[SR-13063] String(decoding:as:) is 10x slower for ASCII than for UTF8

[karwa]

Previous ID	SR-13063
Radar	None
Original Reporter	@karwa
Type	Improvement

Attachment: Download

Environment

Xcode Version 11.5 (11E608c), macOS 10.15.2

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Improvement, Performance
Assignee	None
Priority	Medium

md5: 69390e8b97a32575471cd6fb5ab50fa4

Issue Description:

Take the following code, which is part of a parser for URL schemes:

struct ASCII {
  var codePoint: UInt8
  init(_unchecked: UInt8) { self.codePoint = _unchecked }
  
  static var h: ASCII { ASCII(_unchecked: 0x68) }
  // ...
}

enum Scheme {
        case ftp
        case file
        case http
        case https
        case ws
        case wss
        case other(String)

 static func parse<C>(asciiBytes: C) -> Scheme where C: Collection, C.Element == UInt8 {
            func notRecognised() -> Scheme {
                return .other(String(decoding: asciiBytes, as: Unicode.ASCII.self))
            }

            var iter = asciiBytes.lazy.map { ASCII(_unchecked: $0) }.makeIterator()
            switch iter.next() {
            case .h?:
                guard iter.next() == .t, iter.next() == .t, iter.next() == .p else { return notRecognised() }
                switch iter.next() {
                case .s?:
                    guard iter.next() == nil else { return notRecognised() }
                    return .https
                case .none:
                    return .http
                case .some(_):
                    return notRecognised()
                }
            case .f?:
                switch iter.next() {
                case .i?:
                    guard iter.next() == .l, iter.next() == .e, iter.next() == nil else { return notRecognised() }
                    return .file
                case .t?:
                    guard iter.next() == .p, iter.next() == nil else { return notRecognised() }
                    return .ftp
                default:
                    return notRecognised()
                }

<<<<< snip. Et cetera. Full code is here: https://github.com/karwa/base/blob/eb914b42cf26adaf63b5b65368c53eef07c2903c/Sources/URL/URL.swift#L42 >>>>>

            }
        }
}

While benchmarking, I found that this was taking an incredible amount of time - up to 15% of parsing a URL, with almost all of that coming from String creation:

With one modification: changing decoding to `UTF8.self`, we can get literally an order of magnitude better performance:

I understand that UTF8 is String's native encoding and has certain fast-paths, but ASCII is a restricted subset of UTF8 and should be even easier to validate and repair (if needed). I would certainly not expect it to be 10x slower! That's just too much.