Emitting correct DWARF location descriptor for multi-reg frame pointer

[Senthil Kumar Selvaraj <senthil_kumar dot selvaraj at atmel dot com>]

As a follow-up to this discussion
(http://gcc.gnu.org/ml/gcc/2012-11/msg00307.html), I have a patch that
makes dwarf2out.c emit correct location information for a FP register spanning
multiple hard regs.

Before the patch, the DW_AT_location for a local variable on the AVR
target (whose FP spans registers R29 and R28) looks like this
<4f>   DW_AT_location    : 2 byte block: 8c 1   (DW_OP_breg28 (r28): 1)

After the patch, it looks like this
<4f>   DW_AT_location    : 11 byte block: 6c 93 1 6d 93 1 11 1 22 94 2
(DW_OP_reg28 (r28); DW_OP_piece: 1; DW_OP_reg29 (r29); DW_OP_piece: 1;
DW_OP_consts: 1; DW_OP_plus; DW_OP_deref_size: 2)

What do you guys think?

Regards
Senthil


diff --git gcc/dwarf2out.c gcc/dwarf2out.c
index f0256ae..53a4e56 100644
--- gcc/dwarf2out.c
+++ gcc/dwarf2out.c
@@ -10875,11 +10875,25 @@ based_loc_descr (rtx reg, HOST_WIDE_INT offset,
       return new_loc_descr (DW_OP_fbreg, offset, 0);
     }
 
-  if (regno <= 31)
-    result = new_loc_descr ((enum dwarf_location_atom) (DW_OP_breg0 + regno),
-			    offset, 0);
+  rtx regs = targetm.dwarf_register_span (reg);
+
+  if (hard_regno_nregs[REGNO (reg)][GET_MODE (reg)] > 1 || regs)
+    {
+      result = multiple_reg_loc_descriptor (reg, regs, initialized);
+      add_loc_descr (&result, new_loc_descr (DW_OP_consts, offset, 0));
+      add_loc_descr (&result, new_loc_descr (DW_OP_plus, 0, 0));
+      add_loc_descr (&result, new_loc_descr (DW_OP_deref_size, 
+                  GET_MODE_SIZE (GET_MODE (reg)),
+                  0));
+    }
   else
-    result = new_loc_descr (DW_OP_bregx, regno, offset);
+    {
+      if (regno <= 31)
+          result = new_loc_descr ((enum dwarf_location_atom) (DW_OP_breg0 + regno),
+			    offset, 0);
+      else
+          result = new_loc_descr (DW_OP_bregx, regno, offset);
+    }
 
   if (initialized == VAR_INIT_STATUS_UNINITIALIZED)
     add_loc_descr (&result, new_loc_descr (DW_OP_GNU_uninit, 0, 0));