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

[Andrew Cagney <cagney at gnu dot 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