The 'restart' Bytecode

The 'restart' Bytecode

[Christian Lindig]

To verify that some function is indeed tail recursive I sometimes 
inspect its bytecode using "ocaml -dinstr". The 'appterm' instruction 
implements the desired tail call and thus is what I am looking for. 
However, sometimes the appterm instruction is followed by a restart 
instruction. I could neither figure out what it does (by looking at 
interp.c) nor I was expecting code following an appterm to be reachable 
at all. For example:

L3:     grab 1
         const ' '
         push
         acc 2
         push
         getglobal String!
         getfield 0
         apply 2
         push
         const "\n"
         push
         acc 2
         push
         envacc 1
         apply 2
         push
         envacc 1
         appterm 2, 4
         restart

I would be grateful if some could elaborate what restart does and how 
it interacts with tail calls. Some information about the OCaml Bytecode 
machine can be found here, and also in Xavier's ZINC paper.

	http://pauillac.inria.fr/~lebotlan/docaml_eng.html

-- Christian

	

--
http://www.st.cs.uni-sb.de/~lindig/

Re: [Caml-list] The 'restart' Bytecode

[Xavier Leroy]

> To verify that some function is indeed tail recursive I sometimes
> inspect its bytecode using "ocaml -dinstr". The 'appterm' instruction
> implements the desired tail call and thus is what I am looking for.
> However, sometimes the appterm instruction is followed by a restart
> instruction.

The "restart" is actually part of the following function.  "grab"
instructions that mark the beginning of a multiple-argument function
are always preceded by a "restart" instruction.  If not enough
arguments are provided to the function, "grab" returns a closure
pointing to the preceding "restart" instruction and containing the
values of the provided arguments.  When this partial application
closure is applied, "restart" executes, pushes back the saved
arguments on the stack, and reexecutes (by falling through) the "grab"
instruction.

So, nothing to worry about concerning tail calls.

- Xavier Leroy