Impossible to use wasm:js-string functions in webassembly

[socketpair]

Here you can see an example of WASM code how to use concat JS builtin:

https://developer.mozilla.org/en-US/docs/WebAssembly/Guides/JavaScript_builtins#wasm_module

(func $concat (import "wasm:js-string" "concat")
    (param externref externref) (result (ref extern)))

So, in C I defined it in the following way:

__externref_t js_concat(__externref_t, __externref_t) 
__attribute__((import_module("wasm:js-string"), import_name("concat")));

Next, I'm trying to run the code that uses definition above in the following way:

const { instance } = await WebAssembly.instantiateStreaming(
  fetch('trie_search.wasm'),
  {
      env: {  },
  },
  {
      builtins: ["js-string"],
      importedStringConstants: "Something",
  },
);
instance.exports.my_function();

Browser console says:

Uncaught (in promise) CompileError: WebAssembly.instantiateStreaming():
Imported builtin function "wasm:js-string" "concat" has incorrect signature

I guess, the problem in return value. in WAT language it should look like (ref extern). How to express it in C (Clang)?

https://github.com/WebAssembly/js-string-builtins/blob/main/proposals/js-string-builtins/Overview.md#wasmjs-string-concat
https://github.com/WebAssembly/function-references/blob/master/proposals/function-references/Overview.md#binary-format

(module
  (global $h (import "string_constants" "hello ") externref)
  (global $w (import "string_constants" "world!") externref)

  (func $concat (import "wasm:js-string" "concat")
    (param externref externref) (result (ref extern)))

  (func $log (import "m" "log") (param externref))
  (func (export "main")
    (call $log (call $concat (global.get $h) (global.get $w))))
)

Compiles perfectly well using binryen, but how to write an equivalent using C and clang ?

[llvmbot]

@llvm/issue-subscribers-backend-webassembly

Author: Коренберг Марк (socketpair)

Here you can see an example of WASM code how to use `concat` JS builtin:

https://developer.mozilla.org/en-US/docs/WebAssembly/Guides/JavaScript_builtins#wasm_module

(func $concat (import "wasm:js-string" "concat")
    (param externref externref) (result (ref extern)))

So, in C I defined it in the following way:

__externref_t js_concat(__externref_t, __externref_t) 
__attribute__((import_module("wasm:js-string"), import_name("concat")));

Next, I'm trying to run the code that uses definition above in the following way:

const { instance } = await WebAssembly.instantiateStreaming(
  fetch('trie_search.wasm'),
  {
      env: {  },
  },
  {
      builtins: ["js-string"],
      importedStringConstants: "Something",
  },
);
instance.exports.my_function();

Browser console says:

Uncaught (in promise) CompileError: WebAssembly.instantiateStreaming():
Imported builtin function "wasm:js-string" "concat" has incorrect signature

I guess, the problem in return value. in WAT language it should look like (ref extern). How to express it in C (Clang)?

https://github.com/WebAssembly/js-string-builtins/blob/main/proposals/js-string-builtins/Overview.md#wasmjs-string-concat
https://github.com/WebAssembly/function-references/blob/master/proposals/function-references/Overview.md#binary-format

(module
  (global $h (import "string_constants" "hello ") externref)
  (global $w (import "string_constants" "world!") externref)

  (func $concat (import "wasm:js-string" "concat")
    (param externref externref) (result (ref extern)))

  (func $log (import "m" "log") (param externref))
  (func (export "main")
    (call $log (call $concat (global.get $h) (global.get $w))))
)

Compiles perfectly well using binryen, but how to write an equivalent using C and clang ?

[sbc100]

When you compile the code using clang, what is the type of the resulting import? I.e. what does wasm-dis to wasm-objdump say about the import?

[hoodmane]

I think the problem is that the return type of the import is ref extern (a non nullable reference to extern) whereas __externref_t is externref aka ref null extern. ref null extern is not expressible from C/assembly.

[hoodmane]

It looks like test, charCodeAt, codePointAt, length, equals, and compare have types that make them usable from C/llvm asm whereas cast, fromCharCodeArray, intoCharCodeArray, fromCharCode, fromCodePoint, concat, and substring all have inexpressible types and would require an extra adapter to use.

[socketpair]

@hoodmane who can work on the issue ? Possibly introduce a new built-in type ? Add a new attribute?

[socketpair]

And also, please someone change tags for the issue. It's not a question. Actually, it is a bug or a feature request.

[dschuff]

I don't think anyone is currently working on the extension you are using. Actually I'm not aware of anyone really using either, so I'm interested in what your use case is. Can you say more about that?

[socketpair]

Okay. I just wanted to build logging system from wasm to js. I don't use emscripten, just bare clang and a memory manager. I can share sources, actually.

Now, I just export from js (and import in wasm) own concat function. And it works. I guess, importing builtin is better.

Actually, nothing special. But wasm support in Clang looks incomplete, just faced it. Why not to improve ? JS builtins is finalized standard. Why not to support it? I guess clang changes will be minimal.

Also, I would distribute in Clang additional header file with these definitions. For c++ I would add these functions to wasm::js_string namespace. In C, I would import as _js_string..... Or so.

[dschuff]

Yeah; wasm support in clang is actually quite mature, but primarily for applications which only use linear memory (and therefore use C or C++ strings rather than JS strings). As you've seen there is some limited support for externref (which never really advanced beyond the experimental stage), but the use cases for externref are somewhat limited, since you can't really do anything with them in C/C++, you would just have to pass them back out to JS. So there just hasn't been a lot of user demand for this support, and therefore we've prioritized other things. Having said that, I don't think we'd be opposed to seeing it added. I think it would be reasonable to add something like __nullexternref_t for this; perhaps you could imagine going even further and adding a C type qualifier (like const or volatile) that would compose with externref, but I suspect that would be more involved in the frontend, and IIRC we don't support any other GC types yet, so it may not be worth it. And as you said, once we had that, it should just be a matter of declaring the JS string builtin functions in a header, to make it work.
As for the "who would work on it" part, I don't think we (i.e. the Google wasm tools folks) are likely to prioritize this soon, but if you wanted to give it a try, we (and maybe @pmatos ?) can probably help you with reviews and advice.

[hoodmane]

It would be really great to have more of these types added to llvm assembly first. Then it would be possible to make your own wrappers for functions like this in assembly. It should be possible to cast everything to externref to pass it to C and downcast arguments from externref to the appropriate type. But even assembly level support is quite a lot of work and someone would have to actually do it.