WASM isn't particularly low-level, in that it's a compiler's IR, with a few concomitant restrictions on how control flow is allowed to work. Here's a piece on those restrictions, and how they make implementation more difficult:
> WebAssembly isn’t designed as a front-end language for general programming though, so what’s the problem? Well, WebAssembly does have some constraints, specifically it must produce code that is valid. WebAssembly is a stack machine, and you can’t jump to just any label since that label might point to code that pops too many values off the stack, or pops the wrong types off the stack, or pushes too many values onto the stack.
Also, to summarize another part, when a compiler targeting WASM sees control flow WASM can't directly express, it has to implement it in a loop-and-switch form, something called the Relooper algorithm; therefore, compilers taking WASM down to native code have to understand that, and undo it back into a performant form. This adds up to a lot of work most VMs don't require, hence the proposal to move WASM towards a control-flow model friendlier to tail-call optimization, which would bring it more in line with physical hardware.
http://troubles.md/why-do-we-need-the-relooper-algorithm-aga...
> WebAssembly isn’t designed as a front-end language for general programming though, so what’s the problem? Well, WebAssembly does have some constraints, specifically it must produce code that is valid. WebAssembly is a stack machine, and you can’t jump to just any label since that label might point to code that pops too many values off the stack, or pops the wrong types off the stack, or pushes too many values onto the stack.
Also, to summarize another part, when a compiler targeting WASM sees control flow WASM can't directly express, it has to implement it in a loop-and-switch form, something called the Relooper algorithm; therefore, compilers taking WASM down to native code have to understand that, and undo it back into a performant form. This adds up to a lot of work most VMs don't require, hence the proposal to move WASM towards a control-flow model friendlier to tail-call optimization, which would bring it more in line with physical hardware.