Bug 164582 – [JSC] Default parameters should be retrieved by op_get_argument opcode instead of changing arity

• Status: RESOLVED
• Resolution: FIXED
• Component: JavaScriptCore

[Yusuke Suzuki]

If you have a function like,

function func(a, b, options = {}) {
...
}

`func.length` is 2. But the number of parameters of this function is counted as 3.
So if you call it with the form `func(a, b)`, the arity check always fails.
But in the above case, not passing options is common.

Instead, we should retrieve the argument by using `op_get_argument`. It does not increase the arity.

[Yusuke Suzuki]

Created attachment 304196 [details]
Patch

WIP

[Yusuke Suzuki]

Note that this patch improves six-speed default.es6 by 4.05x.

[Saam Barati]

(In reply to comment #2)
> Note that this patch improves six-speed default.es6 by 4.05x.

nice

[Yusuke Suzuki]

Created attachment 304228 [details]
Patch

[Saam Barati]

Comment on attachment 304228 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=304228&action=review

r=me

> Source/JavaScriptCore/ChangeLog:21
> +        The above is simple. However, it has the side effect that it always increase the arity of the function.
> +        While `function.length` does not increase, internally, the number of parameters of CodeBlock increases.
> +        This effectively prevent our DFG / FTL to perform inlining: currently we only allows DFG to inline
> +        the function with the arity less than or equal the number of passing arguments. It is OK. But when using
> +        default parameters, we frequently do not pass the argument for the parameter with the default value.
> +        Thus, in our current implementation, we frequently need to fixup the arity. And we frequently fail
> +        to inline the function.

Great find.

> Source/JavaScriptCore/bytecompiler/BytecodeGenerator.cpp:1120
> +        if (!nonSimpleArguments)
>              initializeNextParameter();

So our arity is just up to our first default-value parameter?

> Source/JavaScriptCore/bytecompiler/NodesCodegen.cpp:3468
> +        // If the parameter list is non simple one, it is handled in bindValue's code.

This will be true for every parameter?

> Source/JavaScriptCore/dfg/DFGByteCodeParser.cpp:1444
> +    if (codeBlock->numParameters() > argumentCountIncludingThis) {

numParameters() includes |this|?

[Yusuke Suzuki]

Comment on attachment 304228 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=304228&action=review

Thanks!

>> Source/JavaScriptCore/bytecompiler/BytecodeGenerator.cpp:1120
>>              initializeNextParameter();
> 
> So our arity is just up to our first default-value parameter?

Yes. And it is the same to the ECMAScript's function.length.

>> Source/JavaScriptCore/bytecompiler/NodesCodegen.cpp:3468
>> +        // If the parameter list is non simple one, it is handled in bindValue's code.
> 
> This will be true for every parameter?

Yes. BytecodeGenerator::initializeDefaultParameterValuesAndSetupFunctionScopeStack does so.

>> Source/JavaScriptCore/dfg/DFGByteCodeParser.cpp:1444
>> +    if (codeBlock->numParameters() > argumentCountIncludingThis) {
> 
> numParameters() includes |this|?

Yes. CodeBlock's numParameter includes |this| since it is part of arg from the point of virtual register view.

[Yusuke Suzuki]

Committed r214029: <http://trac.webkit.org/changeset/214029>

[Yusuke Suzuki]

I'll soon fix the failures in debug build.

[Yusuke Suzuki]

Committed r214040: <http://trac.webkit.org/changeset/214040>

[Yusuke Suzuki]

Committed r214041: <http://trac.webkit.org/changeset/214041>