[kazinator]

The clang disassembly given in the article sure makes it look like WASM is a nested expression tree, which leaves the choice of stack versus register to the implementation.

      (f32.add
        (f32.add
          (f32.mul
            (f32.load
              (local.get 0))
            (f32.load
              (local.get 1)))
          (f32.mul
            (f32.load offset=4
              (local.get 0))
            (f32.load offset=4
              (local.get 1))))
        (f32.mul
          (f32.load offset=8
            (local.get 0))
          (f32.load offset=8
            (local.get 1))))

The outer f32.add could translate into a byte code instruction that finds its two operands on a stack, or to one which gets them from registers.

The code only says that there is a f32.add call which has two operands that are the result of a f32.add and f32.mul and so on.

The implementations will agree in their treatment of locals: that there are two locals 0 and 1, which support loading at offsets and such.

Both stack and register machines can support locals.

[jsjolen]

That's just the pprinter working its magic :-).

Just read the standard, it's quite clear from the semantics that it's a stack machine: https://webassembly.github.io/spec/core/exec/runtime.html#st... https://webassembly.github.io/spec/core/exec/instructions.ht...

[klodolph]

Yes, that’s exactly true. The “stack machine” here can be seen as nothing more than a way of encoding the expression tree.

[afiori]

The stack machine is a model for the semantics of wasm, in the sense that the safety properties of wasm are defined in terms of stack types rather than SSA of register properties (for those that are unfamiliar with stack machines, this stack is a different type of concept from the call stack, that in wasm is used store the local variables). Whether during execution it is better implemented as a register machine or a stack machine is an implementation detail.