Bug (JSArraySetLength) – optimize jsarray setlength

• Component: JavaScriptCore
• Status: RESOLVED INVALID

[Peng Xinchao]

JSC Engine Optimize 'var ret =[] ; ret.length=1024' .When type of  ret is undefine and it   is setted length , its only public length is setted ,but its vector is not setted . the memory of variable ret is not changed .When the type of variable ret is confirm , its will allocate memory by public length.

[Peng Xinchao]

Created attachment 212944 [details]
optimze jsarray setlength

 JSC Engine Optimize 'var ret =[] ; ret.length=1024' .When type of  ret is undefine and it   is setted length , its only public length is setted ,but its vector is not setted . the memory of variable ret is not changed .When the type of variable ret is confirm , its will allocate memory by public length.

[Geoffrey Garen]

Can you provide a benchmark that demonstrates why you did this?

[Peng Xinchao]

sunsprider and v8benchmark is not changed . It is to improve Drameo/JavaScript Arrays .
 The first item of Arrays is "Array Construction, []". its source code 
:
    for ( var j = 0; j < i * 15; j++ ) {
			ret = [];
			ret.length = 1024;
		}
 
My purpose is here.

[Geoffrey Garen]

I'm pretty skeptical about a change just for this benchmark. Real programs typically use their arrays. Have you seen behavior like this in any real programs?

[Peng Xinchao]

I don't argree with you.  When type of array is undefine and  array is setted length , That allocate memory directly is not necessary . I think that Memory will be allocated when it is really require.

[Geoffrey Garen]

> I don't argree with you.

This isn't a matter of opinion. If a real program or website uses this idiom, you should be able to provide an empirical example.

[Geoffrey Garen]

Comment on attachment 212944 [details]
optimze jsarray setlength

Marking r- because we don't have an example of a website that uses this idiom.