[RS6000] PR81996, __builtin_return_address(0) fails

rs6000_return_addr assumes that the stack link is at frame+0, which is
true for count>0.  For count==0, rs6000_return_addr is called with
frame==frame_pointer_rtx and the stack link is *not* at frame+0 if
-fstack-protector-all or -fsanitize=address because rs6000.h sets
FRAME_GROWS_DOWNWARD for those options.

	PR target/81996
	* gcc/config/rs6000/rs6000.c (rs6000_return_addr): Use
	stack_pointer_rtx for count 0.  Update comments.  Break up
	large rtl expression.

From-SVN: r252901

gcc/ChangeLog

+2017-09-18  Alan Modra  <amodra@gmail.com>
+
+	PR target/81996
+	* gcc/config/rs6000/rs6000.c (rs6000_return_addr): Use
+	stack_pointer_rtx for count 0.  Update comments.  Break up
+	large rtl expression.
+
 2017-09-17  Daniel Santos  <daniel.santos@pobox.com>
 
 	config/i386/i386.c: (xlogue_layout::STUB_NAME_MAX_LEN): Increase to 20

gcc/config/rs6000/rs6000.c

@@ -25009,24 +25009,23 @@ debug_stack_info (rs6000_stack_t *info)
 rtx
 rs6000_return_addr (int count, rtx frame)
 {
-  /* Currently we don't optimize very well between prolog and body
-     code and for PIC code the code can be actually quite bad, so
-     don't try to be too clever here.  */
+  /* We can't use get_hard_reg_initial_val for LR when count == 0 if LR
+     is trashed by the prologue, as it is for PIC on ABI_V4 and Darwin.  */
   if (count != 0
       || ((DEFAULT_ABI == ABI_V4 || DEFAULT_ABI == ABI_DARWIN) && flag_pic))
     {
       cfun->machine->ra_needs_full_frame = 1;
 
-      return
-	gen_rtx_MEM
-	  (Pmode,
-	   memory_address
-	   (Pmode,
-	    plus_constant (Pmode,
-			   copy_to_reg
-			   (gen_rtx_MEM (Pmode,
-					 memory_address (Pmode, frame))),
-			   RETURN_ADDRESS_OFFSET)));
+      if (count == 0)
+	/* FRAME is set to frame_pointer_rtx by the generic code, but that
+	   is good for loading 0(r1) only when !FRAME_GROWS_DOWNWARD.  */
+	frame = stack_pointer_rtx;
+      rtx prev_frame_addr = memory_address (Pmode, frame);
+      rtx prev_frame = copy_to_reg (gen_rtx_MEM (Pmode, prev_frame_addr));
+      rtx lr_save_off = plus_constant (Pmode,
+				       prev_frame, RETURN_ADDRESS_OFFSET);
+      rtx lr_save_addr = memory_address (Pmode, lr_save_off);
+      return gen_rtx_MEM (Pmode, lr_save_addr);
     }
 
   cfun->machine->ra_need_lr = 1;