Call webassembly from embedded v8 without JS - c++

Question

I would like to use webassembly directly from my embedded v8 without the detour via JavaScript. I used the provided hello-world example and the WasmModuleObjectBuilderStreaming class from v8.h. However, I am stuck at how to extract the add function. Help would be appreciated.
#include <include/v8.h>
#include <include/libplatform/libplatform.h>
#include <stdlib.h>
#include <unistd.h>
using v8::HandleScope;
using v8::Isolate;
using v8::Local;
using v8::Promise;
using v8::WasmModuleObjectBuilderStreaming;
int main(int argc, char* argv[]) {
v8::V8::InitializeICUDefaultLocation(argv[0]);
v8::V8::InitializeExternalStartupData(argv[0]);
std::unique_ptr<v8::Platform> platform = v8::platform::NewDefaultPlatform();
v8::V8::InitializePlatform(platform.get());
v8::V8::Initialize();
Isolate::CreateParams create_params;
create_params.array_buffer_allocator = v8::ArrayBuffer::Allocator::NewDefaultAllocator();
Isolate* isolate = Isolate::New(create_params);
Isolate::Scope isolate_scope(isolate);
HandleScope scope(isolate);
WasmModuleObjectBuilderStreaming stream(isolate);
// Use the v8 API to generate a WebAssembly module.
//
// |bytes| contains the binary format for the following module:
//
// (func (export "add") (param i32 i32) (result i32)
// get_local 0
// get_local 1
// i32.add)
//
// taken from: https://github.com/v8/v8/blob/master/samples/hello-world.cc#L66
std::vector<uint8_t> wasmbin {
0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00, 0x01, 0x07, 0x01,
0x60, 0x02, 0x7f, 0x7f, 0x01, 0x7f, 0x03, 0x02, 0x01, 0x00, 0x07,
0x07, 0x01, 0x03, 0x61, 0x64, 0x64, 0x00, 0x00, 0x0a, 0x09, 0x01,
0x07, 0x00, 0x20, 0x00, 0x20, 0x01, 0x6a, 0x0b
};
// write bytes and finish
stream.OnBytesReceived(wasmbin.data(), wasmbin.size());
stream.Finish();
Local<Promise> promise = stream.GetPromise();
// TODO: Get exports, extract `add` & call `add`
}
Build setup:
Follow the instruction in Run the example from the official Getting started with embedding V8. Save the code to sample/wasm.cc and execute following commands:
$ g++ -I. -O2 -Iinclude samples/wasm.cc -o wasm -lv8_monolith -Lout.gn/x64.release.sample/obj/ -pthread -std=c++17`
$ ./wasm`
Solution:
Thanks #liliscent, I adapted my example accordingly. Because we all like, working code:
#include <include/v8.h>
#include <include/libplatform/libplatform.h>
using v8::HandleScope;
using v8::Isolate;
using v8::Local;
using v8::Promise;
using v8::WasmModuleObjectBuilderStreaming;
using v8::WasmCompiledModule;
using v8::Context;
using v8::Local;
using v8::Value;
using v8::String;
using v8::Object;
using v8::Function;
using v8::Int32;
using args_type = Local<Value>[];
int main(int argc, char* argv[]) {
v8::V8::InitializeICUDefaultLocation(argv[0]);
v8::V8::InitializeExternalStartupData(argv[0]);
std::unique_ptr<v8::Platform> platform = v8::platform::NewDefaultPlatform();
v8::V8::InitializePlatform(platform.get());
v8::V8::Initialize();
Isolate::CreateParams create_params;
create_params.array_buffer_allocator = v8::ArrayBuffer::Allocator::NewDefaultAllocator();
Isolate* isolate = Isolate::New(create_params);
Isolate::Scope isolate_scope(isolate);
HandleScope scope(isolate);
Local<Context> context = Context::New(isolate);
Context::Scope context_scope(context);
WasmModuleObjectBuilderStreaming stream(isolate);
// Use the v8 API to generate a WebAssembly module.
//
// |bytes| contains the binary format for the following module: //
// (func (export "add") (param i32 i32) (result i32)
// get_local 0
// get_local 1
// i32.add)
//
// taken from: https://github.com/v8/v8/blob/master/samples/hello-world.cc#L66
std::vector<uint8_t> wasmbin {
0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00, 0x01, 0x07, 0x01,
0x60, 0x02, 0x7f, 0x7f, 0x01, 0x7f, 0x03, 0x02, 0x01, 0x00, 0x07,
0x07, 0x01, 0x03, 0x61, 0x64, 0x64, 0x00, 0x00, 0x0a, 0x09, 0x01,
0x07, 0x00, 0x20, 0x00, 0x20, 0x01, 0x6a, 0x0b
};
// same as calling:
// let module = new WebAssembly.Module(bytes);
Local<WasmCompiledModule> module = WasmCompiledModule::DeserializeOrCompile(isolate,
WasmCompiledModule::BufferReference(0, 0),
WasmCompiledModule::BufferReference(wasmbin.data(), wasmbin.size())
).ToLocalChecked();
// same as calling:
// let module_instance_exports = new WebAssembly.Instance(module).exports;
args_type instance_args{module};
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, 1, instance_args)
.ToLocalChecked().As<Object>()
->Get(context, String::NewFromUtf8(isolate, "exports"))
.ToLocalChecked().As<Object>()
;
// same as calling:
// module_instance_exports.add(77, 88)
args_type add_args{Int32::New(isolate, 77), Int32::New(isolate, 88)};
Local<Int32> adder_res = module_instance_exports
->Get(context, String::NewFromUtf8(isolate, "add"))
.ToLocalChecked().As<Function>()
->Call(context, context->Global(), 2, add_args)
.ToLocalChecked().As<Int32>();
printf("77 + 88 = %d\n", adder_res->Value());
return 0;
}

Answer 1

You can construct a WebAssembly module directly from C++ via v8::WasmCompiledModule class (it will be renamed to v8::WasmModuleObject in next version):
Local<WasmCompiledModule> module = WasmCompiledModule::DeserializeOrCompile(isolate,
WasmCompiledModule::BufferReference(0, 0),
WasmCompiledModule::BufferReference(wasmbin.data(), wasmbin.size())
).ToLocalChecked();
But AFAIK, v8 doesn't expose its webassembly api directly, you have to get them from JS global context. The following code creates a module instance, and gets the exports of the instance:
using args_type = Local<Value>[];
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, 1, args_type{module})
.ToLocalChecked().As<Object>()
->Get(context, String::NewFromUtf8(isolate, "exports"))
.ToLocalChecked().As<Object>()
;
Then you can get the add function from exports object and call it:
Local<Int32> adder_res = module_instance_exports
->Get(context, String::NewFromUtf8(isolate, "add"))
.ToLocalChecked().As<Function>()
->Call(context, context->Global(), 2, args_type{Int32::New(isolate, 77), Int32::New(isolate, 88)})
.ToLocalChecked().As<Int32>();
std::cout << "77 + 88 = " << adder_res->Value() << "\n";

Answer 2

You might be interested in the Wasm C/C++ API proposal, which allows using a Wasm engine directly from C/C++. The design of this API is independent of any particular engine, but the proposal contains a more or less complete prototype implementation on top of V8.
Sample snippet (see e.g. hello.cc):
// ...
auto engine = wasm::Engine::make();
auto store = wasm::Store::make(engine.get());
auto module = wasm::Module::make(store.get(), binary);
auto instance = wasm::Instance::make(store.get(), module.get(), imports);
auto exports = instance->exports();
exports[0]->func()->call();