[Bug target/51784] PIC register not correctly preserved in nested funcs / with non-local goto

["jakub at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51784

--- Comment #23 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-01-14 11:03:26 UTC ---
How would that help?  With nonlocal goto, you need to recompute the PIC
register (if different from the function doing nonlocal goto) in the nonlocal
goto receiver.
Consider:
extern void baz (void (*) (void));
volatile int z, z1;
static volatile int z2, z3;
int
foo (void)
{
  __label__ l;
  void bar ()
  {
    goto l;
  }
  baz (bar);
  return z1 + z3;
l:
  return z + z2;
}
If you attempt to restore the PIC register in bar before doing the jump,
you'd restore it to baz PIC register rather than foo PIC register.
md.texi clearly hints it:
@cindex @code{nonlocal_goto_receiver} instruction pattern
@item @samp{nonlocal_goto_receiver}
This pattern, if defined, contains code needed at the target of a
nonlocal goto after the code already generated by GCC@.  You will not
normally need to define this pattern.  A typical reason why you might
need this pattern is if some value, such as a pointer to a global table,
must be restored when the frame pointer is restored.  Note that a nonlocal
goto only occurs within a unit-of-translation, so a global table pointer
that is shared by all functions of a given module need not be restored.
There are no arguments.

darwin clearly doesn't have a PIC pointer shared by all functions of a given
module, therefore it needs to be restored.