Use meta for IR instruction pattern matching

[bnjbvr]

The meta language is powerful enough to handle the problem of transforming one instruction into a sequence of other instructions using simple declarations. It could also solve the reverse problem of pattern-matching several instructions into one, which could help implementing instruction selection / pipehole optimizations using meta declarations.

[fitzgen]

Also easy for a superoptimizer to generate such patterns ;)

[bjorn3]

^ https://github.com/jubitaneja/codegen

Havent tried it yet, so I dont know what the status is. (cc @jubitaneja)

[bjorn3]

We have peepmatic now, which is roughtly this, right?

[cfallin]

Yes, though there are open questions around how to integrate it or use a Peepmatic-like scheme for instruction lowering in the new backends, so I think we should leave this open to track.

[akirilov-arm]

And I guess we are now discussing this in bytecodealliance/rfcs#15?

[bnjbvr]

Yep.