[SR-11736] Type checking is unacceptably slow, and got significantly worse with Xcode 11

[swift-ci]

Previous ID	SR-11736
Radar	None
Original Reporter	mluisbrown (JIRA User)
Type	Bug

Attachment: Download

Environment

macOS Catalina 10.15.1

Xcode 11.1

MacBook Pro 15 (2018), 2.9 GHz 6-Core Intel Core i9, 32GB of RAM

Additional Detail from JIRA

Votes	1
Component/s	Compiler
Labels	Bug, TypeChecker
Assignee	@harlanhaskins
Priority	Medium

md5: 255a6b937472bfd319b643906829143f

Issue Description:

Xcode 11 / Swift 5.1 introduced significant increases in the time taken to type check Swift code. Even quite simple expressions can now take longer than 100ms.

This problem was first raised on twitter by my colleague David Rodrigues: https://twitter.com/dmcrodrigues/status/1192136776212713472

The example project attached (Type Checking Issues.zip) has a number of examples taken from a larger project where the number of "time to type-check" warnings has skyrocketed from 2 or 3 to 40 or 50 since we migrated to Xcode 11.

An example warning output is this

As you move the selection through these warnings, Xcode also seems to be continuously re-compiling the code (even though nothing has changed) and regenerating new type check time warnings as shown in this video:

XcodeTypeCheck.mov

Two of the worst examples are the following:

import UIKit

class SomeViewController: UIViewController {

    private func updatePreferredContentSize() {
        preferredContentSize = CGSize(
            width: view.bounds.width,
            height: (view.subviews.map { $0.frame.height }.reduce(+) ?? 0) + 20.0
        )
    }
}

and this one:

import Foundation

extension Int {

    public var days: TimeInterval {
        return TimeInterval(self) * 24.hours
    }

    public var hours: TimeInterval {
        return TimeInterval(self) * 60.minutes
    }

    public var minutes: TimeInterval {
        return TimeInterval(self) * 60.seconds
    }

    public var seconds: TimeInterval {
        return TimeInterval(self)
    }
}

These type checker performance degradations have also significantly increased overall build times, which have increased between 15-20% since we moved to Xcode 11.

Note: building the project using xcodebuild -configuration Debug -sdk iphonesimulator the type check times are not as bad, but still there.

[theblixguy]

I think this includes time spent on other things as well (such as deserializing) and not just purely type checking, so it's not accurate or reliable enough. But still, I would be good to track down where that extra time is being spent and why.

[slavapestov]

All of the examples in the zip file are now fast, except this one takes 900ms to emit "failed to produce diagnostic":

import UIKit

class SomeViewController: UIViewController {
    private func updatePreferredContentSize() {
        preferredContentSize = CGSize(
            width: view.bounds.width,
            height: (view.subviews.map { $0.frame.height }.reduce(+) ?? 0) + 20.0
        )
    }
}

I'll add them to the test suite.