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.