Re: [VM] Tail recursion and multiple values

[ludo@gnu.org (Ludovic Courtès)]

ludo@gnu.org (Ludovic Courtès) writes:

> Use of multiple values breaks tail recursion in VM-compiled code:
> 
>   (let loop ((x 1000000))
>     (and (> x 0)
>          (call-with-values
>              (lambda ()
>                (values (1+ x) (1- x)))
>            (lambda (next prev)
>              (loop prev)))))

Actually no: it works with VM-compiled code, but it breaks when using
Guile-VM with `,o interp #t' (which appears to be the default, except at
the REPL).

In this case, `loop' is an interpreter procedure while
`call-with-values' is a program.  It's the implementation of `goto/args'
in this particular case that seems to break tail recursion:

  if (!SCM_FALSEP (scm_procedure_p (x)))
    {
      POP_LIST (nargs);
      SYNC_REGISTER ();
      sp[-1] = scm_apply (x, sp[0], SCM_EOL);

The `scm_apply ()' call introduces a new stack frame instead of re-using
the current one.

At this point I'm not sure how to fix it.  We need better integration of
calls from the VM to subrs/procedures anyway (notably removing
`POP_LIST ()' and argument consing before subr/procedure calls!) so
probably we could postpone this issue until then.  Andy?

Thanks,
Ludo'.