Re: [PATCH/RFC/RFA] Print in-memory struct return values

[Andrew Cagney <cagney@gnu.org>]

   How come it doesn't work?

   Popping the caller's frame should (assuming the unwind info is correct) 
   restore the struct-return address register to the value that the callee 
   expects.  return_value can then use that register value to find the 
   location at which to store the struct?

The problem is in the "assuming the unwind info is correct".  It's not
there.  It will probably never be there.  This isn't a normal register
save/restore.  The incoming address will be on the stack.  The callee
moves it into %eax somewhere, but we don't know where and when.  You
simply can't express this in DWARF2 CFI.
You can.  %eax would start out as on the stack, and then later be marked 
as ``in register''.  It's just the reverse flow of what happens to other 
registers.

Now, we could do some nifty prologue/epilogue analysis and make a good
guess at where the address is stored.  Compilers seem to either do the
move in the prologue (as the SVR4 ABI suggests, see
i386_analyze_struct_return()) or leave the address on the stack until
the epilogue (as GCC does).  However, we can never be sure.  This
would lead to the unreliabilities you were trying to get rid of when
gdbarch_return_value() was instroduced.
Ah.  This is what needs to be added to the return_value documentation.

Andrew