[SR-4761] array.enumerated().map() a lot slower than a simple implementation

[NachoSoto]

Previous ID	SR-4761
Radar	rdar://problem/31921407
Original Reporter	@NachoSoto
Type	Bug

Environment

Swift 3.1

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, Performance
Assignee	None
Priority	Medium

md5: 3c116a7de5b152b0fd7817061ceae539

Issue Description:

I didn't discover myself, I noticed it in this commit: https://github.com/apple/swift-protobuf/pull/497/files#diff-72b84f4e5b31a77724b54d914fd6ab71.

See the docstring for details:

import Foundation

extension Array {

  /// Like map, but calls the transform with the index and value.
  ///
  /// NOTE: It would seem like doing:
  ///   return self.enumerated().map {
  ///     return try transform($0.index, $0.element)
  ///   }
  /// would seem like a simple thing to avoid extension. However as of Xcode 8.3.2
  /// (Swift 3.1), building/running 5000000 interation test (macOS) of the differences
  /// are rather large -
  ///   Release build:
  ///     Using enumerated: 3.694987967
  ///     Using enumeratedMap: 0.961241992
  ///   Debug build:
  ///     Using enumerated: 20.038512905
  ///     Using enumeratedMap: 8.521299144
  func enumeratedMap<T>(_ transform: (Int, Element) throws -> T) rethrows -> [T] {
    var i: Int = -1
    return try map {
      i += 1
      return try transform(i, $0)
    }
  }
}

[jckarter]

@swift-ci create