On Mon, Feb 25, 2019 at 5:12 PM '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"?
>
I'm not familiar with these, but do know about HEAP8, HEAP32, etc. :) The
code I get generated does not have either of those symbols; it might be to
support some C++ structure you have... All the code I compiled was actually
C. The basic C library support all works as expected (fopen for instance).
>
> Was far as I know, the WASM code is basically self contained and doesn't
> immediately provide interface with javascript.
>
I used WASM and compiled a GSM codec (audio).
THis is the script I used to build it...
There's several options to export functions for JS to call. The batch file
includes
`-s EXPORTED_FUNCTIONS="['_gsm_create','_gsm_decode','_gsm_encode']"`
https://github.com/d3x0r/gsm/blob/master/x.bat
The readme here shows basically how to use _gsm_create, _gsm_decode and
_gsm_encode.
https://github.com/d3x0r/gsm-wasm
To get JS arrays/strings into the WASM code you need to malloc in the wasm
heap, and copy the data to the buffers; primitive values (numbers/strings)
pass through simply enough.
And yes, there is very bad support for inter-oping the other direction; The
other thing I was playing with WASMing is a JSOX(JSON variant) parser, but
creating the objects on the .parse() side of things is very hairy.
But most of these issues would be Emscripten; since WASM has to interop
with other JS engines, and not just V8.
>
>
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