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 <dero...@adacore.com>
--
Dwarf-discuss mailing list
Dwarf-discuss@lists.dwarfstd.org
https://lists.dwarfstd.org/mailman/listinfo/dwarf-discuss
