Support OCaml Multicore

[pklehre]

Describe the bug

When compiling code using the OCaml Multicore compiler, I get the following error message:

dune build jsoo js_of_ocaml .js/stdlib/std_exit.cmo.js (exit 1) (cd _build/default/.js/stdlib && /home/pkl/.opam/4.10.0+multicore+no-effect-syntax/bin/js_of_ocaml --pretty --source-map-inline -o std_exit.cmo.js /home/pkl/.opam/4.10.0+multicore+no-effect-syntax/lib/ocaml/std_exit.cmo) /home/pkl/.opam/4.10.0+multicore+no-effect-syntax/bin/js_of_ocaml: You found a bug. Please report it at https://github.com/ocsigen/js_of_ocaml/issues : Error: File "compiler/lib/parse_bytecode.ml", line 693, characters 2-8: Assertion failed make: *** [Makefile:4: all] Error 1

Expected behavior

Code compiles without problems.

Versions

4.10.0+multicore+no-effect-syntax opam switch

[hhugo]

Js_of_ocaml doesn't support ocaml multicore

• ocaml multicore uses a different set of bytecode instructions, it also shuffles instruction codes

-  GETFIELD0, GETFIELD1, GETFIELD2, GETFIELD3, GETFIELD, GETFLOATFIELD,
+  GETFIELD0, GETFIELD1, GETFIELD2, GETFIELD3, GETFIELD,
+  GETMUTABLEFIELD0, GETMUTABLEFIELD1, GETMUTABLEFIELD2, GETMUTABLEFIELD3,
+  GETMUTABLEFIELD, GETFLOATFIELD,

• It's unclear how multicore would work in javascript

[hhugo]

IIRC @Armael and @kayceesrk worked on supporting multicore in jsoo. I dont know what they ended up with

[pklehre]

I thought it might be possible to compile plain OCaml code which does not use any of the multicore features to JS. However given that the bytecode instructions have changed, it is a more substantial task than I expected. I believe multicore will be part of OCaml 5, so this issue might become relevant again soon.

[hhugo]

I've a branch that propagate changes for some of the instructions. But it fail to compile the stdlib due to instruction related to effects.
https://github.com/ocsigen/js_of_ocaml/tree/multicore-410 837566a

[hhugo]

I've opened an issue regarding the instructions/opcodes reuse. ocaml-multicore/ocaml-multicore#475

[Lupus]

It's unclear how multicore would work in javascript

There is relevant prior work done on supporting Golang's goroutines (which have M:N mapping to real threads with smart runtime scheduler on the native Golang compilation target) in Gopherjs project [1]. What they seem to be doing is unwinding stack when blocking call is encountered and reconstruct it later when it is to be resumed. It works well in practice - when playing around with it I was just copy pasting some Go code using threads and watched it somehow work in JS, that felt like magic. May be something similar could be done for js_of_ocaml to support multicore and effects?

[1] https://github.com/gopherjs/gopherjs#goroutines