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.
> Despite the name, register VMs also have a stack.
Out of curiosity what do you think about this - in spite of the name, stack machines also have yet another stack. Ok I don't like that wording, but locals are basically the stack frames people know of from their computer arch class I think.
It doesn't change the fact that Wasm operations have to have the execution stack as one or more of the operands. Seems like a stack machines to me too, though I don't know more details on why the specific design of Wasm would make optimizing compilers harder to write than JVM as the article suggests (I think?).
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.
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.
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.
I dont really disagree with the main premise of the article, which is that WASM is not really a stack language, but this part just gave me pause:
> In textual Wasm, for example, they are instead represented in a LISP-like notation – not any less or more efficient
The Text format, at least when it comes to instructions, it 1 to 1 with the binary format. The LISP-like syntax is mainly just syntax sugar[1].
‘(’ plaininstr instrs ‘)’ ≡ instrs plaininstr
So (in theory, as far as I understand it) you can just do `(local.get 2 local.get 0 local.get 1)` to mean `local.get 0 local.get 1 local.get 2`, and it works for (almost) any instruction.
Unfortunately, in my limited testing, tools like `wat2wasm` and Binaryen's `wasm-as` don't seem to adhere to (my perhaps faulty understanding of) the spec, and demand all instructions in a folded block be folded and have the "correct" amount of arguments, which makes Binaryen do weird things like
(return
(tuple.make ;; Binaryen only pseudoinstruction
(local.get 0) ;; or w/e expression
(local.get 1) ;; or w/e expression
)
)
when this is perfectly valid
local.get 0
local.get 1
return
tl;dr: the LISP syntax is just syntax sugar. The textual format is as "stack-like" as the binary format.
Edit: An example that is easily done with the stack syntax and not with lisp syntax is the following:
call function_that_returns_multivalue
local.set 2 ;; last return
local.set 1 ;;
local.set 0 ;; first return
I am sad about WASM. It was a promise for epic greatness.
It has failed to deliver that - so much is clear now. You
rarely see any awesome success story shown with regard to
WASM nowadays. What happened to the old promises? "Electron
will be SUPER fast thanks to WASM" or "use any language,
WASM unifies it all for the larger browser ecosystem".
It feels as if WASM is on a step towards exctinction. Sure,
it is mentioned, it is used, but let's be honest - only few
people really use it. And that won't change either.
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.
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.
> Despite the name, register VMs also have a stack.
Out of curiosity what do you think about this - in spite of the name, stack machines also have yet another stack. Ok I don't like that wording, but locals are basically the stack frames people know of from their computer arch class I think.
It doesn't change the fact that Wasm operations have to have the execution stack as one or more of the operands. Seems like a stack machines to me too, though I don't know more details on why the specific design of Wasm would make optimizing compilers harder to write than JVM as the article suggests (I think?).
Java does use dup in some cases, e.g.
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.
Compiling WASM to C is a really good option: https://00f.net/2023/12/11/webassembly-compilation-to-c/
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.
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
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 dont really disagree with the main premise of the article, which is that WASM is not really a stack language, but this part just gave me pause:
> In textual Wasm, for example, they are instead represented in a LISP-like notation – not any less or more efficient
The Text format, at least when it comes to instructions, it 1 to 1 with the binary format. The LISP-like syntax is mainly just syntax sugar[1].
So (in theory, as far as I understand it) you can just do `(local.get 2 local.get 0 local.get 1)` to mean `local.get 0 local.get 1 local.get 2`, and it works for (almost) any instruction.Unfortunately, in my limited testing, tools like `wat2wasm` and Binaryen's `wasm-as` don't seem to adhere to (my perhaps faulty understanding of) the spec, and demand all instructions in a folded block be folded and have the "correct" amount of arguments, which makes Binaryen do weird things like
when this is perfectly valid tl;dr: the LISP syntax is just syntax sugar. The textual format is as "stack-like" as the binary format.Edit: An example that is easily done with the stack syntax and not with lisp syntax is the following:
In LISP syntax this would be I have not yet tried this with Binaryen but I doubt it flies.[1]: https://webassembly.github.io/spec/core/text/instructions.ht...
FWIW if you are looking for examples of WebAssembly written in the textual format, take a look at:
https://raw.githubusercontent.com/soegaard/webracket/refs/he...
As a small example, here is a definition of `$car` which extracts the first value from a pair.
I am sad about WASM. It was a promise for epic greatness.
It has failed to deliver that - so much is clear now. You rarely see any awesome success story shown with regard to WASM nowadays. What happened to the old promises? "Electron will be SUPER fast thanks to WASM" or "use any language, WASM unifies it all for the larger browser ecosystem".
It feels as if WASM is on a step towards exctinction. Sure, it is mentioned, it is used, but let's be honest - only few people really use it. And that won't change either.
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.
You could use "local.tee". I kind of is "store" + "duplicate".
`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`)