On Tue, Feb 26, 2019 at 2:12 AM 'Steven Johnson' via v8-users
<v8-users@googlegroups.com> wrote:
> I'm experimenting with embedding V8 in a test app in order to load, compile,
> and run wasm bytecode that has been generated by LLVM (using V8 7.4.x and
> recent trunk versions of LLVM)
>
> Loading and running seems simple enough (via
> WasmModuleObject::DeserializeOrCompile), but calling the resulting functions
> from C++ seems tricky, because, well, it's not clear that there's a public V8
> API for actually looking at what's in the resulting WasmModuleObject; in
> JavaScript, you can apparently do something like
>
> let module = new WebAssembly.Module(buffer)
> let imports = { ... map of funcs that are declared as 'import' by the
> module, if any ... }
> let instance = new WebAssembly.Instance(module, imports);
> let myfunc = instance.exports["myfunc"];
> myfunc(...);
>
> but a similar API doesn't seem to be surfaced for C++, at least not in the
> public API.
>
> I've tried replicating the JavaScript calls from C++, but there's a big
> problem: it appears that the 'imports' argument is required to be
> v8::internal::Handle<v8::internal::JSReceiver>, which (AFAICT) there isn't a
> clean way to create and manipulate directly from the C++ API, at least
> certainly not from the public API:
>
> Local<WasmModuleObject> module =
> WasmModuleObject::DeserializeOrCompile(
> isolate, {nullptr, 0}, {source, source_len}).ToLocalChecked();
>
> v8::internal::Handle<v8::internal::JSReceiver> imports = ???; // Oh
> well
> Local<Value> args[2] = { module, imports };
>
> Local<Object> module_instance_exports = context->Global()
> ->Get(context, String::NewFromUtf8(isolate,
> "WebAssembly")).ToLocalChecked().As<Object>()
> ->Get(context, String::NewFromUtf8(isolate,
> "Instance")).ToLocalChecked().As<Object>()
> ->CallAsConstructor(context, 2,
> args).ToLocalChecked().As<Object>()
> ->Get(context, String::NewFromUtf8(isolate,
> "exports")).ToLocalChecked().As<Object>()
> ;
>
> I suppose I could accomplish this by adding some extra JS-only wrapper code
> that does the necessary magic, but that's going to add another layer of
> indirection that surely shouldn't be necessary here (since the code in
> question here is otherwise free of JavaScript).
>
> The thing that makes this extra-fun is that apparently LLVM injects a couple
> of symbols into the import table of every bit of wasm it generates
> ("__linear_memory" and "__indirect_function_table")... which don't appear to
> be referenced in any code in V8 that I can find, so their purpose is a bit of
> a mystery to me. (Perhaps holdovers from older implementations of wasm?) In
> any event, I'm almost certainly going to have other imports I need to fill in
> here (for glue functions), so figuring out how to to accomplish this is
> probably essential to my task.
>
> So...
>
> (1) Is there an explicit API to access/call wasm-compiled functions from C++?
> Is so, where may I find it? If not, um, shouldn't we have one?
> (2) If there isn't a C++ API for this (and/or isn't going to be one anytime
> in the very near future), is the above approach the most reasonable
> workaround for now?
> (3) If the above mimic-JS-from-C++ approach is reasonable, is there a clean
> way to specify the imports table?
> (4) Finally, just out of curiosity, anyone know what the story is with
> "__linear_memory" and "__indirect_function_table"?
A JSReceiver is (for the sake of this discussion) just a v8::Object.
This is the answer to your third question. :-)
As to (2), consulting context->Global() is reasonable and idiomatic.
And a propos (4), http://andrewsweeney.net/post/llvm-to-wasm/ contains
links to explanations but in a nutshell, one is the Memory object,
whereas the other is to support C semantics that don't map 1-to-1 to
WASM.
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