[Dwarf-discuss] Representing vtables in DWARF for downcasting

[Pierre-Marie de Rodat]

Hello,

On Wed, May 7, 2025 at 2:49 PM Todd Allen via Dwarf-discuss
<dwarf-discuss@lists.dwarfstd.org> wrote:
> In 250506.2, the use of a rnglist is throwing me.  I would expect the lifetime of a vtable to be the whole program.  Or did you envision the rnglist to be the range of data/rodata addresses of the vtable object?  2.17 clarifies that they're code addresses (i.e. text), though.
>
> We did have a discussion sometime in the last year about describing data/rodata address ranges, but that was in .debug_aranges (RIP).  And, IIRC, no actual compiler was generating data/rodata address there either.

If it helps the design: there are languages where vtables are not
necessarily statically allocated. Here is a small Ada example,
involving a tagged type (equivalent to a C++ class) nested in a
procedure, and with a primitive (C++ method) that actually has
up-level references to the procedure locals (so the vtable is actually
tied to the current stack frame):

     1  with Ada.Text_IO; use Ada.Text_IO;
     2
     3  procedure Main is
     4     Msg : constant String := "Hello world";
     5
     6     package Pkg is
     7        type T is tagged null record;
     8        procedure Print (Self : T);
     9     end Pkg;
    10
    11     package body Pkg is
    12        procedure Print (Self : T) is
    13        begin
    14           Put_Line (Msg);
    15        end Print;
    16     end Pkg;
    17
    18     Object : Pkg.T;
    19  begin
    20     Object.Print;
    21  end Main;

GDB allows us to observe where the vtable for T is stored (tested on a
x86_64-linux machine):

$ gdb ./main
(gdb) b main.adb:20
Breakpoint 1 at 0x6ae9: file main.adb, line 20.
(gdb) r
[…]
Breakpoint 1, main () at main.adb:20
20         Object.Print;
(gdb) set lang c
Warning: the current language does not match this frame.
(gdb) print object
$1 = {_tag = 0x7fffffffda30}
(gdb) p $rsp
$2 = (void *) 0x7fffffffd7d0
(gdb) p $rbp
$3 = (void *) 0x7fffffffda70

“_tag” is an artificial component for the record T that GCC
(currently) generates in the debug info to materialize the vtable: it
points to a structure that is in the current stack frame (between $rsp
and $rbp).

-- 
Pierre-Marie de Rodat <derodat@adacore.com>