[VM] Tail recursion and multiple values

[Ludovic Courtès]

Hello!

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)))))

This example yields a stack overflow with the VM, whereas tail recursion
is correctly handled by the interpreter.  Tail recursion for
`call-with-values' is mandated by R5RS (info "(r5rs)Proper tail
recursion"):

  Certain built-in procedures are also required to perform tail calls.
  The first argument passed to `apply' and to
  `call-with-current-continuation', and the second argument passed to
  `call-with-values', must be called via a tail call.  Similarly, `eval'
  must evaluate its argument as if it were in tail position within the
  `eval' procedure.

I'll look into it and report back if no one beats me.  ;-)

Thanks,
Ludo'.