Re: Handling of c++ function members

[Joost van der Sluis <joost@cnoc.nl>]

On Mon, 2011-09-26 at 23:00 +0200, Joost van der Sluis wrote:
> On Mon, 2011-09-26 at 16:38 -0400, Daniel Jacobowitz wrote:
> > On Mon, Sep 26, 2011 at 12:46 PM, Joost van der Sluis <joost@cnoc.nl> wrote:
> > 
> > Can you be a little more specific, maybe an example?
> > 

> What I understood is, that DW_AT_vtable_elem_location should contain a
> Dwarf-block that calculates the location of a pointer in which the
> location of the function member is stored.

If you need an example, you can compile the wikipedia
virtual-function-example (after fixing compilation by adding the
declaration of 'it') http://en.wikipedia.org/wiki/Virtual_function

objdump -W of the function 'eat' in that example:
 <1><1902>: Abbrev Number: 66 (DW_TAG_class_type)
    <1903>   DW_AT_name        : (indirect string, offset: 0x17e6): Animal      
    <1907>   DW_AT_byte_size   : 8      
    <1908>   DW_AT_decl_file   : 2      
    <1909>   DW_AT_decl_line   : 4      
    <190a>   DW_AT_containing_type: <0x1902>    
    <190e>   DW_AT_sibling     : <0x19be>
 <2><1953>: Abbrev Number: 69 (DW_TAG_subprogram)
    <1954>   DW_AT_external    : 1      
    <1955>   DW_AT_name        : eat    
    <1959>   DW_AT_decl_file   : 2      
    <195a>   DW_AT_decl_line   : 6      
    <195b>   DW_AT_MIPS_linkage_name: (indirect string, offset: 0xad3): _ZNK6Animal3eatEv       
    <195f>   DW_AT_virtuality  : 1      (virtual)
    <1960>   DW_AT_vtable_elem_location: 2 byte block: 10 0     (DW_OP_constu: 0)
    <1963>   DW_AT_containing_type: <0x1902>    
    <1967>   DW_AT_accessibility: 1     (public)
    <1968>   DW_AT_declaration : 1      
    <1969>   DW_AT_object_pointer: <0x1971>     
    <196d>   DW_AT_sibling     : <0x1978>       
 <3><1971>: Abbrev Number: 25 (DW_TAG_formal_parameter)
    <1972>   DW_AT_type        : <0x2a55>       
    <1976>   DW_AT_artificial  : 1

> But it seems to me that gcc stores the index of the function member
> within some vtable in DW_AT_vtable_elem_location, instead of the memory
> address itself. In gnu-v2-abi.c there is some code that 'knows' how this
> vtable is organized so it is able to calculate the location of the
> method-pointer.
> 
> And there are two thing's I'm not sure about: gcc stores
> DW_AT_containing_type in the debug-info, with the 'parent' entry of the
> DW_TAG_subprogram entry. To me it looks that this is duplicated
> information, and not specified in the Dwarf-specs. Secondly I do not
> understand where the DW_AT_object_pointer is used for?

DW_AT_containing_type is placed into NFN's Context field by dwarf2read.

Regards,

Joost van der Sluis.