><3D65001C.70609@ges.redhat.com>
Discussed before in:
http://sources.redhat.com/ml/gdb-patches/2001-11/msg00571.html
and
http://sources.redhat.com/ml/gdb-patches/2001-12/msg00325.html
never resolved.
Elena
Andrew Cagney writes:
> > Problem: Find a function's return value when it is a struct
> > returned by reference (thru a pointer).
>
> Hmm,
>
>
> As far as I know, there are two cases:
>
> 1. a normal function forced to return:
> (gdb) break foo
> (gdb) finish
>
> 2. an inferior function call:
> (gdb) print foo()
>
>
> I don't think the first case has a solution (unless the debug info steps
> forward with the answer we need).
>
>
> For the latter case, the inferior function code contains:
>
> /* Figure out the value returned by the function. */
> /* elz: I defined this new macro for the hppa architecture only.
> this gives us a way to get the value returned by the function from
> the stack,
> at the same address we told the function to put it.
> We cannot assume on the pa that r28 still contains the address of
> the returned
> structure. Usually this will be overwritten by the callee.
> I don't know about other architectures, so I defined this macro
> */
>
> #ifdef VALUE_RETURNED_FROM_STACK
> if (struct_return)
> {
> do_cleanups (retbuf_cleanup);
> return VALUE_RETURNED_FROM_STACK (value_type, struct_addr);
> }
> #endif
>
> {
> struct value *retval = value_being_returned (value_type, retbuf,
> struct_return);
> do_cleanups (retbuf_cleanup);
> return retval;
> }
> }
>
> I get the feeling that all that is needed is for the above to be enabled
> for all targets.
>
> enjoy,
> Andrew
>
>
> > Solution level one: Take the value of the register that was
> > used by the caller to pass the struct return address.
> >
> > Shortcoming: that register isn't preserved, so may be clobbered.
> >
> > Solution level two: Save the struct_return address when it
> > is passed to store_struct_return (or push_arguments), and
> > recover it when it is needed by extract_struct_value_address.
> >
> > Shortcoming: Not reentrant. Nested function calls will clobber it.
> >
> > Proposed solution: create a stack structure, and "push" the
> > struct_return address in store_struct_return, popping it in
> > extract_return_address. If you can't find it on the stack,
> > then use the value of the appropriate arg0 register.
> >
> > I think this should work for most targets, so the code for
> > managing the stack can be shared.
> >
> > What do you think?
> >
>