The author seems to complain about a lack of stack manip expressions like dup and rot, but at least for me that's what I would expect from an average programming language stack machine. Even Java, which does have those instructions, doesn't use them --- reuse happens via local variables.

The way I see it, the difference between register and stack vms is all about the instruction encoding. Register VMs have fatter instructions in exchange for needing fewer LOAD and STORE operations. Despite the name, register VMs also have a stack.

The series of articles linked at the end (troubles.md/posts/wasm-is-not-a-stack-machine/) is even more interesting, imo.

Very well articulated and concise critique by somebody who seems to have a great amount of knowledge and experience with the topics.

I'm trying to implement a WASM to C compiler, and because of that not-quite-so-stack behavior, I can actually guarantee that it will always build an expression and I don't have to discard or reset stack value! Everything stays within that function, which is very neat, and I think it is one of the reason WAT, the textual format is so neat, that you can represent it with a S-Expression.

• bsder

  today at 8:43 AM

  But how do you handle arguments or loop index variables? Your liveness is the entire function? You have to compile all the WASM chunks together in order to do any optimization? That seems ... problematic.

  Edit: Yep. In article referenced from the original: http://troubles.md/posts/wasm-is-not-a-stack-machine/

  Double edit: Some of this has already been fixed in WASM: https://github.com/WebAssembly/multi-value

• jedisct1

  today at 8:24 AM

  Compiling WASM to C is a really good option: https://00f.net/2023/12/11/webassembly-compilation-to-c/

  • ncruces

    today at 9:24 AM

    Shameless plug… compiling it to Go is a great option too: https://github.com/ncruces/wasm2go

    I've used it to translate SQLite (with a few extensions) and, that I know of, it's been used (to varying degrees of success) to translate the MARISA trie library (C++), libghostty (Zig), zlib, Perl, and QuickJS.

    More on-topic, I use a mix of an unevaluated expression stack and a stack-to-locals approach to translate Wasm.

The lack of a dup opcode in Wasm as mentioned in the post is quite annoying when trying to generate compact code. I wish something like it had made it into the spec.

• thomasmg

  today at 10:02 AM

  You could use "local.tee". I kind of is "store" + "duplicate".

  • asibahi

    today at 10:11 AM

    `local.tee` doesn't duplicate. it just doesn't remove the value from the stack. (so it is "just" `local.set` followed by `local.get`)