fix rtl-opt/17186

[Richard Henderson <rth at redhat dot com>]

The problem here is directly related to the fall-off-the-end
path in the test case:

	double foo()
	{
	    int i;
	    double d;

	    if (i)
		bar();
	    else
		if (d) return 0;
	}

Combined with the path that *does* return a value, the initial
rtl generated looks like

	...
	(clobber st0)
  return_label:
	(set st0 return_value)
	(use st0)
	(return)

After register allocation, return_value is replaced with st1.
But when it comes time for reg-stack, we can't find a live
value for st1 on all incoming edges, so we abort.

The following adds a jump around the setting of the hard return
register there just before the return.  This means that we aren't
looking for st1 to be live in reg-stack, which means that we do
the right thing and load up a NaN in st0 to satisfy the stack
height requirements of the ABI.

Applied to mainline and 3.4.


r~


        PR rtl-opt/17186
        * function.c (expand_function_end): Have fall-off-the-end
        return path jump around return register setup.

Index: function.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/function.c,v
retrieving revision 1.573
diff -c -p -d -r1.573 function.c
*** function.c	8 Sep 2004 18:44:56 -0000	1.573
--- function.c	8 Sep 2004 19:21:16 -0000
*************** expand_function_end (void)
*** 4411,4430 ****
  
    /* Emit the actual code to clobber return register.  */
    {
!     rtx seq, after;
  
      start_sequence ();
      clobber_return_register ();
      seq = get_insns ();
      end_sequence ();
  
!     after = emit_insn_after (seq, clobber_after);
    }
  
!   /* Output the label for the naked return from the function, if one is
!      expected.  This is currently used only by __builtin_return.  */
!   if (naked_return_label)
!     emit_label (naked_return_label);
  
    /* ??? This should no longer be necessary since stupid is no longer with
       us, but there are some parts of the compiler (eg reload_combine, and
--- 4411,4429 ----
  
    /* Emit the actual code to clobber return register.  */
    {
!     rtx seq;
  
      start_sequence ();
      clobber_return_register ();
+     expand_naked_return ();
      seq = get_insns ();
      end_sequence ();
  
!     emit_insn_after (seq, clobber_after);
    }
  
!   /* Output the label for the naked return from the function.  */
!   emit_label (naked_return_label);
  
    /* ??? This should no longer be necessary since stupid is no longer with
       us, but there are some parts of the compiler (eg reload_combine, and