Optimized String bridging (Swift -> JS) (#816)

Author: sliemeobn

Benchmarks/Sources/Generated/BridgeJS.swift

@@ -2429,20 +2429,20 @@ func _$benchmarkHelperNoopWithNumber(_ n: Double) throws(JSException) -> Void {
 
 #if arch(wasm32)
 @_extern(wasm, module: "Benchmarks", name: "bjs_benchmarkRunner")
-fileprivate func bjs_benchmarkRunner_extern(_ nameBytes: Int32, _ nameLength: Int32, _ body: Int32) -> Void
+fileprivate func bjs_benchmarkRunner_extern(_ nameWord0: Int32, _ nameWord1: Int32, _ nameWord2: Int32, _ body: Int32) -> Void
 #else
-fileprivate func bjs_benchmarkRunner_extern(_ nameBytes: Int32, _ nameLength: Int32, _ body: Int32) -> Void {
+fileprivate func bjs_benchmarkRunner_extern(_ nameWord0: Int32, _ nameWord1: Int32, _ nameWord2: Int32, _ body: Int32) -> Void {
     fatalError("Only available on WebAssembly")
 }
 #endif
-@inline(never) fileprivate func bjs_benchmarkRunner(_ nameBytes: Int32, _ nameLength: Int32, _ body: Int32) -> Void {
-    return bjs_benchmarkRunner_extern(nameBytes, nameLength, body)
+@inline(never) fileprivate func bjs_benchmarkRunner(_ nameWord0: Int32, _ nameWord1: Int32, _ nameWord2: Int32, _ body: Int32) -> Void {
+    return bjs_benchmarkRunner_extern(nameWord0, nameWord1, nameWord2, body)
 }
 
 func _$benchmarkRunner(_ name: String, _ body: JSObject) throws(JSException) -> Void {
-    name.bridgeJSWithLoweredParameter { (nameBytes, nameLength) in
+    name.bridgeJSWithLoweredParameter { (nameWord0, nameWord1, nameWord2) in
         let bodyValue = body.bridgeJSLowerParameter()
-        bjs_benchmarkRunner(nameBytes, nameLength, bodyValue)
+        bjs_benchmarkRunner(nameWord0, nameWord1, nameWord2, bodyValue)
     }
     if let error = _swift_js_take_exception() {
         throw error

Examples/PlayBridgeJS/Sources/PlayBridgeJS/Generated/BridgeJS.swift

@@ -268,20 +268,20 @@ func _$createTS2Swift() throws(JSException) -> TS2Swift {
 
 #if arch(wasm32)
 @_extern(wasm, module: "PlayBridgeJS", name: "bjs_TS2Swift_convert")
-fileprivate func bjs_TS2Swift_convert_extern(_ self: Int32, _ tsBytes: Int32, _ tsLength: Int32) -> Int32
+fileprivate func bjs_TS2Swift_convert_extern(_ self: Int32, _ tsWord0: Int32, _ tsWord1: Int32, _ tsWord2: Int32) -> Int32
 #else
-fileprivate func bjs_TS2Swift_convert_extern(_ self: Int32, _ tsBytes: Int32, _ tsLength: Int32) -> Int32 {
+fileprivate func bjs_TS2Swift_convert_extern(_ self: Int32, _ tsWord0: Int32, _ tsWord1: Int32, _ tsWord2: Int32) -> Int32 {
     fatalError("Only available on WebAssembly")
 }
 #endif
-@inline(never) fileprivate func bjs_TS2Swift_convert(_ self: Int32, _ tsBytes: Int32, _ tsLength: Int32) -> Int32 {
-    return bjs_TS2Swift_convert_extern(self, tsBytes, tsLength)
+@inline(never) fileprivate func bjs_TS2Swift_convert(_ self: Int32, _ tsWord0: Int32, _ tsWord1: Int32, _ tsWord2: Int32) -> Int32 {
+    return bjs_TS2Swift_convert_extern(self, tsWord0, tsWord1, tsWord2)
 }
 
 func _$TS2Swift_convert(_ self: JSObject, _ ts: String) throws(JSException) -> String {
-    let ret0 = ts.bridgeJSWithLoweredParameter { (tsBytes, tsLength) in
+    let ret0 = ts.bridgeJSWithLoweredParameter { (tsWord0, tsWord1, tsWord2) in
         let selfValue = self.bridgeJSLowerParameter()
-        let ret = bjs_TS2Swift_convert(selfValue, tsBytes, tsLength)
+        let ret = bjs_TS2Swift_convert(selfValue, tsWord0, tsWord1, tsWord2)
         return ret
     }
     let ret = ret0

Plugins/BridgeJS/README.md

@@ -139,7 +139,8 @@ The ABI will not be stable, and not meant to be interposed by other tools.
 Parameter passing follows Wasm calling conventions, with custom handling for complex types:
 
 - **Primitives**: Passed directly as Wasm arguments (`i32`, `i64`, `f32`, `f64`)
-- **Strings**: UTF-8 bytes stored in `swift.memory`, ID + length passed as Wasm arguments
+- **Strings (JS → Swift)**: UTF-8 bytes stored in `swift.memory`, ID + length passed as Wasm arguments
+- **Strings (Swift → JS)**: the wasm32 12-byte `String` layout passed as three `i32` words. JS fast-decodes small strings, caches large immortals by a 53-bit `ptr + len` pack when `len < 2**21`, and otherwise reads UTF-8 at `word1 + 20` (`nativeBias`). This layout is a Swift stdlib implementation detail and can change with the toolchain.
 - **Swift Classes**: Raw Swift heap pointer passed as `i32`
 - **JSObjects**: Object stored in `swift.memory.heap`, object ID passed as `i32`
 - **Structs/Arrays**: Fields/elements pushed to type-specific stacks, Swift pops in reverse order
@@ -150,7 +151,7 @@ Parameter passing follows Wasm calling conventions, with custom handling for com
 Return values use direct Wasm returns for primitives, and imported intrinsic functions for complex types:
 
 - **Primitives**: Returned directly via Wasm return value
-- **Strings**: Swift writes UTF-8 bytes to shared memory, JS decodes
+- **Strings**: Swift passes the 12-byte `String` layout; JS decodes (small / immortal-cached / UTF-8)
 - **Swift Classes**: Pointer returned directly, JS wraps in `SwiftHeapObject` with `FinalizationRegistry`
 - **Structs/Arrays**: Swift pushes fields/elements to type-specific stacks, JS reconstructs
 

Plugins/BridgeJS/Sources/BridgeJSCore/ExportSwift.swift

@@ -744,9 +744,7 @@ public class ExportSwift {
     /// public func _bjs_Greeter_greet(pointer: UnsafeMutableRawPointer) -> Void {
     ///     let _self = Unmanaged<Greeter>.fromOpaque(pointer).takeUnretainedValue()
     ///     var ret = _self.greet()
-    ///     return ret.withUTF8 { ptr in
-    ///         _swift_js_return_string(ptr.baseAddress, Int32(ptr.count))
-    ///     }
+    ///     return ret.bridgeJSLowerReturn()
     /// }
     /// @_expose(wasm, "bjs_Greeter_deinit")
     /// @_cdecl("bjs_Greeter_deinit")

Plugins/BridgeJS/Sources/BridgeJSCore/ImportTS.swift

@@ -927,7 +927,7 @@ extension BridgeType {
         static let float = LoweringParameterInfo(loweredParameters: [("value", .f32)])
         static let double = LoweringParameterInfo(loweredParameters: [("value", .f64)])
         static let string = LoweringParameterInfo(
-            loweredParameters: [("bytes", .i32), ("length", .i32)],
+            loweredParameters: [("word0", .i32), ("word1", .i32), ("word2", .i32)],
             useBorrowing: true
         )
         static let jsObject = LoweringParameterInfo(loweredParameters: [("value", .i32)])