performance issue with ES6 class inheritance

[seth.b...@microsoft.com]

Hello,

I've recently been looking through the JetStream 2 test suite, and I found a pretty interesting behavior in the FlightPlanner benchmark. It defines a class constructor for a class named Leg as follows:

    constructor(fix, location)
    {
        this.previous = undefined;
        this.next = undefined;
        this.fix = fix;
        this.location = location;
        this.course = 0;
        this.distance = 0;
        this.trueAirspeed = 0;
        this.windDirection = 0;
        this.windSpeed = 0;
        this.heading = 0;
        this.estGS = 0;
        this.startFlightTiming = false;
        this.stopFlightTiming = false;
        this.engineConfig = EngineConfig.Cruise;
        this.fuelFlow = 0;
        this.distanceRemaining = 0;
        this.estimatedTimeEnroute = undefined;
        this.estTimeRemaining = 0;
        this.estFuel = 0;
    }

At a glance, this seems like good code, written following best practices: it defines all of the properties for the object, in a consistent order, with reasonable default values. However, a bunch of subclasses extend from Leg, which makes all of these stores megamorphic. I tried wrapping the contents of this constructor with "if (this.constructor === Leg)", and copying all of the property-initialization code into the leaf subclass constructors, to enforce monomorphic behavior during construction; this yielded a 17% improvement to average speed.

Of course, storing properties during construction is only part of the problem; there are also many megamorphic loads and stores of these properties elsewhere.

This seems like a pretty common case that might have come up previously, but I found nothing in a quick search through the bug tracker and v8-dev group. Have there been previous discussions, or are there existing design docs regarding faster property access when using ES6 class inheritance?

Thanks!

[Jakob Gruber]

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[Leszek Swirski]

+bmeurer@

Thanks Seth,

This is a known issue to a certain extent (bit.ly/v8-investigation-of-class-performance), though I'm not sure we've measured the impact of the issue on Jetstream specifically. It's a tricky problem -- the object model is optimised for "struct-like" use of objects, rather than "class-like", and it's not obvious how to optimise the latter without sacrificing performance on the former. There are some ideas on the linked doc, I don't think any have been investigated much further than in that doc though.

- Leszek