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> It is a stack machine, no registers; instructions generally push and pop 32/64-bit ints and floats onto that stack. One thing to note here is that WebAssembl

> It is a stack machine, no registers; instructions generally push and pop 32/64-bit ints and floats onto that stack. One thing to note here is that WebAssembly is not just a stack machine but a stack machine encoding of an instruction tree. You can still execute it as like a stack machine but it doesn't look like a typical stack machine. In particular there are no generic stack manipulation operations at all (`drop` is the sole exception), so it is trivial to convert this encoding back to the instruction tree. This greatly simplifies the validation step. (Edited to clarify that WebAssembly is still a stack machine, just that it lacks usual stack operations.)

one-more-minute 4y ago
I dislike referring to WASM as a "stack machine" for this reason. It is useful to remember that the text format started out as a low-level lisp-y expression tree with local variables and nested control flow. It still basically is that, but with the AST syntactically flattened in a way that looks a bit like a stack machine. (Admittedly, multi-value and block returns have since made it trickier to recover the AST.) Things like Forth or the JVM byte code are true stack machines: all locals can be on the stack, and the stack shape can vary at runtime (which in turn makes validation harder). I seem to be in the minority, but I think wasm is quite different and the distinction is useful to maintain.
evmar 4y ago
Thanks for this comment. In a previous draft of the post I had more about this, but I decided to omit it because I didn't have much insight about it beyond some handwaving around how it's just encoding the instructions and the optimizer will do something different. Your comments (this and the other one in this thread) are better than what I would have written, so I learned something today! (Edit: I updated the post to link to your comment.)

one-more-minute 4y ago

I dislike referring to WASM as a "stack machine" for this reason. It is useful to remember that the text format started out as a low-level lisp-y expression tree with local variables and nested control flow. It still basically is that, but with the AST syntactically flattened in a way that looks a bit like a stack machine. (Admittedly, multi-value and block returns have since made it trickier to recover the AST.) Things like Forth or the JVM byte code are true stack machines: all locals can be on the stack, and the stack shape can vary at runtime (which in turn makes validation harder). I seem to be in the minority, but I think wasm is quite different and the distinction is useful to maintain.

evmar 4y ago

Thanks for this comment. In a previous draft of the post I had more about this, but I decided to omit it because I didn't have much insight about it beyond some handwaving around how it's just encoding the instructions and the optimizer will do something different. Your comments (this and the other one in this thread) are better than what I would have written, so I learned something today! (Edit: I updated the post to link to your comment.)