buffer.length performance regressed in 6.5.0

[Nibbler999]

• Version: v6.7.0
• Platform: Linux x86_64
• Subsystem: Buffer

[nibbler@nibbler ~]$ cat buffer.js 
'use strict';

var data = new Buffer(1024);

console.time('test');

for (var i = 0; i < 1000000; i++) {
    for (var j = 0; j < data.length; j++) {

    }
}

console.timeEnd('test');

[nibbler@nibbler ~]$ ~/Downloads/node-v6.4.0-linux-x64/bin/node buffer.js 
test: 623.471ms
[nibbler@nibbler ~]$ ~/Downloads/node-v6.5.0-linux-x64/bin/node buffer.js 
test: 2837.938ms

[mscdex]

I think about the only change that would have affected that was the V8 5.0->5.1 upgrade.

EDIT: FWIW v6.4.0 and master have the same results, but I can confirm that v6.5.0 is a little over twice as slow when running that test.

[mscdex]

/cc @nodejs/v8 ?

[targos]

I think the fix would be v8/v8@3c927e0 but the diff is huge.

[jeisinger]

@bmeurer

[bmeurer]

Yeah, I guess it would be fixed by v8/v8@3c927e0.

[apapirovski]

ping @targos @bmeurer — do you know if this was ever addressed in v6.x?

[addaleax]

@apapirovski I can reproduce this using the latest v6.x release (6.11.4), so I’d assume “no”. It might be something we’d just have to accept as a wontfix if nobody thinks they can put in the work to fix it?

[bmeurer]

This was never addressed for v6.x, no. Not sure it's worth spending time on this, esp. since this code is fairly different in V8 nowadays.

[apapirovski]

Given the feedback last time and the lack of any movement, I'm going to go ahead and close. If anyone feels like they want to take it on or this is an important issue, feel free to reopen.