[ping][RFA] Return trampoline generic pieces

["Maciej W. Rozycki" <macro at codesourcery dot com>]

Hi,

 Would anyone please have a look at the infrun.c changes made as a part of 
this submission:

http://sourceware.org/ml/gdb-patches/2012-04/msg00197.html

and tell me if they are all right or not?  Here's the relevant excerpt 
from the full patch description:

"4. A handler is now installed to skip over return thunks while
   single-stepping at the source level, reusing the shared library return
   trampoline hook.  As per the comment included, there is no overlap here
   as no MIPS ABI uses shared library return trampolines.

   This modification, however, requires a change to generic code -- in
   handle_inferior_event (infrun.c).  This is because a return thunk is
   called like an ordinary function and therefore once reached, things
   looks as if a new frame has been created.  As a result code that checks
   for subroutine calls triggers, at which point there is no way to back
   out -- while the trampoline handler may still be used there (like for
   call thunks) to skip over the return thunk, the subroutine call handler
   insists on skipping over code further, to the next source line.
   Which, given that we're about to return from a function, means the
   point after the return instruction.  Now placing the single-stepping
   breakpoint there -- doesn't quite work as expected.

   Therefore my proposal is to reverse the order of the checks made in
   handle_inferior_event -- it looks to me there have been no particular
   reason for why they have been put in the current order, except that one
   must have been chosen.  Currently there are two users of the shared
   library return trampoline hook: hppa-hpux-tdep.c and rs6000-tdep.c.
   The latter is rather trivial and makes me almost sure that the order of
   the checks does not matter.  The former is less obvious to me, but I
   think it does not care either.  Input from people knowledgeable on
   these platforms will be appreciated."

 Thanks.

  Maciej