Re: [RFC] Struct return values

[Mark Kettenis <kettenis@chello.nl>]

   Date: Sun, 18 Jan 2004 11:30:54 -0500
   From: Andrew Cagney <cagney@gnu.org>

   When looking at the code I found two cases:

   "return VALUE":
   GDB first pops the callers frame, and second stores the return VALUE. 
   This means that the method will see caller's frame just after the callee 
   has been forceably "returned".

   "finish"
   GDB first finishes the function, and second extracts the return VALUE. 
   This again means that the method will see the caller's frame just after 
   the callee has returned.

   So perhaphs something like:
   The target should only define this method if it has a reliable way of 
   extracting the struct-convention return-value address using only 
   information obtained from the caller's frame just after the callee has 
   returned.  [insert something about how this is impossible on most ABIs :-)]

Hmm, OK, but this means that there are really three classes of ABIs here:

a) The return value address is passed to the callee, and the callee
   may clobber the location where the address is stored (i.e. it's
   stored in the callee's stack frame, or in a scratch register).

b) The return value address is passed to the callee, and the callee
   may clobber the location where the address is stored (i.e. it's
   stored in the callee's stack frame, or in a scratch register).  We
   can't "return VALUE" or display the return value with "finish".

c) The return value address is passed to the callee in a location that
   isn't clobbered by the callee (i.e. in the caller's stack frame, or
   in a preserved register).

This means that

a) We cannot "return VALUE" or display the return value with "finish".

b) We cannot "return VALUE" but we can display the return values with
   "finish".

c) We can both "return VALUE" and display the return value with
   "finish".

Examples are:

a) The PPC System V psABI: The return value address is passed in r3,
   which is "volatile".

b) The AMD64 System V psABI: The return values address is passed in
   %rdi.  On return %rax will contain the address that has been passed
   in by the caller in %rdi.

c) The (32-bit) SPARC System V psABI: The return address is stored in
   a reserved slot in the caller's frame.

Personally I find it very usefull that GDB prints the return value
when I say "finish", so I'd like to make that work too.

Mark