Hacker News Reader

Comment by readittwice | Hacker News Reader

I have to admit this line seems arbitrary to me. So WASM is a register machine to you but if they would simply add those 2 instruction would it suddenly become a stack machine then? Those instructions would actually be trivial to add. I think those terms are relatively well defined and when you argue that WASM is a register machine even though the inventors explicitly claim it's a stack machine you should have really good arguments for that. Personally I would be surprised if you could point me to any literature that supports your definition.

View on HN

Turing completeness sounds like an arbitrary distinction to those outside of the field of CS, but it’s not.

To me, the distinction here is that the stack machine in WASM is restricted to the point that it corresponds 1:1 with an expression tree—not even a graph, just a tree. This means that every function in Web Assembly can be thought of as a collections of statements and expressions, and the stack machine abstraction is nothing more than a serialization format for the expressions.

Maybe dial it back a bit with the challenge to point at literature. The literature has not really caught up with the existence of WASM yet.

> Maybe dial it back a bit with the challenge to point at literature. The literature has not really caught up with the existence of WASM yet.

It wasn't me who claimed that WASM is "obviously" a register machine, despite the inventors saying otherwise. They even explicitly state that they decided against a register machine. I guess it's then reasonable for me to ask on what definition of stack vs register you are basing this opinion on. Let me be clear: I was not asking here for literature about WASM specifically but a definition of register/stack machines that supports your claim.

WASM's instruction encoding is very much based on a stack machine. Even with the initial limitations you mentioned I don't think it qualifies as "obviously a register machine". As already mentioned in multiple comments those restrictions were already lifted with the multi value proposal.

I understand that there is a grey area, but simply claiming "obviously a register machine" doesn't seem right to me. Implementation-wise WASM is a stack machine even if it needs/needed locals to be turing-complete.

The multi-value proposal breaks the ability to turn Wasm into expressions easily, and thus makes it even more of a stack machine than it already is. Dup and swap may still be added in the future.

A defining feature of a register machine is that the actual instruction encoding has direct references to source and destination registers in it. Wasm doesn't have those, it has explicit get_local instructions instead.

That said, if you turn off LLVM's WebAssemblyRegStackify pass, all LLVM IR's values will end up in locals, with little to no stack usage. Still no register machine, but a bit more of a grey area :)

dup and swap can be implemented with locals in wasm (on well typed stacks)

Reply on news.ycombinator.com