Lifetime of local variables

[Martin Baulig <martin@gnome.org>]

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Hi,

I was recently working a bit on debugging support for C# (using Mono)
and I need a way to tell GDB about the lifetime of local variables. In
C#, the JIT engine may decide to store two different variables at the
same stack offset if they aren't used throughout the whole function.

Now I was wondering how to do this - DWARF 2 already has a
`DW_AT_begin_scope' but unfortunately no `DW_AT_end_scope' - can we
add this or something similar as a GNU extension ?

After looking at the code, I found out that `struct symbol' contains a
`ranges' field which seems to do exactly what I want - but this field
isn't used anywhere.

The following patch

* adds a new DWARF 2 attribute DW_AT_end_scope to specify the end of
  a local variable's scope.

* initializes SYMBOL_RANGES() in dwarf2read.c if DW_AT_begin_scope and
  DW_AT_end_scope are specified.

  [FIXME: According to the DWARF 2 specification, this attribute takes
          an offset (DW_FROM_data), not an address - but I found now
          way to get the current frame's address in new_symbol(), this
          needs to be fixed.]

* checks whether the current PC is within the symbol's SYMBOL_RANGES()
  when listing the local variables of a function and when printing a
  variable.


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Index: include/elf/ChangeLog
===================================================================
RCS file: /cvs/src/src/include/elf/ChangeLog,v
retrieving revision 1.121
diff -u -u -p -r1.121 ChangeLog
--- include/elf/ChangeLog	13 Feb 2002 18:14:48 -0000	1.121
+++ include/elf/ChangeLog	12 Apr 2002 19:50:31 -0000
@@ -1,3 +1,7 @@
+2002-04-12  Martin Baulig  <martin@gnome.org>
+
+	* dwarf2.h (DW_AT_end_scope): Added as GNU extension.
+
 2002-02-13  Matt Fredette  <fredette@netbsd.org>
 
 	* m68k.h (EF_M68000): Define.
Index: include/elf/dwarf2.h
===================================================================
RCS file: /cvs/src/src/include/elf/dwarf2.h,v
retrieving revision 1.8
diff -u -u -p -r1.8 dwarf2.h
--- include/elf/dwarf2.h	28 Jan 2002 23:26:53 -0000	1.8
+++ include/elf/dwarf2.h	12 Apr 2002 19:50:32 -0000
@@ -328,6 +328,8 @@ enum dwarf_attribute
     DW_AT_src_coords = 0x2104,
     DW_AT_body_begin = 0x2105,
     DW_AT_body_end   = 0x2106,
+    DW_AT_end_scope  = 0x2121,
+    
     /* VMS Extensions.  */
     DW_AT_VMS_rtnbeg_pd_address = 0x2201
   };
Index: gdb/ChangeLog
===================================================================
RCS file: /cvs/src/src/gdb/ChangeLog,v
retrieving revision 1.2421
diff -u -u -p -r1.2421 ChangeLog
--- gdb/ChangeLog	12 Apr 2002 07:37:17 -0000	1.2421
+++ gdb/ChangeLog	12 Apr 2002 19:50:38 -0000
@@ -1,3 +1,14 @@
+2002-04-12  Martin Baulig  <martin@gnome.org>
+
+	* dwarf2read.c (new_symbol): If DW_AT_start_scope and DW_AT_end_scope
+	are specified, set SYMBOL_RANGES().
+
+	* findvar.c (read_var_value): Check whether the current PC is within
+	the SYMBOL_RANGES(), return NULL if not.
+
+	* stack.c (print_block_frame_locals): Only print vars if the current PC
+	is in their SYMBOL_RANGES().
+
 2002-04-12  Kevin Buettner  <kevinb@redhat.com>
 
 	From Jimi X <jimix@watson.ibm.com>:
Index: gdb/dwarf2read.c
===================================================================
RCS file: /cvs/src/src/gdb/dwarf2read.c,v
retrieving revision 1.52
diff -u -u -p -r1.52 dwarf2read.c
--- gdb/dwarf2read.c	4 Apr 2002 22:26:43 -0000	1.52
+++ gdb/dwarf2read.c	12 Apr 2002 19:50:44 -0000
@@ -4394,6 +4394,19 @@ new_symbol (struct die_info *die, struct
 		add_symbol_to_list (sym, list_in_scope);
 	      break;
 	    }
+	  attr = dwarf_attr (die, DW_AT_start_scope);
+	  attr2 = dwarf_attr (die, DW_AT_end_scope);
+	  if (attr && attr2)
+	    {
+	      struct range_list *r = (struct range_list *)
+		obstack_alloc (&objfile->type_obstack,
+			       sizeof (struct range_list));
+
+	      r->start = DW_ADDR (attr);
+	      r->end = DW_ADDR (attr2);
+
+	      SYMBOL_RANGES (sym) = r;
+	    }
 	  attr = dwarf_attr (die, DW_AT_location);
 	  if (attr)
 	    {
Index: gdb/findvar.c
===================================================================
RCS file: /cvs/src/src/gdb/findvar.c,v
retrieving revision 1.31
diff -u -u -p -r1.31 findvar.c
--- gdb/findvar.c	9 Apr 2002 03:06:13 -0000	1.31
+++ gdb/findvar.c	12 Apr 2002 19:50:45 -0000
@@ -417,9 +417,11 @@ struct value *
 read_var_value (register struct symbol *var, struct frame_info *frame)
 {
   register struct value *v;
+  register struct range_list *r;
   struct type *type = SYMBOL_TYPE (var);
   CORE_ADDR addr;
   register int len;
+  int range_ok = 0;
 
   v = allocate_value (type);
   VALUE_LVAL (v) = lval_memory;	/* The most likely possibility.  */
@@ -429,6 +431,23 @@ read_var_value (register struct symbol *
 
   if (frame == NULL)
     frame = selected_frame;
+
+  if (!SYMBOL_RANGES (var))
+    range_ok = 1;
+  else
+    {
+      for (r = SYMBOL_RANGES (var); r; r = r->next)
+	{
+	  if (r->start <= frame->pc && r->end > frame->pc)
+	    {
+	      range_ok = 1;
+	      break;
+	    }
+	}
+    }
+
+  if (!range_ok)
+    return NULL;
 
   switch (SYMBOL_CLASS (var))
     {
Index: gdb/stack.c
===================================================================
RCS file: /cvs/src/src/gdb/stack.c,v
retrieving revision 1.33
diff -u -u -p -r1.33 stack.c
--- gdb/stack.c	10 Apr 2002 23:32:33 -0000	1.33
+++ gdb/stack.c	12 Apr 2002 19:50:47 -0000
@@ -1173,14 +1173,36 @@ print_block_frame_locals (struct block *
 	case LOC_REGISTER:
 	case LOC_STATIC:
 	case LOC_BASEREG:
-	  values_printed = 1;
-	  for (j = 0; j < num_tabs; j++)
-	    fputs_filtered ("\t", stream);
-	  fputs_filtered (SYMBOL_SOURCE_NAME (sym), stream);
-	  fputs_filtered (" = ", stream);
-	  print_variable_value (sym, fi, stream);
-	  fprintf_filtered (stream, "\n");
-	  break;
+	  {
+	    struct range_list *r;
+	    int range_ok = 0;
+
+	    if (!SYMBOL_RANGES (sym))
+	      range_ok = 1;
+	    else
+	      {
+		for (r = SYMBOL_RANGES (sym); r; r = r->next)
+		  {
+		    if (r->start <= fi->pc && r->end > fi->pc)
+		      {
+			range_ok = 1;
+			break;
+		      }
+		  }
+	      }
+
+	    if (range_ok)
+	      {
+		values_printed = 1;
+		for (j = 0; j < num_tabs; j++)
+		  fputs_filtered ("\t", stream);
+		fputs_filtered (SYMBOL_SOURCE_NAME (sym), stream);
+		fputs_filtered (" = ", stream);
+		print_variable_value (sym, fi, stream);
+		fprintf_filtered (stream, "\n");
+	      }
+	    break;
+	  }
 
 	default:
 	  /* Ignore symbols which are not locals.  */

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-- 
Martin Baulig
martin@gnome.org