Improve the speed of comparing memory buffers by using a workaround t… (

#812)

* Improve the speed of comparing memory buffers by using a workaround to a missed compiler optimization

Co-authored-by: Eric Jensen <eric@yetilabs.org>

* Update NSImage.swift

* Update UIImage.swift

* fix

---------

Co-authored-by: Eric Jensen <eric@yetilabs.org>
Co-authored-by: Stephen Celis <stephen@stephencelis.com>

Sources/SnapshotTesting/Snapshotting/NSImage.swift

@@ -118,8 +118,14 @@
       let newRep = NSBitmapImageRep(cgImage: newerCgImage).bitmapData!
       let byteCountThreshold = Int((1 - precision) * Float(byteCount))
       var differentByteCount = 0
-      for offset in 0..<byteCount {
-        if oldRep[offset] != newRep[offset] {
+      // NB: We are purposely using a verbose 'while' loop instead of a 'for in' loop.  When the
+      //     compiler doesn't have optimizations enabled, like in test targets, a `while` loop is
+      //     significantly faster than a `for` loop for iterating through the elements of a memory
+      //     buffer. Details can be found in [SR-6983](https://github.com/apple/swift/issues/49531)
+      var index = 0
+      while index < byteCount {
+        defer { index += 1 }
+        if oldRep[index] != newRep[index] {
           differentByteCount += 1
         }
       }

Sources/SnapshotTesting/Snapshotting/UIImage.swift

@@ -140,8 +140,14 @@
     } else {
       let byteCountThreshold = Int((1 - precision) * Float(byteCount))
       var differentByteCount = 0
-      for offset in 0..<byteCount {
-        if oldBytes[offset] != newerBytes[offset] {
+      // NB: We are purposely using a verbose 'while' loop instead of a 'for in' loop.  When the
+      //     compiler doesn't have optimizations enabled, like in test targets, a `while` loop is
+      //     significantly faster than a `for` loop for iterating through the elements of a memory
+      //     buffer. Details can be found in [SR-6983](https://github.com/apple/swift/issues/49531)
+      var index = 0
+      while index < byteCount {
+        defer { index += 1 }
+        if oldBytes[index] != newerBytes[index] {
           differentByteCount += 1
         }
       }