[dart2wasm] Remove special casing of noSuchMethod in dispatch table builder

So far we force-included the `noSuchMethod` selector in the dispatch
table, making us have an entry for each class id (i.e. a dense row).

This is wasteful for most cases, because there may be few (if any)
overrides of `noSuchMethod`.

So instead we use the same policy on how to call `noSuchMethod` as
with all the other selectors, namely if there's only a single target
we call it directly, if there's a few we may use a static polymorphic
dispatcher function and otherwise use dispatch table.

Change-Id: I5067fb54908d905396c3b93b824bf251dc73ff50
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/428521
Reviewed-by: Ömer Ağacan <[email protected]>
Commit-Queue: Martin Kustermann <[email protected]>

Author: mkustermann

pkg/dart2wasm/lib/code_generator.dart

@@ -1953,31 +1953,37 @@ abstract class AstCodeGenerator
     List<({Range range, Reference target})> targetRanges = targets.targetRanges;
     List<({Range range, Reference target})> staticDispatchRanges =
         targets.staticDispatchRanges;
-    if (!selector.isDynamicSubmoduleOverridable) {
-      if (targetRanges.length == 1) {
-        final target = translator.getFunctionEntry(targetRanges[0].target,
-            uncheckedEntry: useUncheckedEntry);
-        final directCallSignature = translator.signatureForDirectCall(target);
-        final paramInfo = translator.paramInfoForDirectCall(target);
-        pushArguments(directCallSignature, paramInfo);
-        return translator.outputOrVoid(call(target));
-      }
-      if (targetRanges.isEmpty) {
-        // Unreachable call
-        b.comment("Virtual call of $name with no targets"
-            " at ${node.location}");
-        pushArguments(signature, selector.paramInfo);
-        for (int i = 0; i < signature.inputs.length; ++i) {
-          b.drop();
-        }
-        b.block(const [], signature.outputs);
-        b.unreachable();
-        b.end();
-        return translator.outputOrVoid(signature.outputs);
+
+    // NOTE: Keep this in sync with
+    // `dynamic_forwarders.dart:generateNoSuchMethodCall`.
+    final bool noTarget =
+        targetRanges.isEmpty && !selector.isDynamicSubmoduleOverridable;
+    final bool directCall =
+        targetRanges.length == 1 && staticDispatchRanges.length == 1;
+    final callPolymorphicDispatcher =
+        !directCall && staticDispatchRanges.isNotEmpty;
+
+    if (noTarget) {
+      // Unreachable call
+      b.comment("Virtual call of $name with no targets"
+          " at ${node.location}");
+      pushArguments(signature, selector.paramInfo);
+      for (int i = 0; i < signature.inputs.length; ++i) {
+        b.drop();
       }
+      b.block(const [], signature.outputs);
+      b.unreachable();
+      b.end();
+      return translator.outputOrVoid(signature.outputs);
+    }
+    if (directCall) {
+      final target = translator.getFunctionEntry(targetRanges[0].target,
+          uncheckedEntry: useUncheckedEntry);
+      final directCallSignature = translator.signatureForDirectCall(target);
+      final paramInfo = translator.paramInfoForDirectCall(target);
+      pushArguments(directCallSignature, paramInfo);
+      return translator.outputOrVoid(call(target));
     }
-
-    final callPolymorphicDispatcher = staticDispatchRanges.isNotEmpty;
 
     // Receiver is already on stack.
     w.Local receiverVar = addLocal(signature.inputs.first);

pkg/dart2wasm/lib/dispatch_table.dart

@@ -61,11 +61,6 @@ class SelectorInfo {
   /// The selector's member's name.
   final String name;
 
-  /// Whether the selector is for `Object.noSuchMethod`. We introduce instance
-  /// invocations of `noSuchMethod` in dynamic calls, so we always add
-  /// `noSuchMethod` overrides to the dispatch table.
-  final bool isNoSuchMethod;
-
   SelectorTargets? _checked;
   SelectorTargets? _unchecked;
   SelectorTargets? _normal;
@@ -88,7 +83,6 @@ class SelectorInfo {
     this.callCount,
     this.paramInfo, {
     required this.isSetter,
-    required this.isNoSuchMethod,
   });
 
   void serialize(DataSerializer sink) {
@@ -100,7 +94,6 @@ class SelectorInfo {
       useMultipleEntryPoints,
       isDynamicSubmoduleOverridable,
       isDynamicSubmoduleCallable,
-      isNoSuchMethod
     ]);
     sink.writeNullable(_checked, (targets) => targets.serialize(sink));
     sink.writeNullable(_unchecked, (targets) => targets.serialize(sink));
@@ -118,7 +111,6 @@ class SelectorInfo {
       useMultipleEntryPoints,
       isDynamicSubmoduleOverridable,
       isDynamicSubmoduleCallable,
-      isNoSuchMethod
     ] = source.readBoolList();
     final checked =
         source.readNullable(() => SelectorTargets.deserialize(source));
@@ -132,7 +124,7 @@ class SelectorInfo {
         ParameterInfo.fromMember(references.first),
         (p, e) => p..merge(ParameterInfo.fromMember(e)));
     return SelectorInfo._(dispatchTable, id, name, callCount, paramInfo,
-        isSetter: isSetter, isNoSuchMethod: isNoSuchMethod)
+        isSetter: isSetter)
       ..useMultipleEntryPoints = useMultipleEntryPoints
       ..isDynamicSubmoduleCallable = isDynamicSubmoduleCallable
       ..isDynamicSubmoduleOverridable = isDynamicSubmoduleOverridable
@@ -484,16 +476,16 @@ class DispatchTable {
     final isDynamicSubmoduleOverridable =
         member.isDynamicSubmoduleOverridable(translator.coreTypes);
 
+    // The compiler will generate calls to `noSuchMethod` in the dynamic
+    // invocation forwarders. So we ensure that the call count is positive.
+    final isNoSuchMethod = member == translator.objectNoSuchMethod;
+    final callCount =
+        _selectorMetadata[selectorId].callCount + (isNoSuchMethod ? 1 : 0);
     final selector = _selectorInfo.putIfAbsent(
         selectorId,
-        () => SelectorInfo._(
-            this,
-            selectorId,
-            member.name.text,
-            _selectorMetadata[selectorId].callCount,
+        () => SelectorInfo._(this, selectorId, member.name.text, callCount,
             ParameterInfo.fromMember(target),
-            isSetter: isSetter,
-            isNoSuchMethod: member == translator.objectNoSuchMethod));
+            isSetter: isSetter));
     assert(selector.isSetter == isSetter);
     final useMultipleEntryPoints = !member.isAbstract &&
         !member.isExternal &&
@@ -872,17 +864,8 @@ class DispatchTable {
 }
 
 bool _isUsedViaDispatchTableCall(SelectorInfo selector) {
-  // Special case for `objectNoSuchMethod`: we introduce instance
-  // invocations of `objectNoSuchMethod` in dynamic calls, so keep it alive
-  // even if there was no references to it from the Dart code.
-  if (selector.isNoSuchMethod) {
-    return true;
-  }
   if (selector.isDynamicSubmoduleCallable) return true;
-
-  if (selector.callCount == 0) {
-    return false;
-  }
+  if (selector.callCount == 0) return false;
   if (selector.isDynamicSubmoduleOverridable) return true;
 
   final targets = selector.targets(unchecked: false);

pkg/dart2wasm/lib/dynamic_forwarders.dart

@@ -782,10 +782,26 @@ void generateNoSuchMethodCall(
   translator.functions.recordSelectorUse(noSuchMethodSelector, false);
   final signature = noSuchMethodSelector.signature;
 
+  final targetRanges =
+      noSuchMethodSelector.targets(unchecked: false).targetRanges;
+  final staticDispatchRanges =
+      noSuchMethodSelector.targets(unchecked: false).staticDispatchRanges;
+
+  // NOTE: Keep this in sync with
+  // `code_generator.dart:AstCodeGenerator._virtualCall`.
+  final bool directCall =
+      targetRanges.length == 1 && staticDispatchRanges.length == 1;
+  final callPolymorphicDispatcher =
+      !directCall && staticDispatchRanges.isNotEmpty;
+
   final noSuchMethodParamInfo = noSuchMethodSelector.paramInfo;
   final noSuchMethodWasmFunctionType = signature;
 
   pushReceiver();
+  if (callPolymorphicDispatcher) {
+    b.struct_get(translator.topInfo.struct, FieldIndex.classId);
+    pushReceiver();
+  }
   pushInvocationObject();
 
   final invocationFactory = translator.functions
@@ -817,11 +833,19 @@ void generateNoSuchMethodCall(
 
   assert(wasmArgIdx == noSuchMethodWasmFunctionType.inputs.length);
 
-  // Get class id for virtual call
-  pushReceiver();
-  translator.callDispatchTable(b, noSuchMethodSelector,
-      interfaceTarget: translator.objectNoSuchMethod.reference,
-      useUncheckedEntry: false);
+  if (directCall) {
+    translator.callReference(targetRanges[0].target, b);
+  } else if (callPolymorphicDispatcher) {
+    b.invoke(translator
+        .getPolymorphicDispatchersForModule(b.module)
+        .getPolymorphicDispatcher(noSuchMethodSelector,
+            useUncheckedEntry: false));
+  } else {
+    pushReceiver();
+    translator.callDispatchTable(b, noSuchMethodSelector,
+        interfaceTarget: translator.objectNoSuchMethod.reference,
+        useUncheckedEntry: false);
+  }
 }
 
 class ClassIdRange {