Re: Support constants for DW_AT_data_member_location

[Jim Blandy <jimb@codesourcery.com>]

Vladimir Prus <vladimir at codesourcery.com> writes:
> This is wrong, per DWARF standard:
>
>        Some encodings are members of more than one class; in that case, 
>         the list of classes allowed by the applicable attribute in 
>         Figure 18 determines the class of the form. DW_FORM_data4 and 
>         DW_FORM_data8 may be members of classes constant, lineptr, 
>         loclistptr, macptr and  rangelistptr. They are members of the 
>         class constant if used for the value of an attribute that allows class
>         constant but not class lineptr, loclistptr, macptr or rangelistptr.
>         They are members of the class lineptr, loclistptr, macptr or 
>         rangelistptr if used for the value of an attribute that allows
>         one of those classes.
>
> Referring to Figure 18, I see:
>
>      Attribute name                  Classes
>     DW_AT_data_member_location       block, constant, loclistptr
>
> So, DW_FORM_data4 used for DW_AT_data_member_location is never interepreted
> as constant. Maybe we should have "is_surely_constant" function?

Oh --- great catch, thanks.  (We don't support location lists for
DW_AT_data_member_location yet, but we certainly should warn, not
randomly misinterpret the value.)

How's this, then?

gdb/ChangeLog:
2007-11-30  Jim Blandy  <jimb@codesourcery.com>

	* dwarf2read.c (attr_form_is_constant): New function.
	(dwarf2_add_field): Use it and attr_form_is_section_offset to
	recognize DW_AT_data_member_location attributes.  Use
	dwarf2_get_attr_constant_value when the attribute is a constant.

	* dwarf2read.c (attr_form_is_section_offset): New function.
	(dwarf_add_member_fn, read_common_block, read_partial_die)
	(dwarf2_symbol_mark_computed): Use it, instead of writing it out.

diff -r c4f654de59cf gdb/dwarf2read.c
--- a/gdb/dwarf2read.c	Thu Nov 29 11:28:59 2007 -0800
+++ b/gdb/dwarf2read.c	Fri Nov 30 17:32:46 2007 -0800
@@ -1039,6 +1039,10 @@ static void dwarf_decode_macros (struct 
                                  char *, bfd *, struct dwarf2_cu *);
 
 static int attr_form_is_block (struct attribute *);
+
+static int attr_form_is_section_offset (struct attribute *);
+
+static int attr_form_is_constant (struct attribute *);
 
 static void dwarf2_symbol_mark_computed (struct attribute *attr,
 					 struct symbol *sym,
@@ -3380,8 +3384,16 @@ dwarf2_add_field (struct field_info *fip
       attr = dwarf2_attr (die, DW_AT_data_member_location, cu);
       if (attr)
 	{
-	  FIELD_BITPOS (*fp) =
-	    decode_locdesc (DW_BLOCK (attr), cu) * bits_per_byte;
+          if (attr_form_is_section_offset (attr))
+            {
+              dwarf2_complex_location_expr_complaint ();
+              FIELD_BITPOS (*fp) = 0;
+            }
+          else if (attr_form_is_constant (attr))
+            FIELD_BITPOS (*fp) = dwarf2_get_attr_constant_value (attr, 0);
+          else
+            FIELD_BITPOS (*fp) =
+              decode_locdesc (DW_BLOCK (attr), cu) * bits_per_byte;
 	}
       else
 	FIELD_BITPOS (*fp) = 0;
@@ -3700,7 +3712,7 @@ dwarf2_add_member_fn (struct field_info 
         {
           fnp->voffset = decode_locdesc (DW_BLOCK (attr), cu) + 2;
         }
-      else if (attr->form == DW_FORM_data4 || attr->form == DW_FORM_data8)
+      else if (attr_form_is_section_offset (attr))
         {
 	  dwarf2_complex_location_expr_complaint ();
         }
@@ -4416,7 +4428,7 @@ read_common_block (struct die_info *die,
         {
           base = decode_locdesc (DW_BLOCK (attr), cu);
         }
-      else if (attr->form == DW_FORM_data4 || attr->form == DW_FORM_data8)
+      else if (attr_form_is_section_offset (attr))
         {
 	  dwarf2_complex_location_expr_complaint ();
         }
@@ -5598,7 +5610,7 @@ read_partial_die (struct partial_die_inf
             {
 	       part_die->locdesc = DW_BLOCK (&attr);
             }
-          else if (attr.form == DW_FORM_data4 || attr.form == DW_FORM_data8)
+          else if (attr_form_is_section_offset (&attr))
             {
 	      dwarf2_complex_location_expr_complaint ();
             }
@@ -9755,6 +9767,46 @@ attr_form_is_block (struct attribute *at
       || attr->form == DW_FORM_block);
 }
 
+/* Return non-zero if ATTR's value is a section offset (classes
+   lineptr, loclistptr, macptr or rangelistptr).  In this case,
+   you may use DW_UNSND (attr) to retrieve the offset.  */
+static int
+attr_form_is_section_offset (struct attribute *attr)
+{
+  return (attr->form == DW_FORM_data4
+          || attr->form == DW_FORM_data8);
+}
+
+
+/* Return non-zero if ATTR's value falls in the 'constant' class, or
+   zero otherwise.  When this function returns true, you can apply
+   dwarf2_get_attr_constant_value to it.
+
+   However, note that for some attributes you must check
+   attr_form_is_section_offset before using this test.  DW_FORM_data4
+   and DW_FORM_data8 are members of both the constant class, and of
+   the classes that contain offsets into other debug sections
+   (lineptr, loclistptr, macptr or rangelistptr).  The DWARF spec says
+   that, if an attribute's can be either a constant or one of the
+   section offset classes, DW_FORM_data4 and DW_FORM_data8 should be
+   taken as section offsets, not constants.  */
+static int
+attr_form_is_constant (struct attribute *attr)
+{
+  switch (attr->form)
+    {
+    case DW_FORM_sdata:
+    case DW_FORM_udata:
+    case DW_FORM_data1:
+    case DW_FORM_data2:
+    case DW_FORM_data4:
+    case DW_FORM_data8:
+      return 1;
+    default:
+      return 0;
+    }
+}
+
 static void
 dwarf2_symbol_mark_computed (struct attribute *attr, struct symbol *sym,
 			     struct dwarf2_cu *cu)
@@ -9766,7 +9818,7 @@ dwarf2_symbol_mark_computed (struct attr
   if (objfile->separate_debug_objfile_backlink)
     objfile = objfile->separate_debug_objfile_backlink;
 
-  if ((attr->form == DW_FORM_data4 || attr->form == DW_FORM_data8)
+  if (attr_form_is_section_offset (attr)
       /* ".debug_loc" may not exist at all, or the offset may be outside
 	 the section.  If so, fall through to the complaint in the
 	 other branch.  */