Making the Menhir parser more friendly to Js_of_ocaml

[hhugo]

• lookahead : https://github.com/ocsigen/js_of_ocaml/blob/master/compiler/js_parser.mly#L205
• stack overflow : https://github.com/protz/mezzo/blob/master/web/break-mutual-recursion-in-menhir.patch

[gasche]

Is it necessary given your previous work on trampolines for mutual recursion?

[hhugo]

here is the pattern used by menhir and it is not optimized with trampolines

let other f g h = ... (* tailcall f,g or h*) 

let rec f () =
  other f g h
and g () =
  other f g h
and h () = other f g h

[dbuenzli]

Does it mean that in general continuation passing style will still blow the stack ?

[hhugo]

yes, tail call is not optimized in general

[Simn]

Where is this issue at with regards to the stack overflow? I read that there's a trampoline implementation now, but with js_of_ocaml 2.8.4 and menhir 20170101 we are still observing stack overflows when parsing large files. I would love to provide more information regarding the actual cause, but I'm having a hard time debugging this or even getting a sensible stacktrace.

[ejgallego]

@Simn just out of curiosity, do you see that with recent (beta!) Chrome and the chrome://flags/#enable-javascript-harmony flag enabled ? [you need to restart for the flag to take effect]

[Simn]

Sorry, I should have mentioned that we are running the generated code through node.js, not the browser.

[ejgallego]

I think you can still enable node's tail call stuff with node --harmony (and a very recent build I think)

[Simn]

Unfortunately, that doesn't seem to make a difference:

node --version
v7.7.3

node --harmony cli.js
[248,Stack_overflow,-9]

[ejgallego]

Maybe you need node nightly or some different tweaks to --harmony flag, I am not sure about Chrome.

But indeed, latest versions of Chrome's v8 can perform some limited tail-call optimization and that proven to be of great help with jsCoq. Further context: http://stackoverflow.com/questions/38272392/why-does-enable-javascript-harmony-prevent-stackoverflows-in-web-workers