Re: [Caml-list] Possible ephemeron bug?

[Stephen Dolan <stephen.dolan@cl.cam.ac.uk>]

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On Thu, 20 Dec 2018 at 00:30, Lindsay Errington <lindsay.errington@gmail.com>
wrote:

> If however, I use the standalone bytecode compiler or the native compiler
> (4.07.1), then the entries are not nullified. Is this a bug or is there
> another way to persuade the garbage collector to clobber the entries?
>

Thanks for the detailed example!

The issue is with the test code at the end:

    let (root,map) =
      let map = [] in
      let (k0,map) = intern 0 (str 1) map in
      let (k1,map) = intern 1 (Link k0) map in
      let (k2,map) = intern 2 (Link k1) map in
      (k2,map);;

    Fmt.printf "root=%a, map=@[<v>%a@]@." pp_key root pp_map map;;

    let map = upd root (str 2) map;;

    Fmt.printf "root=%a map1=@[<v>%a@]@." pp_key root pp_map map;;

Both the bytecode and native code compilers take any value bound to a
global to be reachable forever, even if that global is later shadowed by
another global of the same name. The ephemerons don't get cleared, because
the original map is still alive.

You can change the code to avoid putting the original map in a global
constant:

    let (root, map) =
      let map = [] in
      let (k0,map) = intern 0 (str 1) map in
      let (k1,map) = intern 1 (Link k0) map in
      let (k2,map) = intern 2 (Link k1) map in
      Fmt.printf "root=%a, map=@[<v>%a@]@." pp_key k2 pp_map map;
      let map = upd k2 (str 2) map in
      (k2, map);;

    Fmt.printf "root=%a map1=@[<v>%a@]@." pp_key root pp_map map;;

With this version, the original map becomes unreachable, so the GC can
clear the ephemerons.

Stephen

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