[dart2wasm] Implement identityHashCode via br_table.

This saves the space for the `_identityHashCode` method in the
dispatch table and allows customizing `identityHashCode` for classes
not in `dart:core`.

Change-Id: I829fb0f53dea1f3baf92e12079b9772ce2ff8c2a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/292820
Commit-Queue: Aske Simon Christensen <askesc@google.com>
Reviewed-by: Joshua Litt <joshualitt@google.com>
Auto-Submit: Aske Simon Christensen <askesc@google.com>

pkg/dart2wasm/lib/class_info.dart

@@ -452,6 +452,11 @@ class ClassInfoCollector {
     // parameters.
     _initialize(translator.typeClass);
 
+    // Initialize value classes to make sure they have low class IDs.
+    for (Class cls in translator.valueClasses.keys) {
+      _initialize(cls);
+    }
+
     // Initialize masquerade classes to make sure they have low class IDs.
     for (Class cls in _masquerades.values) {
       _initialize(cls);

pkg/dart2wasm/lib/intrinsics.dart

@@ -1305,6 +1305,57 @@ class Intrinsifier {
       return true;
     }
 
+    if (member.enclosingLibrary == translator.coreTypes.coreLibrary &&
+        name == "identityHashCode") {
+      final w.Local arg = paramLocals[0];
+      final w.Local nonNullArg =
+          function.addLocal(translator.topInfo.nonNullableType);
+      final List<int> classIds = translator.valueClasses.keys
+          .map((cls) => translator.classInfo[cls]!.classId)
+          .toList()
+        ..sort();
+
+      // If the argument is `null`, return the hash code of `null`.
+      final w.Label notNull =
+          b.block(const [], [translator.topInfo.nonNullableType]);
+      b.local_get(arg);
+      b.br_on_non_null(notNull);
+      b.i64_const(null.hashCode);
+      b.return_();
+      b.end(); // notNull
+      b.local_set(nonNullArg);
+
+      // Branch on class ID.
+      final w.Label defaultLabel = b.block();
+      final List<w.Label> labels =
+          List.generate(classIds.length, (_) => b.block());
+      b.local_get(nonNullArg);
+      b.struct_get(translator.topInfo.struct, FieldIndex.classId);
+      int labelIndex = 0;
+      final List<w.Label> targets = List.generate(classIds.last + 1, (id) {
+        return id == classIds[labelIndex] ? labels[labelIndex++] : defaultLabel;
+      });
+      b.br_table(targets, defaultLabel);
+
+      // For value classes, dispatch to their `hashCode` implementation.
+      for (final int id in classIds.reversed) {
+        final Class cls = translator.valueClasses[translator.classes[id].cls!]!;
+        final Procedure hashCodeProcedure =
+            cls.procedures.firstWhere((p) => p.name.text == "hashCode");
+        b.end(); // Jump target for class ID
+        b.local_get(nonNullArg);
+        codeGen.call(hashCodeProcedure.reference);
+        b.return_();
+      }
+
+      // For all other classes, dispatch to the `hashCode` implementation in
+      // `Object`.
+      b.end(); // defaultLabel
+      b.local_get(nonNullArg);
+      codeGen.call(translator.objectHashCode.reference);
+      return true;
+    }
+
     // _typeArguments
     if (member.name.text == "_typeArguments") {
       Class cls = member.enclosingClass!;

pkg/dart2wasm/lib/kernel_nodes.dart

@@ -160,6 +160,8 @@ mixin KernelNodes {
       "dart:collection", "_HashAbstractImmutableBase", "get:_indexNullable");
 
   // dart:core various procedures
+  late final Procedure objectHashCode =
+      index.getProcedure("dart:core", "Object", "get:hashCode");
   late final Procedure objectNoSuchMethod =
       index.getProcedure("dart:core", "Object", "noSuchMethod");
   late final Procedure objectGetTypeArguments =

pkg/dart2wasm/lib/translator.dart

@@ -151,6 +151,17 @@ class Translator with KernelNodes {
     w.NumType.f64: boxedDoubleClass,
   };
 
+  /// Classes whose identity hash code is their hash code rather than the
+  /// identity hash code field in the struct. Each implementation class maps to
+  /// the class containing the implementation of its `hashCode` getter.
+  late final Map<Class, Class> valueClasses = {
+    boxedIntClass: boxedIntClass,
+    boxedDoubleClass: boxedDoubleClass,
+    boxedBoolClass: coreTypes.boolClass,
+    oneByteStringClass: stringBaseClass,
+    twoByteStringClass: stringBaseClass,
+  };
+
   /// Type for vtable entries for dynamic calls. These entries are used in
   /// dynamic invocations and `Function.apply`.
   late final w.FunctionType dynamicCallVtableEntryFunctionType =

sdk/lib/_internal/wasm/lib/double.dart

@@ -68,7 +68,6 @@ final class _BoxedDouble extends double {
   static const int _mantissaMask = 0x000FFFFFFFFFFFFF;
 
   int get hashCode => _doubleHashCode(this);
-  int get _identityHashCode => _doubleHashCode(this);
 
   static int _doubleHashCode(double value) {
     const int maxInt = 0x7FFFFFFFFFFFFFFF;

sdk/lib/_internal/wasm/lib/identical_patch.dart

@@ -8,5 +8,4 @@ part of "core_patch.dart";
 external bool identical(Object? a, Object? b);
 
 @patch
-int identityHashCode(Object? object) =>
-    object == null ? Null._HASH_CODE : object._identityHashCode;
+external int identityHashCode(Object? object);

sdk/lib/_internal/wasm/lib/int.dart

@@ -517,7 +517,6 @@ final class _BoxedInt extends int {
   }
 
   int get hashCode => _intHashCode(this);
-  int get _identityHashCode => _intHashCode(this);
 
   static int _intHashCode(int value) {
     const int magic = 0x2D51;

sdk/lib/_internal/wasm/lib/object_patch.dart

@@ -32,7 +32,6 @@ class Object {
 
   @patch
   int get hashCode => _objectHashCode(this);
-  int get _identityHashCode => _objectHashCode(this);
 
   /// Concrete subclasses of [Object] will have overrides of [_typeArguments]
   /// which return their type arguments.

sdk/lib/_internal/wasm/lib/string_patch.dart

@@ -105,8 +105,6 @@ abstract final class _StringBase implements String {
 
   int _computeHashCode();
 
-  int get _identityHashCode => hashCode;
-
   bool get _isOneByte {
     // Alternatively return false and override it on one-byte string classes.
     return this is _OneByteString;