What's the difference between these two JS codes in V8?

[haochen shi]

I try to create a codeCache via V8.

```
ScriptCompiler::CachedData* CreateCachedData(Local<String> sourceContent, Local<String> sourceName) {
    ScriptOrigin* scriptOriginPtr = nullptr;
    scriptOriginPtr = new ScriptOrigin(
        String::NewFromTwoByte(isolate_, sourceName, NewStringType::kNormal).ToLocalChecked(),
        Integer::New(isolate_, 0), Integer::New(isolate_, 0));
    ScriptCompiler::Source* source = nullptr;
    source = new ScriptCompiler::Source(sourceContent, *scriptOriginPtr);

    Local<Script> script;
    ScriptCompiler::Compile(context_, source, ScriptCompiler::CompileOptions::kEagerCompile).ToLocal(&script);

    ScriptCompiler::CachedData* data = ScriptCompiler::CreateCodeCache(script->GetUnboundScript());

    return data;
}
```

When I use these code, the value of data is nullptr.

```
function A() {
    "use asm"
    function R() {}
    return R
}
```

When I use these code, the value of data isn't nullptr.

```
function A() {
    "use asm"
    function R() {};
    return R
}
```

What's the difference between these two JS codes? In V8? And In JS?

[Clemens Backes]

The difference is that the first snippet is valid asm.js while the second snippet is not.

d8 prints a message if you try to instantiate the second function (via `A()`):

test.js:3: Invalid asm.js: Unexpected token

asm.js code is internally converted to Wasm and then run through the Wasm pipeline. There is a TODO in the code to enable serialization of asm.js modules, but this has not been worked on in the last years. If this is something you need, feel free to open a bug and describe your use case.

-Clemens

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Clemens Backes

Software Engineer

clem...@google.com

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[haochen shi]

Thanks for the reply.

But I'm curious about how you got this error message 'test.js:3: Invalid asm.js: Unexpected token' via d8.

When I run these two snippets, I don't get any error message.

130|HWSEA-A:/data/local/tmp/v8/bin $ ./d8
V8 version 9.7.0 (candidate)
d8> function A() {"use asm"; function R() {} return R}
undefined
d8> function B() {"use asm"; function R() {}; return R}
undefined

d8>

[Clemens Backes]

You need to invoke the function (to instantiate the asm.js module). Try this:

function B() {"use asm"; function R() {}; return R}; B()

To view this discussion on the web visit https://groups.google.com/d/msgid/v8-dev/05c51192-6fad-4a27-9ea7-82392845f5dcn%40googlegroups.com.