[External] : Re: Rehabilitating switch -- a scorecard

[John Rose]

On May 18, 2021, at 12:33 PM, Guy Steele <guy.steele at oracle.com<mailto:guy.steele at oracle.com>> wrote:

If only we had tail calls. Then instead of writing

(int q, int r) = quotientAndRemainder(m, n);
whatever

we could write (using my preferred syntax for a mandatory tail call in Java)

goto quotientAndRemainder(m, n, (int q, int r) -> whatever);

(the method quotientAndRemainder would of course tail-call its third argument), with no need of tuples or even records to get multiple values out of a method, and then everyone would be happy, right?  RIGHT?  :-) :-/ :-P

As it happens, this week I have been enjoying “Compiling
without Continuations” by Maurer at al [1].  Their work is
in Haskell which already has tail calls, but needs (for some
key optimizations) a primitive which is a non-local jump
with arguments.  (They mention Rabbit but not Lambda
the Ultimate Goto. :-)  It basically throws, not an exception
but an argument bundle, to a statically (not dynamically)
determined catch point; it compiles to a goto, assuming
it the special catch and jump points are only introduced
after inlining.

Using their jump primitive, the Q&R call looks something
like this:

QR: join(int q, int r) {
  quotientAndRemainder(m, n, QR);
}

and

AlwaysThrows quotientAndRemainder(int m, int n, BiConsumer<int,int> QR) {
  int q = m/n, r = m-n*q;
  throw QR.apply(q, r);
}

If it could compile to JVM-level LET-JUMP-POINT (“join”) and
GOTO-JUMP-POINT (“jump”) IR commands, then you could
get the right kind of “ultimate goto” by the route of throws
instead of tail calls.

Threading the types through exceptions is very icky though.

Also, tail calls, proper, allow you to jump to any dynamically
specified next-callee, not just a selection of catch points already
on the stack.  Still, in this example, a statically present catch
point is enough.

— John

[1]: https://www.microsoft.com/en-us/research/wp-content/uploads/2016/11/compiling-without-continuations.pdf

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