Microwasm: Why does BrIf have an else_ ?

[tiborvass]

Out of curiosity I was wondering why the BrIf instruction has an else_ target when the it could simply be moving on to the next instruction.

[eira-fransham]

Great question! So basically, it's a way to make dealing with control flow easier. So basically in Wasm there are 2 kinds of intra-function control flow - conditional (you might jump or you might not) and unconditional (you always jump). However, since these are both control flow, all the same conditions apply. If you had a br_table with 1 target and 1 default, that's exactly the same as a br_if except that it jumps unconditionally, for example. So we make all control flow unconditional and we model conditional control flow by creating a new block directly after the current one and then emitting an unconditional jump where the "then" is the Wasm br_if's target, and the "else" is that newly-created block. By unconditional I here mean that we must jump, but where we jump is defined at runtime. Doing it this way means that we don't have any special-cases, we handle both unconditional and conditional control flow the same, which has a few added benefits. For example, unconditional control flow where one of the targets is the location directly after the jump (for example, a br or br_table followed immediately by an end) can be compiled to "fall through" just as br_if is, which produces better code while only having a single code-path.

Hope that helps, I'm actually writing up some documents describing some of the design decisions, constraints, and proposals for Lightbeam over at https://github.com/paritytech/lightbeam-stammtisch, but it's very unfinished and doesn't have a great deal of information on Microwasm itself right now.

Since this is a question and not an issue, I'm going to close it, but if you have further questions about Lightbeam's design I highly recommend that you post an issue over at lightbeam-stammtisch since it will help me learn what I haven't explained properly, or other holes that need to be filled.