Add a fast-path for equality checking when the backing is empty or pointer equal (#1322)

* Add a fast-path for equality checking when the backing is empty or pointer-equal

* Address review feedback

Author: parkera

Benchmarks/Benchmarks/Essentials/BenchmarkEssentials.swift

@@ -34,4 +34,91 @@ let benchmarks = {
             assert(u1 != u2)
         }
     }
+    
+    // MARK: Data
+    
+    func createSomeData(_ length: Int) -> Data {
+        var d = Data(repeating: 42, count: length)
+        // Set a byte to be another value just so we know we have a unique pointer to the backing
+        // For maximum inefficiency in the not equal case, set the last byte
+        d[length - 1] = UInt8.random(in: UInt8.min..<UInt8.max)
+        return d
+    }
+    
+    /// A box `Data`. Intentionally turns the value type into a reference, so we can make a promise that the inner value is not copied due to mutation during a test of insertion or replacing.
+    class TwoDatasBox {
+        var d1: Data
+        var d2: Data
+        
+        init(d1: Data, d2: Data) {
+            self.d1 = d1
+            self.d2 = d2
+        }
+    }
+    
+    // MARK: -
+    
+    Benchmark("DataEqualEmpty", closure: { benchmark, box in
+        blackHole(box.d1 == box.d2)
+    }, setup: { () -> TwoDatasBox in
+        let d1 = Data()
+        let d2 = d1
+        let box = TwoDatasBox(d1: d1, d2: d2)
+        return box
+    })
+
+    Benchmark("DataEqualInline", closure: { benchmark, box in
+        blackHole(box.d1 == box.d2)
+    }, setup: { () -> TwoDatasBox in
+        let d1 = createSomeData(12) // Less than size of InlineData.Buffer
+        let d2 = d1
+        let box = TwoDatasBox(d1: d1, d2: d2)
+        return box
+    })
+    
+    Benchmark("DataNotEqualInline", closure: { benchmark, box in
+        blackHole(box.d1 != box.d2)
+    }, setup: { () -> TwoDatasBox in
+        let d1 = createSomeData(12) // Less than size of InlineData.Buffer
+        let d2 = createSomeData(12)
+        let box = TwoDatasBox(d1: d1, d2: d2)
+        return box
+    })
+    
+    Benchmark("DataEqualLarge", closure: { benchmark, box in
+        blackHole(box.d1 == box.d2)
+    }, setup: { () -> TwoDatasBox in
+        let d1 = createSomeData(1024 * 8)
+        let d2 = d1
+        let box = TwoDatasBox(d1: d1, d2: d2)
+        return box
+    })
+    
+    Benchmark("DataNotEqualLarge", closure: { benchmark, box in
+        blackHole(box.d1 != box.d2)
+    }, setup: { () -> TwoDatasBox in
+        let d1 = createSomeData(1024 * 8)
+        let d2 = createSomeData(1024 * 8)
+        let box = TwoDatasBox(d1: d1, d2: d2)
+        return box
+    })
+
+    Benchmark("DataEqualReallyLarge", closure: { benchmark, box in
+        blackHole(box.d1 == box.d2)
+    }, setup: { () -> TwoDatasBox in
+        let d1 = createSomeData(1024 * 1024 * 8)
+        let d2 = d1
+        let box = TwoDatasBox(d1: d1, d2: d2)
+        return box
+    })
+
+    Benchmark("DataNotEqualReallyLarge", closure: { benchmark, box in
+        blackHole(box.d1 != box.d2)
+    }, setup: { () -> TwoDatasBox in
+        let d1 = createSomeData(1024 * 1024 * 8)
+        let d2 = createSomeData(1024 * 1024 * 8)
+        let box = TwoDatasBox(d1: d1, d2: d2)
+        return box
+    })
+
 }

Sources/FoundationEssentials/Data/Data.swift

@@ -2709,14 +2709,36 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
     /// Returns `true` if the two `Data` arguments are equal.
     @inlinable // This is @inlinable as emission into clients is safe -- the concept of equality on Data will not change.
     public static func ==(d1 : Data, d2 : Data) -> Bool {
+        // See if both are empty
+        switch (d1._representation, d2._representation) {
+        case (.empty, .empty):
+            return true
+        default:
+            // Continue on to checks below
+            break
+        }
+        
         let length1 = d1.count
-        if length1 != d2.count {
+        let length2 = d2.count
+        
+        // Unequal length data can never be equal
+        guard length1 == length2 else {
             return false
         }
+        
         if length1 > 0 {
             return d1.withUnsafeBytes { (b1: UnsafeRawBufferPointer) in
                 return d2.withUnsafeBytes { (b2: UnsafeRawBufferPointer) in
-                    return memcmp(b1.baseAddress!, b2.baseAddress!, b2.count) == 0
+                    // If they have the same base address and same count, it is equal
+                    let b1Address = b1.baseAddress!
+                    let b2Address = b2.baseAddress!
+                    
+                    guard b1Address != b2Address else {
+                        return true
+                    }
+
+                    // Compare the contents
+                    return memcmp(b1Address, b2Address, b2.count) == 0
                 }
             }
         }