c++/7081: DWARF enhancements for C++

[jcownie@etnus.com]

>Number:         7081
>Category:       c++
>Synopsis:       DWARF enhancements for C++
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          change-request
>Submitter-Id:   net
>Arrival-Date:   Thu Jun 20 02:16:04 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     James Cownie
>Release:        3.1
>Organization:
Etnus LLC
>Environment:
System: Linux pc4 2.4.18 #1 Tue Apr 2 10:17:01 BST 2002 i686 unknown
Architecture: i686

	
host: i686-pc-linux-gnu
build: i686-pc-linux-gnu
target: i686-pc-linux-gnu
configured with: /home1/GNU/gcc-3.1/configure 
>Description:
1) g++ emits classes in DWARF with the DW_TAG_structure_type rather
   than the DW_TAG_class_type which is suggested in the DWARF V3
   specification. This makes it hard for a debugger to show the type
   name correctly since it can't tell whether the class was _really_
   introduced by the class or struct keywords.

2) g++ does not emit any debug information for the vtable pointer in
   classes with virtual functions. It would be much nicer if this
   information were emitted (tagged with AT_artificial), since it's
   often very useful to be able to see the vtable pointer while
   debugging, as that's how one can work out which class an object
   _actually_ is when one is looking at a base class pointer.

>How-To-Repeat:
Using code like this

#define class_body(classname)					\
{								\
   char * classname##_data;					\
public:								\
  classname () { classname##_data = #classname " data"; }	\
  virtual char * get_##classname##_data () { return classname##_data; } \
}

class A class_body (A);
class B class_body (B);
class foo: virtual public A, public B class_body (foo);

int main (int argc, char **argv)
{
  foo f00;
  int i = 1;
}

Compile it and use readelf to look at the dwarf. One can then observe

1) <1><cb>: Abbrev Number: 7 (DW_TAG_structure_type)
       DW_AT_sibling     : <1b3>	
       DW_AT_name        : foo	
       DW_AT_byte_size   : 20	
       DW_AT_decl_file   : 1	
       DW_AT_decl_line   : 12	
       DW_AT_containing_type: <2e1>	
   
   So "foo" is a "struct", not a "class".

2) Eliding the member functions, the contents of class foo are
   <1><cb>: Abbrev Number: 7 (DW_TAG_structure_type)
       DW_AT_sibling     : <1b3>	
       DW_AT_name        : foo	
       DW_AT_byte_size   : 20	
       DW_AT_decl_file   : 1	
       DW_AT_decl_line   : 12	
       DW_AT_containing_type: <2e1>	
   <2><db>: Abbrev Number: 8 (DW_TAG_inheritance)
       DW_AT_type        : <227>	
       DW_AT_data_member_location: 6 byte block: 12 6 3c 1c 6 22 	(DW_OP_dup; DW_OP_deref; DW_OP_lit12; DW_OP_minus; DW_OP_deref; DW_OP_plus; )
       DW_AT_virtuality  : 1	(virtual)
       DW_AT_accessibility: 1	(public)
   <2><e9>: Abbrev Number: 9 (DW_TAG_inheritance)
       DW_AT_type        : <2e1>	
       DW_AT_data_member_location: 2 byte block: 23 0 	(DW_OP_plus_uconst: 0; )
       DW_AT_accessibility: 1	(public)
   <2><f2>: Abbrev Number: 10 (DW_TAG_member)
       DW_AT_name        : foo_data	
       DW_AT_decl_file   : 1	
       DW_AT_decl_line   : 12	
       DW_AT_type        : <bd>	
       DW_AT_data_member_location: 2 byte block: 23 8 	(DW_OP_plus_uconst: 8; )
     DW_AT_accessibility: 3	(private)

   So there is no information about the vtable pointer.

>Fix:
	
>Release-Note:
>Audit-Trail:
>Unformatted: