[dart2wasm] Remove support for not using non-nullable locals.

With the decision to support non-nullable locals in WasmGC as per
WebAssembly/function-references#44 the
support in dart2wasm for forcing all locals to be nullable is no
longer needed.

This CL removes that support and cleans up some related nullability
issues. Specifically:

- Remove the `--local-nullability` and `--parameter-nullability`
  commandline options. These are now always enabled.
- Clean out special cases around forced nullable locals throughout the
  compiler.
- Make `thisLocal` and `preciseThisLocal` always non-nullable.
- Make `returnValueLocal` (for storing the return value of `return`
  statements inside `try` blocks with `finally`) always defaultable,
  since its initialization flow crosses control constructs.
- Make type argument parameters non-nullable.
- Make non-nullable `FutureOr` translate to a non-nullable Wasm type.
- Implement the "initialized until end of block" validation scheme
  in the Wasm instruction validator.
- Run tests with the `--experimental-wasm-nn-locals` option. This
  is likely going away soon, but for now we need it.

Change-Id: I05873dd70510af5944d86d37cb5765c7bdef73a9
Cq-Include-Trybots: luci.dart.try:dart2wasm-linux-x64-d8-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/252600
Commit-Queue: Aske Simon Christensen <askesc@google.com>
Reviewed-by: Joshua Litt <joshualitt@google.com>
Reviewed-by: William Hesse <whesse@google.com>

pkg/dart2wasm/bin/dart2wasm.dart

@@ -28,16 +28,10 @@ final List<Option> options = [
   Flag(
       "lazy-constants", (o, value) => o.translatorOptions.lazyConstants = value,
       defaultsTo: _d.translatorOptions.lazyConstants),
-  Flag("local-nullability",
-      (o, value) => o.translatorOptions.localNullability = value,
-      defaultsTo: _d.translatorOptions.localNullability),
   Flag("name-section", (o, value) => o.translatorOptions.nameSection = value,
       defaultsTo: _d.translatorOptions.nameSection),
   Flag("nominal-types", (o, value) => o.translatorOptions.nominalTypes = value,
       defaultsTo: _d.translatorOptions.nominalTypes),
-  Flag("parameter-nullability",
-      (o, value) => o.translatorOptions.parameterNullability = value,
-      defaultsTo: _d.translatorOptions.parameterNullability),
   Flag("polymorphic-specialization",
       (o, value) => o.translatorOptions.polymorphicSpecialization = value,
       defaultsTo: _d.translatorOptions.polymorphicSpecialization),

pkg/dart2wasm/bin/run_wasm.js

@@ -6,7 +6,7 @@
 //
 // Run as follows:
 //
-// $> d8 --experimental-wasm-gc --wasm-gc-js-interop run_wasm.js -- <dart_module>.wasm [<ffi_module>.wasm]
+// $> d8 --experimental-wasm-gc --experimental-wasm-nn-locals --wasm-gc-js-interop run_wasm.js -- <dart_module>.wasm [<ffi_module>.wasm]
 //
 // If an FFI module is specified, it will be instantiated first, and its
 // exports will be supplied as imports to the Dart module under the 'ffi'

pkg/dart2wasm/dart2wasm.md

@@ -16,10 +16,8 @@ where *options* include:
 | `--`[`no-`]`import-shared-memory`       | no      | Import a shared memory buffer. If this is on, `--shared-memory-max-pages` must also be specified.
 | `--`[`no-`]`inlining`                   | no      | Inline small functions.
 | `--`[`no-`]`lazy-constants`             | no      | Instantiate constants lazily.
-| `--`[`no-`]`local-nullability`          | no      | Use non-nullable types for non-nullable locals and temporaries.
 | `--`[`no-`]`name-section`               | yes     | Emit Name Section with function names.
 | `--`[`no-`]`nominal-types`              | yes     | Emit nominal types.
-| `--`[`no-`]`parameter-nullability`      | yes     | Use non-nullable types for non-nullable parameters and return values.
 | `--`[`no-`]`polymorphic-specialization` | no      | Do virtual calls by switching on the class ID instead of using `call_indirect`.
 | `--`[`no-`]`print-kernel`               | no      | Print IR for each function before compiling it.
 | `--`[`no-`]`print-wasm`                 | no      | Print Wasm instructions of each compiled function.
@@ -30,7 +28,7 @@ where *options* include:
 
 The resulting `.wasm` file can be run with:
 
-`d8 --experimental-wasm-gc --wasm-gc-js-interop pkg/dart2wasm/bin/run_wasm.js -- `*outfile*`.wasm`
+`d8 --experimental-wasm-gc --experimental-wasm-nn-locals --wasm-gc-js-interop pkg/dart2wasm/bin/run_wasm.js -- `*outfile*`.wasm`
 
 ## Imports and exports
 

pkg/dart2wasm/lib/code_generator.dart

@@ -88,7 +88,7 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
   w.ValueType translateType(DartType type) => translator.translateType(type);
 
   w.Local addLocal(w.ValueType type) {
-    return function.addLocal(translator.typeForLocal(type));
+    return function.addLocal(type);
   }
 
   DartType dartTypeOf(Expression exp) {
@@ -205,7 +205,7 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
 
     void getThis() {
       w.Local thisLocal = paramLocals[0];
-      w.RefType structType = w.RefType.def(struct, nullable: true);
+      w.RefType structType = w.RefType.def(struct, nullable: false);
       b.local_get(thisLocal);
       translator.convertType(function, thisLocal.type, structType);
     }
@@ -338,16 +338,16 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
         if (context.parent == null) break;
 
         b.struct_get(context.struct, context.parentFieldIndex);
-        if (options.localNullability) {
-          b.ref_as_non_null();
-        }
+        b.ref_as_non_null();
         context = context.parent!;
       }
       if (context.containsThis) {
-        thisLocal = addLocal(
-            context.struct.fields[context.thisFieldIndex].type.unpacked);
+        thisLocal = addLocal(context
+            .struct.fields[context.thisFieldIndex].type.unpacked
+            .withNullability(false));
         preciseThisLocal = thisLocal;
         b.struct_get(context.struct, context.thisFieldIndex);
+        b.ref_as_non_null();
         b.local_set(thisLocal!);
       }
     }
@@ -487,9 +487,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
   void visitRedirectingInitializer(RedirectingInitializer node) {
     Class cls = (node.parent as Constructor).enclosingClass;
     b.local_get(thisLocal!);
-    if (options.parameterNullability && thisLocal!.type.nullable) {
-      b.ref_as_non_null();
-    }
     for (TypeParameter typeParam in cls.typeParameters) {
       types.makeType(
           this, TypeParameterType(typeParam, Nullability.nonNullable));
@@ -507,9 +504,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
       return;
     }
     b.local_get(thisLocal!);
-    if (options.parameterNullability && thisLocal!.type.nullable) {
-      b.ref_as_non_null();
-    }
     for (DartType typeArg in supertype!.typeArguments) {
       types.makeType(this, typeArg);
     }
@@ -882,7 +876,10 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
         finalizer.mustHandleReturn = true;
       }
       if (returnType != voidMarker) {
-        returnValueLocal ??= addLocal(returnType);
+        // Since the flow of the return value through the returnValueLocal
+        // crosses control-flow constructs, the local needs to always have a
+        // defaultable type in order for the Wasm code to validate.
+        returnValueLocal ??= addLocal(returnType.withNullability(true));
         b.local_set(returnValueLocal!);
       }
       b.br(finalizers.last.label);
@@ -1001,8 +998,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
         } else {
           wrap(exp, nonNullableType);
           b.local_get(switchValueNonNullableLocal);
-          translator.convertType(
-              function, switchValueNonNullableLocal.type, nonNullableType);
           compare();
           b.br_if(switchLabels[c]!);
         }
@@ -1095,15 +1090,17 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
   }
 
   w.ValueType visitThis(w.ValueType expectedType) {
-    w.ValueType thisType = thisLocal!.type.withNullability(false);
-    w.ValueType preciseThisType = preciseThisLocal!.type.withNullability(false);
+    w.ValueType thisType = thisLocal!.type;
+    w.ValueType preciseThisType = preciseThisLocal!.type;
+    assert(!thisType.nullable);
+    assert(!preciseThisType.nullable);
     if (!thisType.isSubtypeOf(expectedType) &&
         preciseThisType.isSubtypeOf(expectedType)) {
       b.local_get(preciseThisLocal!);
-      return preciseThisLocal!.type;
+      return preciseThisType;
     } else {
       b.local_get(thisLocal!);
-      return thisLocal!.type;
+      return thisType;
     }
   }
 
@@ -1122,9 +1119,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
     b.local_get(temp);
     b.i32_const(info.classId);
     b.struct_set(info.struct, FieldIndex.classId);
-    if (options.parameterNullability && temp.type.nullable) {
-      b.ref_as_non_null();
-    }
     _visitArguments(node.arguments, node.targetReference, 1);
     call(node.targetReference);
     if (expectedType != voidMarker) {
@@ -1158,8 +1152,7 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
         _lookupSuperTarget(node.interfaceTarget, setter: false).reference;
     w.BaseFunction targetFunction = translator.functions.getFunction(target);
     w.ValueType receiverType = targetFunction.type.inputs.first;
-    w.ValueType thisType = visitThis(receiverType);
-    translator.convertType(function, thisType, receiverType);
+    visitThis(receiverType);
     _visitArguments(node.arguments, target, 1);
     return call(target);
   }
@@ -1261,14 +1254,12 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
         b.local_get(leftLocal);
         if (leftNullable) {
           b.br_on_null(operandNull!);
-        } else if (leftLocal.type.nullable) {
-          b.ref_as_non_null();
         }
       }
 
       void right([_]) {
         b.local_get(rightLocal);
-        if (rightLocal.type.nullable) {
+        if (rightNullable) {
           b.ref_as_non_null();
         }
       }
@@ -1339,10 +1330,8 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
 
     // Receiver is already on stack.
     w.Local receiverVar = addLocal(selector.signature.inputs.first);
+    assert(!receiverVar.type.nullable);
     b.local_tee(receiverVar);
-    if (options.parameterNullability && receiverVar.type.nullable) {
-      b.ref_as_non_null();
-    }
     pushArguments(selector.signature);
 
     if (options.polymorphicSpecialization) {
@@ -1610,10 +1599,8 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
     // Evaluate receiver
     wrap(node.receiver, selector.signature.inputs.first);
     w.Local receiverVar = addLocal(selector.signature.inputs.first);
+    assert(!receiverVar.type.nullable);
     b.local_tee(receiverVar);
-    if (options.parameterNullability && receiverVar.type.nullable) {
-      b.ref_as_non_null();
-    }
 
     // Dispatch table call
     b.comment("Dynamic get of '${selector.name}'");
@@ -1714,7 +1701,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
       if (preserved) {
         temp = addLocal(paramType);
         b.local_tee(temp);
-        translator.convertType(function, temp.type, paramType);
       }
       call(target.reference);
     }
@@ -1779,9 +1765,7 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
     Context? context = closures.contexts[functionNode]?.parent;
     if (context != null) {
       b.local_get(context.currentLocal);
-      if (context.currentLocal.type.nullable) {
-        b.ref_as_non_null();
-      }
+      assert(!context.currentLocal.type.nullable);
     } else {
       b.global_get(translator.globals.dummyGlobal); // Dummy context
     }
@@ -1908,7 +1892,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
           signature.inputs[signatureOffset + paramInfo.nameIndex[name]!];
       if (namedLocal != null) {
         b.local_get(namedLocal);
-        translator.convertType(function, namedLocal.type, type);
       } else {
         translator.constants
             .instantiateConstant(function, b, paramInfo.named[name]!, type);
@@ -2029,9 +2012,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
           b.array_set(arrayType);
         }
         b.local_get(arrayLocal);
-        if (arrayLocal.type.nullable) {
-          b.ref_as_non_null();
-        }
       }
     } else {
       for (int i = 0; i < length; i++) {
@@ -2072,7 +2052,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
     b.local_set(mapLocal);
     for (MapLiteralEntry entry in node.entries) {
       b.local_get(mapLocal);
-      translator.convertType(function, mapLocal.type, putReceiverType);
       wrap(entry.key, putKeyType);
       wrap(entry.value, putValueType);
       b.call(mapPut);
@@ -2100,7 +2079,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
     b.local_set(setLocal);
     for (Expression element in node.expressions) {
       b.local_get(setLocal);
-      translator.convertType(function, setLocal.type, addReceiverType);
       wrap(element, addKeyType);
       b.call(setAdd);
       b.drop();
@@ -2161,8 +2139,7 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
       Class cls = parameter.parent as Class;
       ClassInfo info = translator.classInfo[cls]!;
       int fieldIndex = translator.typeParameterIndex[parameter]!;
-      w.ValueType thisType = visitThis(info.nonNullableType);
-      translator.convertType(function, thisType, info.nonNullableType);
+      visitThis(info.nonNullableType);
       b.struct_get(info.struct, fieldIndex);
       resultType = info.struct.fields[fieldIndex].type.unpacked;
     }

pkg/dart2wasm/lib/constants.dart

@@ -212,8 +212,8 @@ class Constants {
 
     w.Local offset = function.locals[0];
     w.Local length = function.locals[1];
-    w.Local array = function.addLocal(
-        translator.typeForLocal(w.RefType.def(arrayType, nullable: false)));
+    w.Local array =
+        function.addLocal(w.RefType.def(arrayType, nullable: false));
     w.Local index = function.addLocal(w.NumType.i32);
 
     w.Instructions b = function.body;
@@ -422,12 +422,11 @@ class ConstantCreator extends ConstantVisitor<ConstantInfo?> {
       w.FunctionType ftype = translator.functionType(const [], [type]);
       w.DefinedFunction function = m.addFunction(ftype, "$constant");
       generator(function, function.body);
-      w.Local temp = function.addLocal(translator.typeForLocal(type));
+      w.Local temp = function.addLocal(type);
       w.Instructions b2 = function.body;
       b2.local_tee(temp);
       b2.global_set(global);
       b2.local_get(temp);
-      translator.convertType(function, temp.type, type);
       b2.end();
 
       return ConstantInfo(constant, global, function);
@@ -579,8 +578,7 @@ class ConstantCreator extends ConstantVisitor<ConstantInfo?> {
       b.i64_const(length);
       if (lazyConstants) {
         // Allocate array and set each entry to the corresponding sub-constant.
-        w.Local arrayLocal = function!.addLocal(
-            refType.withNullability(!translator.options.localNullability));
+        w.Local arrayLocal = function!.addLocal(refType.withNullability(false));
         b.i32_const(length);
         translator.array_new_default(b, arrayType);
         b.local_set(arrayLocal);
@@ -592,9 +590,6 @@ class ConstantCreator extends ConstantVisitor<ConstantInfo?> {
           b.array_set(arrayType);
         }
         b.local_get(arrayLocal);
-        if (arrayLocal.type.nullable) {
-          b.ref_as_non_null();
-        }
       } else {
         // Push all sub-constants on the stack and initialize array from them.
         for (int i = 0; i < length; i++) {

pkg/dart2wasm/lib/dispatch_table.dart

@@ -121,7 +121,7 @@ class SelectorInfo {
     });
 
     List<w.ValueType> typeParameters = List.filled(paramInfo.typeParamCount,
-        translator.classInfo[translator.typeClass]!.nullableType);
+        translator.classInfo[translator.typeClass]!.nonNullableType);
     List<w.ValueType> inputs = List.generate(
         inputSets.length,
         (i) => translator.typeForInfo(

pkg/dart2wasm/lib/functions.dart

@@ -195,7 +195,7 @@ class FunctionCollector extends MemberVisitor1<w.FunctionType, Reference> {
     }
 
     List<w.ValueType> typeParameters = List.filled(typeParamCount,
-        translator.classInfo[translator.typeClass]!.nullableType);
+        translator.classInfo[translator.typeClass]!.nonNullableType);
 
     // The JS embedder will not accept Wasm struct types as parameter or return
     // types for functions called from JS. We need to use eqref instead.

pkg/dart2wasm/lib/intrinsics.dart

@@ -811,7 +811,7 @@ class Intrinsifier {
           codeGen.wrap(
               node.arguments.positional.single, receiverInfo.nonNullableType);
           w.Local receiverLocal =
-              codeGen.function.addLocal(receiverInfo.nullableType);
+              codeGen.function.addLocal(receiverInfo.nonNullableType);
           b.local_tee(receiverLocal);
           // We ignore the type argument and just update the classID of the
           // receiver.
@@ -823,7 +823,6 @@ class Intrinsifier {
           b.i32_const(fixedLengthListInfo.classId);
           b.struct_set(topInfo.struct, FieldIndex.classId);
           b.local_get(receiverLocal);
-          b.ref_as_non_null();
           return fixedLengthListInfo.nonNullableType;
       }
     }