Increasing Performance by Reducing Dynamic Dispatch

https://developer.apple.com/swift/blog/?id=39

Increasing Performance by Reducing Dynamic Dispatch

Like many other languages, Swift allows a class to override methods and properties declared in its superclasses. This means that the program has to determine at runtime which method or property is being referred to and then perform an indirect call or indirect access. This technique, called dynamic dispatch, increases language expressivity at the cost of a constant amount of runtime overhead for each indirect usage. In performance sensitive code such overhead is often undesirable. This blog post showcases three ways to improve performance by eliminating such dynamism: final, private, and Whole Module Optimization.

Consider the following example:

class ParticleModel {
	var point = ( 0.0, 0.0 )
	var velocity = 100.0

	func updatePoint(newPoint: (Double, Double), newVelocity: Double) {
		point = newPoint
		velocity = newVelocity
	}

	func update(newP: (Double, Double), newV: Double) {
		updatePoint(newP, newVelocity: newV)
	}
}

var p = ParticleModel()
for i in stride(from: 0.0, through: 360, by: 1.0) {
	p.update((i * sin(i), i), newV:i*1000)
}

As written, the compiler will emit a dynamically dispatched call to:

1. Call update on p.
2. Call updatePoint on p.
3. Get the property point tuple of p.
4. Get the property velocity of p.

This might not be what you would expect when looking at this code. The dynamic calls are necessary because a subclass of ParticleModel might override point or velocity with a computed property or override updatePoint() or update() with new implementations.

In Swift, dynamic dispatch calls are implemented by looking up a function from a method table and then performing an indirect call. This is slower than performing a direct call. Additionally, indirect calls also prevent many compiler optimizations, making the indirect call even more expensive. In performance critical code there are techniques you can use to restrict this dynamic behavior when it isn’t needed to improve performance.

Use final when you know that a declaration does not need to be overridden.

The final keyword is a restriction on a class, method, or property that indicates that the declaration cannot be overridden. This allows the compiler to safely elide dynamic dispatch indirection. For instance, in the following point and velocity will be accessed directly through a load from the object’s stored property and updatePoint() will be called via a direct function call. On the other hand, update() will still be called via dynamic dispatch, allowing for subclasses to override update() with customized functionality.

class ParticleModel {
	final var point = ( x: 0.0, y: 0.0 )
	final var velocity = 100.0

	final func updatePoint(newPoint: (Double, Double), newVelocity: Double) {
		point = newPoint
		velocity = newVelocity
	}

	func update(newP: (Double, Double), newV: Double) {
		updatePoint(newP, newVelocity: newV)
	}
}

It is possible to mark an entire class as final by attaching the attribute to the class itself. This forbids subclassing the class, implying that all functions and properties of the class are final as well.

final class ParticleModel {
	var point = ( x: 0.0, y: 0.0 )
	var velocity = 100.0
	// ...
}

Infer final on declarations referenced in one file by applying the private keyword.

Applying the private keyword to a declaration restricts the visibility of the declaration to the current file. This allows the compiler to find all potentially overriding declarations. The absence of any such overriding declarations enables the compiler to infer the final keyword automatically and remove indirect calls for methods and property accesses.

Assuming there is no class overriding ParticleModel in the current file, the compiler can replace all dynamically dispatched calls to private declarations with direct calls.

class ParticleModel {
	private var point = ( x: 0.0, y: 0.0 )
	private var velocity = 100.0

	private func updatePoint(newPoint: (Double, Double), newVelocity: Double) {
		point = newPoint
		velocity = newVelocity
	}

	func update(newP: (Double, Double), newV: Double) {
		updatePoint(newP, newVelocity: newV)
	}
}

As in the previous example, point and velocity are accessed directly and updatePoint() is called directly. Again, update() will be invoked indirectly due to update() not being private.

Just like with final, it is possible to apply the private attribute to the class declaration itself causing the class to be private and thus all of the properties and methods of the class as well.

private class ParticleModel {
	var point = ( x: 0.0, y: 0.0 )
	var velocity = 100.0
	// ...
}

Use Whole Module Optimization to infer final on internal declarations.

Declarations with internal access (the default if nothing is declared) are only visible within the module where they are declared. Because Swift normally compiles the files that make up a module separately, the compiler cannot ascertain whether or not an internal declaration is overridden in a different file. However, if Whole Module Optimization is enabled, all of the module is compiled together at the same time. This allows the compiler to make inferences about the entire module together and infer final on declarations with internal if there are no visible overrides.

Let’s go back to the original code snippet, this time adding some extra public keywords to ParticleModel.

public class ParticleModel {
	var point = ( x: 0.0, y: 0.0 )
	var velocity = 100.0

	func updatePoint(newPoint: (Double, Double), newVelocity: Double) {
		point = newPoint
		velocity = newVelocity
	}

	public func update(newP: (Double, Double), newV: Double) {
		updatePoint(newP, newVelocity: newV)
	}
}

var p = ParticleModel()
for i in stride(from: 0.0, through: times, by: 1.0) {
	p.update((i * sin(i), i), newV:i*1000)
}

When compiling this snippet with Whole Module Optimization the compiler can infer final on the properties point, velocity, and the method call updatePoint(). In contrast, it can not be inferred that update() is final since update() has public access.