[dart2wasm] Some refactorings of the compiler

a) Usage of `ClassInfo.*`

We have an abstraction layer that translates dart types
to wasm types - which is the `Translator` object (it
has e.g. `Translator.translateType`).

=> We remove direct use of `ClassInfo.*` in most places
=> Instead of exposing `ClassInfo` for specific types
   (e.g. top type), we expose the wasm representation type

b) Type of `ClassInfo.repr`

Currently `ClassInfo.repr` is itself a `ClassInfo`, which is
rather confusing. We use `ClassInfo.repr` to determine how to
translate `InterfaceType`s of that class to the wasm type system.

=> We make `Class.repr` be an actual wasm representation type
   (we make it a `w.RefType`)

c) We create a macro assembler `loadClassId` method so we have
   one place where we load the class id and we verify in
   assertions that the receiver type is `ref #Top`.

Change-Id: Ie24008a98131dae34948406a1f5aec881499c1f0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/431080
Commit-Queue: Martin Kustermann <[email protected]>
Reviewed-by: Ömer Ağacan <[email protected]>

Author: mkustermann

pkg/dart2wasm/lib/async.dart

@@ -55,9 +55,9 @@ mixin AsyncCodeGeneratorMixin on StateMachineEntryAstCodeGenerator {
     // here.
 
     b.local_get(asyncStateLocal);
-    b.ref_null(translator.topInfo.struct); // await value
-    b.ref_null(translator.topInfo.struct); // error value
-    b.ref_null(translator.stackTraceInfo.repr.struct); // stack trace
+    b.ref_null(translator.topType.heapType); // await value
+    b.ref_null(translator.topType.heapType); // error value
+    b.ref_null(translator.stackTraceType.heapType); // stack trace
     translator.callFunction(resumeFun, b);
     b.drop(); // drop null
 
@@ -79,15 +79,14 @@ mixin AsyncCodeGeneratorMixin on StateMachineEntryAstCodeGenerator {
       b.module.functions.define(
           translator.typesBuilder.defineFunction([
             asyncSuspendStateInfo.nonNullableType, // _AsyncSuspendState
-            translator.topInfo.nullableType, // Object?, await value
-            translator.topInfo.nullableType, // Object?, error value
-            translator.stackTraceInfo.repr
-                .nullableType // StackTrace?, error stack trace
+            translator.topType, // Object?, await value
+            translator.topType, // Object?, error value
+            translator.stackTraceTypeNullable // StackTrace?, error stack trace
           ], [
             // Inner function does not return a value, but it's Dart type is
             // `void Function(...)` and all Dart functions return a value, so we
             // add a return type.
-            translator.topInfo.nullableType
+            translator.topType
           ]),
           "${function.functionName} inner");
 }
@@ -254,17 +253,15 @@ class AsyncStateMachineCodeGenerator extends StateMachineCodeGenerator {
     // Final state: return.
     emitTargetLabel(targets.last);
     b.local_get(_suspendStateLocal);
-    b.ref_null(translator.topInfo.struct);
+    b.ref_null(translator.topType.heapType);
     call(translator.getFunctionEntry(
         translator.asyncSuspendStateComplete.reference,
         uncheckedEntry: true));
     b.return_();
     b.end(); // masterLoop
 
-    final stackTraceLocal =
-        addLocal(translator.stackTraceInfo.repr.nonNullableType);
-
-    final exceptionLocal = addLocal(translator.topInfo.nonNullableType);
+    final stackTraceLocal = addLocal(translator.stackTraceType);
+    final exceptionLocal = addLocal(translator.topTypeNonNullable);
 
     void callCompleteError() {
       b.local_get(_suspendStateLocal);

pkg/dart2wasm/lib/class_info.dart

@@ -183,11 +183,11 @@ class ClassInfo {
   /// the superclass.
   final Map<TypeParameter, TypeParameter> typeParameterMatch;
 
-  /// The class whose struct is used as the type for variables of this type.
-  /// This is a type which is a superclass of all subtypes of this type.
-  ClassInfo get repr => _repr!;
+  /// The wasm type used to represent values of a dart interface type of this
+  /// class.
+  w.RefType get repr => _repr!;
 
-  ClassInfo? _repr;
+  w.RefType? _repr;
 
   /// Nullabe Wasm ref type for this class.
   final w.RefType nullableType;
@@ -478,7 +478,7 @@ class ClassInfoCollector {
       }
 
       for (Field field in info.cls!.fields) {
-        info._addField(w.FieldType(topInfo.nullableType),
+        info._addField(w.FieldType(translator.topType),
             fieldName: field.name.text);
       }
     }
@@ -548,7 +548,7 @@ class ClassInfoCollector {
         // class maps to topInfo because boxed values are a subtype of Object in
         // Dart but not of the object struct.
         representation = cls == translator.coreTypes.objectClass
-            ? translator.topInfo
+            ? topInfo
             : translator.objectInfo;
       } else {
         void addRanges(List<Range> ranges) {
@@ -581,7 +581,13 @@ class ClassInfoCollector {
         }
       }
       final info = translator.classInfo[cls]!;
-      info._repr = representation ?? info;
+      representation ??= info;
+
+      if (representation == topInfo) {
+        info._repr = translator.topTypeNonNullable;
+      } else {
+        info._repr = representation!.nonNullableType;
+      }
     }
 
     // Now that the representation types for all classes have been computed,

pkg/dart2wasm/lib/closures.dart

@@ -356,7 +356,7 @@ class ClosureLayouter extends RecursiveVisitor {
         superType: superType);
   }
 
-  w.ValueType get topType => translator.topInfo.nullableType;
+  w.ValueType get topType => translator.topType;
 
   ClosureLayouter(this.translator)
       : procedureAttributeMetadata =