[PATCH] Gracefully handle not being able to access a DWARF register number

[Matthew Gretton-Dann <matthew dot gretton-dann at arm dot com>]

Hi,

There are cases when gcc outputs a valid DWARF register number in a
DWARF expression, but gdb can't access that register.  In theory this is
a generic problem, but the specific case I'm interested in is debugging
ARM VFP code on Linux with a kernel prior to 2.6.30. In this case ptrace
does not expose the contents of a processes' VFP registers although gcc
accurately describes the location of floating point values as being in
the VFP registers.

If you try to access such a DWARF register number gdb reports an
internal error.

The attached patch changes these internal errors into warnings, and
tries to return something sensible to the user.

Please could someone review and comment on the patch, and then if this
is approved can they please commit it as I don't have commit rights.

Suggested ChangeLog:

2010-04-26  Matthew Gretton-Dann  <matthew.gretton-dann@arm.com>

	* dwarf2loc.c (read_pieced_value, write_pieced_value,
	dwarf2_evaluate_loc_desc): Handle not being able to access DWARF
	registers gracefully.

Thanks,

Matt

-- 
Matthew Gretton-Dann
Principal Engineer - Tools, PD Software
ARM Limited

Index: gdb/dwarf2loc.c
===================================================================
RCS file: /cvs/src/src/gdb/dwarf2loc.c,v
retrieving revision 1.75
diff -u -p -r1.75 dwarf2loc.c
--- gdb/dwarf2loc.c	20 Apr 2010 18:52:59 -0000	1.75
+++ gdb/dwarf2loc.c	26 Apr 2010 14:51:25 -0000
@@ -284,8 +284,17 @@ read_pieced_value (struct value *v)
 	      /* Big-endian, and we want less than full size.  */
 	      reg_offset = register_size (arch, gdb_regnum) - p->size;
 
-	    get_frame_register_bytes (frame, gdb_regnum, reg_offset, p->size,
-				      contents + offset);
+	    if (gdb_regnum != -1)
+	      {
+		get_frame_register_bytes (frame, gdb_regnum, reg_offset, 
+					  p->size, contents + offset);
+	      }
+	    else
+	      {
+		warning (_("Unable to access DWARF register number %" BFD_VMA_FMT "d"),
+			 p->v.expr.value);
+		memset (contents + offset, '\0', p->size);
+	      }
 	  }
 	  break;
 
@@ -356,8 +365,16 @@ write_pieced_value (struct value *to, st
 	      /* Big-endian, and we want less than full size.  */
 	      reg_offset = register_size (arch, gdb_regnum) - p->size;
 
-	    put_frame_register_bytes (frame, gdb_regnum, reg_offset, p->size,
-				      contents + offset);
+	    if (gdb_regnum != -1)
+	      {
+		put_frame_register_bytes (frame, gdb_regnum, reg_offset, 
+					  p->size, contents + offset);
+	      }
+	    else
+	      {
+		warning (_("Unable to write to DWARF Register %" BFD_VMA_FMT "d"),
+			 p->v.expr.value);
+	      }
 	  }
 	  break;
 	case DWARF_VALUE_MEMORY:
@@ -454,7 +471,20 @@ dwarf2_evaluate_loc_desc (struct symbol 
 	    struct gdbarch *arch = get_frame_arch (frame);
 	    CORE_ADDR dwarf_regnum = dwarf_expr_fetch (ctx, 0);
 	    int gdb_regnum = gdbarch_dwarf2_reg_to_regnum (arch, dwarf_regnum);
-	    retval = value_from_register (SYMBOL_TYPE (var), gdb_regnum, frame);
+	    if (gdb_regnum != -1)
+	      {
+		retval = value_from_register (SYMBOL_TYPE (var),
+					      gdb_regnum, frame);
+	      }
+	    else
+	      {
+		warning (_("Unable to access DWARF register number %" BFD_VMA_FMT "d"),
+			 dwarf_regnum);
+		retval = allocate_value (SYMBOL_TYPE (var));
+		VALUE_LVAL (retval) = not_lval;
+		set_value_optimized_out (retval, 1);
+		return retval;
+	      }
 	  }
 	  break;