[BUG] Miscompilation of code involving references and exceptions

[vouillon]

I have found out what the block handler was for... This following piece of code is miscompiled. The issue is that the OCaml bytecode program does not allocate a reference but update the stack in place. But we no longer keep track of this update.

let x = ref 0 in
try
  incr x;
  raise Exit
with Exit ->
  Format.eprintf "%d@." !x

You have to compile it without the -g option. For instance:

ocamlc bug.ml -o bug.byte && js_of_ocaml bug.byte && ./bug.byte && node bug.js

The above command outputs 1 (correct) then 0 (wrong):

[vouillon]

I think that this piece of code, with nested exception handlers, has always been miscompiled.

let x = ref 0 in
try
  (try incr x ; raise Exit with _ -> ());
  raise Exit
with Exit ->
  Format.eprintf "%d@." !x

[hhugo]

I wonder if we could make that logic more explicit by keeping track of ASSIGNed variable that have been previously captured in an exception handler. Maybe using a new instruction

type instr =
  | Let of Var.t * expr
  | Set_field of Var.t * int * Var.t
  | Offset_ref of Var.t * int
  | Array_set of Var.t * Var.t * Var.t
  | Update_pushtrap_cont  of Var.t * Var.t

[hhugo]

The following program was never compiled correctly.

let x = ref 0 in
try
  incr x ;
  (try incr x ; raise Exit with _ -> Format.eprintf "%d@." !x);
  raise Exit
with Exit ->
  Format.eprintf "%d@." !x

I have a wip fix in #1355