1

I was doing some experimentation and noted that type inference results in much better compilation time, when compared with explicit type declaration.
For example A.swift is

import UIKit
class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        let label0 = UILabel()
        label0.font = .systemFont(ofSize: 16, weight: .bold)
        label0.textColor = .red
        label0.textAlignment = .center
        label0.lineBreakMode = .byWordWrapping
    }
}

and B.swift is

import UIKit
class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        let label0 = UILabel()
        label0.font = UIFont.systemFont(ofSize: 16, weight: UIFont.Weight.bold)
        label0.textColor = UIColor.red
        label0.textAlignment = NSTextAlignment.center
        label0.lineBreakMode = NSLineBreakMode.byWordWrapping
    }
}

I added 1000 labels in each viewDidLoad function and same properties are being set for these. This is because with a single label, there wasn't much difference in compilation time. After averaging out 20 runs of swiftc -c I got the inferred file A.swift to be 451.9 ms, whereas the explicit file B.swift was at 623.7 ms.

I thought the explicit one will be faster to compile, as there would be no inference overhead. Could anyone explain why the opposite is happening? Was my initial assumption wrong?

2

There was a similar discussion several months (6? I can’t tell time anymore.) ago prompted by similar tests and results. Here’s Holly’s eventual response (read the whole thread for lots of random performance testing): Regarding Swift type inference compile-time performance - #30 by hborla

3 Likes
3

Thanks, will check it out.

4

I went through the thread, and @hborla response makes sense for that scenario, where the RHS has to be convertible to LHS when explicit types are introduced in LHS. But in my example, I am passing the enum with the explicit type and there's no change in LHS. Will the same type checking overhead apply here as well?