Olly misreporting GC time

[kayceesrk]

See description at https://roscidus.com/blog/blog/2024/07/22/performance-2/#:~:text=olly%20can%20be%20used%20get%20summary%20statistics%3A.

Todo: get reproduction steps.

CC @talex5.

[talex5]

Looks like Olly collects stats when it gets EV_RING_STOP, but the child might not have exited by then. This reproduces it for me:

let () =
  let domains = List.init 5 (fun _ ->
      Domain.spawn (fun () ->
          for _ = 1 to 10000000000 do
            ignore (Sys.opaque_identity (ref ()))
          done;
        )
    )
  in
  List.iter Domain.join domains;
  Printf.printf "gdb attach -p %d\n%!" (Unix.getpid ());
  Printf.printf "Then 'break exit' and 'continue'\n%!";
  Printf.printf "Quit gdb when it hits the break-point.\n%!";
  Unix.sleep 5

(the gdb thing is just to slow down the exit call a bit)

$ olly gc-stats './_build/default/main.exe'
gdb attach -p 271475             
Then 'break exit' and 'continue'
Quit gdb when it hits the break-point.

Execution times:
Wall time (s):	21.18
CPU time (s):	1.01
GC time (s):	16.17
GC overhead (% of CPU time):	1605.17%

[kayceesrk]

This was fixed at 6c61583, and the broken again at bc56d7b, both in the same PR #19. I'll reapply the fix.