Re: [RFC] fixing extract_struct_value_address

[Elena Zannoni <ezannoni at redhat dot 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?
 > > 
 >