Allow inlining jump table

[ultimaweapon]

Background

I'm working on a port of Lua to Rust and currently hit a road block to make Rust performance to match with C version due to Rust currently does not support computed goto. For people who does not familiar with computed goto it is a GCC extension to allows getting the address of a label and goto statement can jump into that label dynamically:

static void *array[] = { &&foo, &&bar, &&hack };

goto *array[i];

This technique can drastically increase the performance of bytecode interpreter due to a jump to next handler can be inlined instead of jumping back to the beginning on the loop then do a next jump.

Proposal

I propose adding an attribute on continue statement to inline the jump like this:

loop {
    match inst.op() {
        Op::Bar => {
           inst = instructions.next();

            #[inline]
            continue;
        }
        Op::Foo => {
            inst = instructions.next();

            #[inline]
            continue;
        }
    }
}

[zachs18]

cc rust-lang/rust#132306 which is for a feature that seems similar to your proposed syntax, though not exactly equivalent

[ultimaweapon]

Looking good! After some reading it seems like some people disagree with the syntax so I think my proposal can be an alternative in case of that one does not made into stable.

[kennytm]

i still think #3720 should use something like finite_state_machine! { ... } rather than the current experimental syntax #[loop_match] loop { match { ... }} 🙂

[tgross35]

Please feel free to propose your syntax as a review comment on the RFC #3720. I'm going to close this since discussion is already happening elsewhere.