[m68k] Fix __builtin_return_address

[Paul Brook <paul at codesourcery dot com>]

The default definition of RETURN_ADDR_RTX assumes the frame pointer points to 
a (frame backlink, return address) pair. This is not true for m68k when 
-fomit-frame-pointer is used.

The attached patch fixes this by using a similar definiton to the one in 
i386.c.  We know the return address will always be immediately before the 
functions arguments.

In the process I tripped over the FIXME in m68k_initial_elimination_offset.
The "obscure reason" is that the old values don't take FIRST_PARM_OFFSET into 
account. The old values worked mostly by chance because we never eliminate 
arg->sp when frame_pointer_needed, and never eliminate arg->fp 
when !frame_pointer_needed.

This fixes gcc.c-torture/execute/pr17377.c

Tested with cross to m68k-elf/-m5200
Ok?

Paul

2005-12-23  Paul Brook  <paul@codesourcery.com>

	* config/m68k/m68k.h (RETURN_ADDR_RTX): Define.
	* config/m68k/m68k.c (m68k_initial_elimination_offset): Remove FIXME.
	Include offset due to FIRST_PARM_OFFSET.

Index: gcc/config/m68k/m68k.c
===================================================================
--- gcc/config/m68k/m68k.c	(revision 108911)
+++ gcc/config/m68k/m68k.c	(working copy)
@@ -434,11 +434,13 @@
 HOST_WIDE_INT
 m68k_initial_elimination_offset (int from, int to)
 {
-  /* FIXME: The correct offset to compute here would appear to be
-       (frame_pointer_needed ? -UNITS_PER_WORD * 2 : -UNITS_PER_WORD);
-     but for some obscure reason, this must be 0 to get correct code.  */
+  int argptr_offset;
+  /* The arg pointer points 8 bytes before the start of the arguments,
+     as defined by FIRST_PARM_OFFSET.  This makes it coincident with the
+     frame pointer in most frames.  */
+  argptr_offset = frame_pointer_needed ? 0 : UNITS_PER_WORD;
   if (from == ARG_POINTER_REGNUM && to == FRAME_POINTER_REGNUM)
-    return 0;
+    return argptr_offset;
 
   m68k_compute_frame_layout ();
 
@@ -446,8 +448,7 @@
   switch (from)
     {
     case ARG_POINTER_REGNUM:
-      return (current_frame.offset + current_frame.size
-	      + (frame_pointer_needed ? -UNITS_PER_WORD * 2 : -UNITS_PER_WORD));
+      return current_frame.offset + current_frame.size - argptr_offset;
     case FRAME_POINTER_REGNUM:
       return current_frame.offset + current_frame.size;
     default:
Index: gcc/config/m68k/m68k.h
===================================================================
--- gcc/config/m68k/m68k.h	(revision 108911)
+++ gcc/config/m68k/m68k.h	(working copy)
@@ -890,6 +890,12 @@
 #define INCOMING_RETURN_ADDR_RTX \
   gen_rtx_MEM (VOIDmode, gen_rtx_REG (VOIDmode, STACK_POINTER_REGNUM))
 
+/* After the prologue, RA is at 4(AP) in the current frame.  */
+#define RETURN_ADDR_RTX(COUNT, FRAME)					   \
+  ((COUNT) == 0								   \
+   ? gen_rtx_MEM (Pmode, plus_constant (arg_pointer_rtx, UNITS_PER_WORD)) \
+   : gen_rtx_MEM (Pmode, plus_constant (FRAME, UNITS_PER_WORD)))
+
 /* We must not use the DBX register numbers for the DWARF 2 CFA column
    numbers because that maps to numbers beyond FIRST_PSEUDO_REGISTER.
    Instead use the identity mapping.  */