[dart2js] Specialize strong 'is Object'

- o is Object --> o != null.
- Avoid specializing 'is Object' to instanceof.

Change-Id: Id882666d547ee2e06c6b92dec5a8070302dd9415
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/147686
Reviewed-by: Joshua Litt <joshualitt@google.com>
Commit-Queue: Stephen Adams <sra@google.com>

pkg/compiler/lib/src/js_backend/specialized_checks.dart

@@ -12,7 +12,8 @@ import '../universe/class_hierarchy.dart' show ClassHierarchy;
 import '../world.dart' show JClosedWorld;
 
 enum IsTestSpecialization {
-  null_,
+  isNull,
+  notNull,
   string,
   bool,
   num,
@@ -27,8 +28,7 @@ class SpecializedChecks {
     if (dartType is LegacyType) {
       DartType base = dartType.baseType;
       // `Never*` accepts only `null`.
-      if (base is NeverType) return IsTestSpecialization.null_;
-      // TODO(sra): Handle strong checking 'x is Object' --> `x != null`.
+      if (base is NeverType) return IsTestSpecialization.isNull;
       // `Object*` is top and should be handled by constant folding.
       if (base.isObject) return null;
       return _findIsTestSpecialization(base, graph, closedWorld);
@@ -44,7 +44,7 @@ class SpecializedChecks {
 
       if (element == commonElements.nullClass ||
           element == commonElements.jsNullClass) {
-        return IsTestSpecialization.null_;
+        return IsTestSpecialization.isNull;
       }
 
       if (element == commonElements.jsStringClass ||
@@ -73,6 +73,9 @@ class SpecializedChecks {
       }
 
       DartTypes dartTypes = closedWorld.dartTypes;
+      // Top types (here it could be Object in non-NNBD mode) should be constant
+      // folded outside the specializer. This test protects logic below.
+      if (dartTypes.isTopType(dartType)) return null;
       ElementEnvironment elementEnvironment = closedWorld.elementEnvironment;
       if (!dartTypes.isSubtype(
           elementEnvironment.getClassInstantiationToBounds(element),
@@ -84,6 +87,11 @@ class SpecializedChecks {
         return IsTestSpecialization.arrayTop;
       }
 
+      if (dartType.isObject) {
+        assert(!dartTypes.isTopType(dartType)); // Checked above.
+        return IsTestSpecialization.notNull;
+      }
+
       ClassHierarchy classHierarchy = closedWorld.classHierarchy;
       InterceptorData interceptorData = closedWorld.interceptorData;
       OutputUnitData outputUnitData = closedWorld.outputUnitData;
@@ -92,6 +100,7 @@ class SpecializedChecks {
           !interceptorData.isInterceptedClass(element) &&
           outputUnitData.hasOnlyNonDeferredImportPathsToClass(
               graph.element, element)) {
+        assert(!dartType.isObject); // Checked above.
         return IsTestSpecialization.instanceof;
       }
     }

pkg/compiler/lib/src/ssa/codegen.dart

@@ -3492,9 +3492,10 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
 
     js.Expression test;
     switch (node.specialization) {
-      case IsTestSpecialization.null_:
-        // This case should be lowered to [HIdentity] during optimization.
-        test = js.Binary(relation, value, js.LiteralNull());
+      case IsTestSpecialization.isNull:
+      case IsTestSpecialization.notNull:
+        // These cases should be lowered using [HIdentity] during optimization.
+        failedAt(node, 'Missing lowering');
         break;
 
       case IsTestSpecialization.string:

pkg/compiler/lib/src/ssa/optimize.dart

@@ -2176,12 +2176,17 @@ class SsaInstructionSimplifier extends HBaseVisitor
           SpecializedChecks.findIsTestSpecialization(
               dartType, _graph, _closedWorld);
 
-      if (specialization == IsTestSpecialization.null_) {
-        return HIdentity(
+      if (specialization == IsTestSpecialization.isNull ||
+          specialization == IsTestSpecialization.notNull) {
+        HInstruction nullTest = HIdentity(
             node.checkedInput,
             _graph.addConstantNull(_closedWorld),
             null,
             _abstractValueDomain.boolType);
+        if (specialization == IsTestSpecialization.isNull) return nullTest;
+        nullTest.sourceInformation = node.sourceInformation;
+        node.block.addBefore(node, nullTest);
+        return HNot(nullTest, _abstractValueDomain.boolType);
       }
 
       if (specialization != null) {