feat: Add opt-in pointer identity mode for SwiftHeapObject wrappers

Benchmarks/README.md

@@ -4,8 +4,6 @@ This directory contains performance benchmarks for JavaScriptKit.
 
 ## Building Benchmarks
 
-Before running the benchmarks, you need to build the test suite:
-
 ```bash
 swift package --swift-sdk $SWIFT_SDK_ID js -c release
 ```
@@ -19,19 +17,38 @@ node run.js
 # Save results to a JSON file
 node run.js --output=results.json
 
-# Specify number of iterations
-node run.js --runs=20
-
 # Run in adaptive mode until results stabilize
 node run.js --adaptive --output=stable-results.json
 
-# Run benchmarks and compare with previous results
+# Compare with previous results
 node run.js --baseline=previous-results.json
 
-# Run only a subset of benchmarks
-# Substring match
+# Filter benchmarks by name
 node run.js --filter=Call
-# Regex (with flags)
 node run.js --filter=/^Property access\//
-node run.js --filter=/string/i
 ```
+
+## Identity Mode Benchmarks
+
+The benchmark suite includes identity-mode variants (`@JS(identityMode: true)`) of the core classes to measure pointer identity caching. Both variants are in the same build and run as regular benchmarks alongside everything else.
+
+```bash
+# Run only identity benchmarks
+node --expose-gc run.js --filter=Identity
+
+# Run only pointer-mode identity benchmarks
+node --expose-gc run.js --filter=Identity/pointer
+
+# Run only non-identity baseline
+node --expose-gc run.js --filter=Identity/none
+```
+
+### Identity Scenarios
+
+| Scenario | What it measures |
+|----------|-----------------|
+| `passBothWaysRoundtrip` | Same object crossing boundary repeatedly (cache hit path) |
+| `getPoolRepeated_100` | Bulk return of 100 cached objects (model collection pattern) |
+| `churnObjects` | Create, roundtrip, release cycle (FinalizationRegistry cleanup pressure) |
+| `swiftConsumesSameObject` | JS passes same object to Swift repeatedly |
+| `swiftCreatesObject` | Fresh object creation overhead (cache miss path) |

Benchmarks/Sources/Benchmarks.swift

@@ -257,6 +257,109 @@ enum ComplexResult {
     }
 }
 
+// MARK: - Class Array Performance Tests
+
+nonisolated(unsafe) var _classArrayPool: [SimpleClass] = []
+
+@JS class ClassArrayRoundtrip {
+    @JS init() {}
+
+    @JS func setupPool(_ count: Int) {
+        _classArrayPool = (0..<count).map {
+            SimpleClass(name: "Item \($0)", count: $0, flag: true, rate: 0.5, precise: 3.14)
+        }
+    }
+
+    @JS func getPool() -> [SimpleClass] {
+        return _classArrayPool
+    }
+
+    @JS func makeClassArray() -> [SimpleClass] {
+        return (0..<100).map {
+            SimpleClass(name: "Item \($0)", count: $0, flag: true, rate: 0.5, precise: 3.14)
+        }
+    }
+
+    @JS func takeClassArray(_ values: [SimpleClass]) {}
+
+    @JS func roundtripClassArray(_ values: [SimpleClass]) -> [SimpleClass] {
+        return values
+    }
+}
+
+// MARK: - Identity Cache Benchmark
+
+nonisolated(unsafe) var _cachedPool: [SimpleClass] = []
+
+@JS class IdentityCacheBenchmark {
+    @JS init() {}
+
+    @JS func setupPool(_ count: Int) {
+        _cachedPool = (0..<count).map {
+            SimpleClass(name: "Item \($0)", count: $0, flag: true, rate: 0.5, precise: 3.14)
+        }
+    }
+
+    @JS func getPoolRepeated() -> [SimpleClass] {
+        return _cachedPool
+    }
+}
+
+// MARK: - Identity Mode Benchmark Variants
+// These classes use @JS(identityMode: true) so that identity cache benchmarks
+// can run in the SAME build alongside the non-identity classes above.
+
+@JS(identityMode: true)
+class SimpleClassIdentity {
+    @JS var name: String
+    @JS var count: Int
+    @JS var flag: Bool
+    @JS var rate: Float
+    @JS var precise: Double
+
+    @JS init(name: String, count: Int, flag: Bool, rate: Float, precise: Double) {
+        self.name = name
+        self.count = count
+        self.flag = flag
+        self.rate = rate
+        self.precise = precise
+    }
+}
+
+@JS(identityMode: true)
+class ClassRoundtripIdentity {
+    @JS init() {}
+
+    @JS func roundtripSimpleClassIdentity(_ obj: SimpleClassIdentity) -> SimpleClassIdentity {
+        return obj
+    }
+
+    @JS func makeSimpleClassIdentity() -> SimpleClassIdentity {
+        return SimpleClassIdentity(name: "Hello", count: 42, flag: true, rate: 0.5, precise: 3.14159)
+    }
+
+    @JS func takeSimpleClassIdentity(_ obj: SimpleClassIdentity) {
+        // consume without returning
+    }
+}
+
+nonisolated(unsafe) var _cachedPoolIdentity: [SimpleClassIdentity] = []
+
+@JS(identityMode: true)
+class IdentityCacheBenchmarkIdentity {
+    @JS init() {}
+
+    @JS func setupPool(_ count: Int) {
+        _cachedPoolIdentity = (0..<count).map {
+            SimpleClassIdentity(name: "Item \($0)", count: $0, flag: true, rate: 0.5, precise: 3.14)
+        }
+    }
+
+    @JS func getPoolRepeated() -> [SimpleClassIdentity] {
+        return _cachedPoolIdentity
+    }
+}
+
 // MARK: - Array Performance Tests
 
 @JS struct Point {