read_structure_type and virtual base types

["Thomson, Patrick S" <patrick dot s dot thomson at intel dot com>]

Howdy Folks,

I'm attempting to use gdb 6.3 on an executable generated from the 
latest intel c++ compiler (9.0). That particular compiler does not 
generate child DIE information for member functions which are 
declared but not implemented. If the class is a derived class and I 
try to print an instantiated object of that class, gdb crashes.

I tracked what I think is the issue to 
dwarf2read.c::read_structure_type().

At line read_structure_type()+119 we set TYPE_VPTR_BASETYPE(type), 
but only if we have 1 or more member functions (see line 
read_structure_type()+107 for fi.nfnfields test, this field is set at 
line read_structure_type()+94). 

Because I have no child DIEs for member functions, TYPE_VPTR_BASETYPE 
remains 0, and I crash later on because this is eventually 
dereferenced. Enough other data remains in the type structure to 
indicate that the class is a derived class, so gdb expects 
TYPE_VPTR_BASETYPE(type) to be set.

Since g++ does output the child die, this isn't generally a problem. 
Ultimately my question is this: Is there a reason why the logic is 
structured the way it is, and what is it? I can fix it easily enough 
in my local copy by moving the closing brace for the 
'if(fi.nfnfields){}' block so that it doesn't encompass the 
DW_AT_containing_type attribute test a few lines later, but I'm not 
sure what the consequences would be.

Thanks!
Pat Thomson
Intel Corporation

p.s. example class with associated DWARF DIE

class A : virtual public V{
    int a;
    int aa;
public:
    void af(int); // declared, not implmented
};

Has the following DIE
<1><b64>: Abbrev Number: 17 (DW_TAG_class_type)
     DW_AT_decl_line   : 11	
     DW_AT_decl_column : 7	
     DW_AT_decl_file   : 1	
     DW_AT_accessibility: 1	(public)
     DW_AT_byte_size   : 20	
     DW_AT_name        : A	
     DW_AT_containing_type: <b64>	
 <2><b70>: Abbrev Number: 18 (DW_TAG_inheritance)
     DW_AT_decl_line   : 11	
     DW_AT_decl_column : 26	
     DW_AT_decl_file   : 1	
     DW_AT_accessibility: 1	(public)
     DW_AT_data_member_location: 7 byte block: 12 6 10 c 1c 6 22
(DW_OP_dup; DW_OP_deref; DW_OP_constu: 12; DW_OP_minus; DW_OP_deref;
DW_OP_plus)
     DW_AT_type        : <a75>	
     DW_AT_virtuality  : 1	(virtual)
 <2><b82>: Abbrev Number: 19 (DW_TAG_member)
     DW_AT_name        : _vptr.A	
     DW_AT_data_member_location: 2 byte block: 23 0
(DW_OP_plus_uconst: 0)
     DW_AT_artificial  : 1	
     DW_AT_type        : <baf>	
 <2><b93>: Abbrev Number: 3 (DW_TAG_member)
     DW_AT_decl_line   : 12	
     DW_AT_decl_column : 9	
     DW_AT_decl_file   : 1	
     DW_AT_data_member_location: 2 byte block: 23 4
(DW_OP_plus_uconst: 4)
     DW_AT_name        : a	
     DW_AT_type        : <ab0>	
 <2><ba0>: Abbrev Number: 3 (DW_TAG_member)
     DW_AT_decl_line   : 13	
     DW_AT_decl_column : 9	
     DW_AT_decl_file   : 1	
     DW_AT_data_member_location: 2 byte block: 23 8
(DW_OP_plus_uconst: 8)
     DW_AT_name        : aa	
     DW_AT_type        : <ab0>