wasm gc: support type nullability and global mutability

core/src/main/java/org/teavm/backend/wasm/WasmGCModuleGenerator.java

@@ -180,7 +180,7 @@ public class WasmGCModuleGenerator {
         initializer = new WasmFunction(functionType);
         initializer.setReferenced(true);
         declarationsGenerator.module.functions.add(initializer);
-        initializerRef = new WasmGlobal("teavm@initializer", functionType.getReference(),
+        initializerRef = new WasmGlobal("teavm@initializer", functionType.getNonNullReference(),
                 new WasmFunctionReference(initializer));
         declarationsGenerator.module.globals.add(initializerRef);
         initializer.getBody().add(new WasmSetGlobal(initializerRef,

core/src/main/java/org/teavm/backend/wasm/disasm/BaseDisassemblyListener.java

@@ -69,7 +69,9 @@ public abstract class BaseDisassemblyListener  {
                         break;
                 }
             } else if (type instanceof WasmHollowType.SpecialReference) {
-                switch (((WasmHollowType.SpecialReference) type).kind) {
+                var refType = (WasmHollowType.SpecialReference) type;
+                if (refType.isNullable()) {
+                    switch (refType.kind) {
                         case ANY:
                             writer.write("anyref");
                             return;
@@ -91,9 +93,38 @@ public abstract class BaseDisassemblyListener  {
                         default:
                             throw new IllegalArgumentException();
                     }
+                } else {
+                    writer.write("(ref ");
+                    switch (refType.kind) {
+                        case ANY:
+                            writer.write("any");
+                            return;
+                        case FUNC:
+                            writer.write("func");
+                            return;
+                        case ARRAY:
+                            writer.write("array");
+                            return;
+                        case EXTERN:
+                            writer.write("extern");
+                            return;
+                        case STRUCT:
+                            writer.write("struct");
+                            return;
+                        case I31:
+                            writer.write("i31");
+                            return;
+                        default:
+                            throw new IllegalArgumentException();
+                    }
+                }
             } else if (type instanceof WasmHollowType.CompositeReference) {
-                writer.write("(ref null ");
+                var refType = (WasmHollowType.CompositeReference) type;
-                writeTypeRef(((WasmHollowType.CompositeReference) type).index);
+                writer.write("(ref ");
+                if (refType.isNullable()) {
+                    writer.write("null");
+                }
+                writeTypeRef(refType.index);
                 writer.write(")");
                 return;
             }

core/src/main/java/org/teavm/backend/wasm/disasm/DisassemblyCodeListener.java

@@ -755,23 +755,17 @@ public class DisassemblyCodeListener extends BaseDisassemblyListener implements
     }
 
     @Override
-    public void cast(WasmHollowType.Reference type, boolean nullable) {
+    public void cast(WasmHollowType.Reference type) {
         writer.address().write("ref.cast (ref ");
-        if (!nullable) {
-            writer.write("null ");
-        }
         writeType(type);
-        writer.write(")").eol();
+        writer.eol();
     }
 
     @Override
-    public void test(WasmHollowType.Reference type, boolean nullable) {
+    public void test(WasmHollowType.Reference type) {
-        writer.address().write("ref.test (ref ");
+        writer.address().write("ref.test ");
-        if (!nullable) {
-            writer.write("null ");
-        }
         writeType(type);
-        writer.write(")").eol();
+        writer.eol();
     }
 
     @Override

core/src/main/java/org/teavm/backend/wasm/generate/gc/classes/WasmGCClassGenerator.java

@@ -281,8 +281,9 @@ public class WasmGCClassGenerator implements WasmGCClassInfoProvider, WasmGCInit
             }
 
             var classStructure = classInfo.virtualTableStructure;
-            classInfo.pointer = new WasmGlobal(pointerName, classStructure.getReference(),
+            classInfo.pointer = new WasmGlobal(pointerName, classStructure.getNonNullReference(),
                     new WasmStructNewDefault(classStructure));
+            classInfo.pointer.setImmutable(true);
             module.globals.add(classInfo.pointer);
             if (type instanceof ValueType.Primitive) {
                 initPrimitiveClass(classInfo, (ValueType.Primitive) type);
@@ -390,10 +391,8 @@ public class WasmGCClassGenerator implements WasmGCClassInfoProvider, WasmGCInit
             if (cls != null && cls.getMethod(CLINIT_METHOD_DESC) != null) {
                 var clinitType = functionTypes.of(null);
                 var wasmName = names.topLevel(names.suggestForClass(name) + "@initializer");
-                var initFunction = functionProvider.forStaticMethod(new MethodReference(name, CLINIT_METHOD_DESC));
+                classInfo.initializerPointer = new WasmGlobal(wasmName, clinitType.getReference(),
-                initFunction.setReferenced(true);
+                        new WasmNullConstant(clinitType.getReference()));
-                var ref = new WasmFunctionReference(initFunction);
-                classInfo.initializerPointer = new WasmGlobal(wasmName, clinitType.getReference(), ref);
                 module.globals.add(classInfo.initializerPointer);
             }
         }
@@ -420,6 +419,11 @@ public class WasmGCClassGenerator implements WasmGCClassInfoProvider, WasmGCInit
             if (virtualTable != null && virtualTable.isConcrete()) {
                 fillVirtualTableMethods(target, classStructure, classInfo.pointer, virtualTable);
             }
+            if (classInfo.initializerPointer != null) {
+                var initFunction = functionProvider.forStaticMethod(new MethodReference(name, CLINIT_METHOD_DESC));
+                initFunction.setReferenced(true);
+                classInfo.initializerPointer.setInitialValue(new WasmFunctionReference(initFunction));
+            }
         };
     }
 
@@ -490,7 +494,7 @@ public class WasmGCClassGenerator implements WasmGCClassInfoProvider, WasmGCInit
         var objectLocal = new WasmLocal(standardClasses.objectClass().getType(), "object");
         function.add(objectLocal);
 
-        var castObject = new WasmCast(new WasmGetLocal(objectLocal), objectStructure.getReference());
+        var castObject = new WasmCast(new WasmGetLocal(objectLocal), objectStructure.getNonNullReference());
         var arrayField = new WasmStructGet(objectStructure, castObject, ARRAY_DATA_FIELD_OFFSET);
         var result = new WasmArrayLength(arrayField);
         function.getBody().add(new WasmReturn(result));
@@ -521,7 +525,7 @@ public class WasmGCClassGenerator implements WasmGCClassInfoProvider, WasmGCInit
             arrayGetObjectFunction.add(objectLocal);
             arrayGetObjectFunction.add(indexLocal);
 
-            var array = new WasmCast(new WasmGetLocal(objectLocal), arrayStruct.getReference());
+            var array = new WasmCast(new WasmGetLocal(objectLocal), arrayStruct.getNonNullReference());
             var arrayData = new WasmStructGet(arrayStruct, array, ARRAY_DATA_FIELD_OFFSET);
             var result = new WasmArrayGet(arrayDataType, arrayData, new WasmGetLocal(indexLocal));
             arrayGetObjectFunction.getBody().add(new WasmReturn(result));
@@ -545,7 +549,7 @@ public class WasmGCClassGenerator implements WasmGCClassInfoProvider, WasmGCInit
         function.add(objectLocal);
         function.add(indexLocal);
 
-        var array = new WasmCast(new WasmGetLocal(objectLocal), arrayStruct.getReference());
+        var array = new WasmCast(new WasmGetLocal(objectLocal), arrayStruct.getNonNullReference());
         var arrayData = new WasmStructGet(arrayStruct, array, ARRAY_DATA_FIELD_OFFSET);
         var result = new WasmArrayGet(arrayDataType, arrayData, new WasmGetLocal(indexLocal));
         Class<?> primitiveType;
@@ -626,7 +630,7 @@ public class WasmGCClassGenerator implements WasmGCClassInfoProvider, WasmGCInit
                 var call = new WasmCall(function);
                 var instanceParam = new WasmLocal(getClassInfo(virtualTable.getClassName()).getType());
                 wrapperFunction.add(instanceParam);
-                var castTarget = getClassInfo(implementor.getClassName()).getType();
+                var castTarget = getClassInfo(implementor.getClassName()).getStructure().getNonNullReference();
                 call.getArguments().add(new WasmCast(new WasmGetLocal(instanceParam), castTarget));
                 var params = new WasmLocal[entry.getMethod().parameterCount()];
                 for (var i = 0; i < entry.getMethod().parameterCount(); ++i) {
@@ -664,7 +668,7 @@ public class WasmGCClassGenerator implements WasmGCClassInfoProvider, WasmGCInit
         function.add(dataCopyLocal);
 
         function.getBody().add(new WasmSetLocal(originalLocal,
-                new WasmCast(new WasmGetLocal(instanceLocal), objectStructure.getReference())));
+                new WasmCast(new WasmGetLocal(instanceLocal), objectStructure.getNonNullReference())));
         function.getBody().add(new WasmSetLocal(resultLocal, new WasmStructNewDefault(objectStructure)));
 
         var classValue = new WasmStructGet(objectStructure, new WasmGetLocal(originalLocal),

core/src/main/java/org/teavm/backend/wasm/generate/gc/methods/WasmGCGenerationVisitor.java

@@ -279,7 +279,7 @@ public class WasmGCGenerationVisitor extends BaseWasmGenerationVisitor {
         var index = context.classInfoProvider().getVirtualMethodsOffset() + entry.getIndex();
         var expectedInstanceClassInfo = context.classInfoProvider().getClassInfo(vtable.getClassName());
         var vtableStruct = expectedInstanceClassInfo.getVirtualTableStructure();
-        classRef = new WasmCast(classRef, vtableStruct.getReference());
+        classRef = new WasmCast(classRef, vtableStruct.getNonNullReference());
 
         var functionRef = new WasmStructGet(vtableStruct, classRef, index);
         var functionTypeRef = (WasmType.CompositeReference) vtableStruct.getFields().get(index).getUnpackedType();
@@ -288,7 +288,7 @@ public class WasmGCGenerationVisitor extends BaseWasmGenerationVisitor {
         var instanceType = (WasmType.CompositeReference) instance.getType();
         var instanceStruct = (WasmStructure) instanceType.composite;
         if (!expectedInstanceClassInfo.getStructure().isSupertypeOf(instanceStruct)) {
-            instanceRef = new WasmCast(instanceRef, expectedInstanceClassInfo.getType());
+            instanceRef = new WasmCast(instanceRef, expectedInstanceClassInfo.getStructure().getNonNullReference());
         }
 
         invoke.getArguments().add(instanceRef);
@@ -381,7 +381,7 @@ public class WasmGCGenerationVisitor extends BaseWasmGenerationVisitor {
             WasmExpression exception = new WasmGetGlobal(context.exceptionGlobal());
             if (exceptionClass != null && !exceptionClass.equals("java.lang.Throwable")) {
                 exception = new WasmCast(exception, context.classInfoProvider().getClassInfo(exceptionClass)
-                        .getStructure().getReference());
+                        .getStructure().getNonNullReference());
             }
             var save = new WasmSetLocal(local, exception);
             save.setLocation(location);

core/src/main/java/org/teavm/backend/wasm/generate/gc/strings/WasmGCStringPool.java

@@ -86,9 +86,10 @@ public class WasmGCStringPool implements WasmGCStringProvider, WasmGCInitializer
             var brief = string.length() > 16 ? string.substring(0, 16) : string;
             var globalName = names.topLevel("teavm@string<" + stringMap.size() + ">"
                     + WasmGCNameProvider.sanitize(brief));
-            var globalType = standardClasses.stringClass().getType();
+            var globalType = standardClasses.stringClass().getStructure().getNonNullReference();
             var global = new WasmGlobal(globalName, globalType,
                     new WasmStructNewDefault(standardClasses.stringClass().getStructure()));
+            global.setImmutable(true);
             module.globals.add(global);
             return new WasmGCStringConstant(stringMap.size(), global);
         });

core/src/main/java/org/teavm/backend/wasm/intrinsics/gc/ArrayIntrinsic.java

@@ -59,7 +59,7 @@ public class ArrayIntrinsic implements WasmGCIntrinsic {
         var object = context.exprCache().create(originalObject, objectStruct.getReference(),
                 invocation.getLocation(), block.getBody());
         var classRef = new WasmStructGet(objectStruct, object.expr(), WasmGCClassInfoProvider.CLASS_FIELD_OFFSET);
-        var vt = new WasmCast(classRef, vtStruct.getReference());
+        var vt = new WasmCast(classRef, vtStruct.getNonNullReference());
         var function = new WasmStructGet(vtStruct, vt, offset);
         var call = new WasmCallReference(function, functionType);
         call.getArguments().add(object.expr());

core/src/main/java/org/teavm/backend/wasm/intrinsics/gc/ObjectIntrinsic.java

@@ -66,8 +66,10 @@ public class ObjectIntrinsic implements WasmGCIntrinsic {
     }
 
     private WasmExpression generateGetMonitor(InvocationExpr invocation, WasmGCIntrinsicContext context) {
-        var monitorType = context.classInfoProvider().getClassInfo(ValueType.object("java.lang.Object$Monitor"))
+        var monitorStruct = context.classInfoProvider().getClassInfo(ValueType.object("java.lang.Object$Monitor"))
-                .getStructure().getReference();
+                .getStructure();
+        var monitorType = monitorStruct.getReference();
+        var monitorNotNullType = monitorStruct.getNonNullReference();
         var objectStruct = context.classInfoProvider().getClassInfo(ValueType.object("java.lang.Object"))
                 .getStructure();
         var block = new WasmBlock(false);
@@ -77,13 +79,13 @@ public class ObjectIntrinsic implements WasmGCIntrinsic {
         block.getBody().add(new WasmSetLocal(tmpVar, new WasmStructGet(objectStruct, instance,
                 WasmGCClassInfoProvider.MONITOR_FIELD_OFFSET)));
 
-        WasmExpression test = new WasmTest(new WasmGetLocal(tmpVar), monitorType, false);
+        WasmExpression test = new WasmTest(new WasmGetLocal(tmpVar), monitorNotNullType);
         test = new WasmIntUnary(WasmIntType.INT32, WasmIntUnaryOperation.EQZ, test);
         var branch = new WasmBranch(test, block);
         branch.setResult(new WasmNullConstant(monitorType));
         block.getBody().add(new WasmDrop(branch));
 
-        block.getBody().add(new WasmCast(new WasmGetLocal(tmpVar), monitorType));
+        block.getBody().add(new WasmCast(new WasmGetLocal(tmpVar), monitorNotNullType));
         context.tempVars().release(tmpVar);
         return block;
     }
@@ -106,13 +108,14 @@ public class ObjectIntrinsic implements WasmGCIntrinsic {
         block.getBody().add(new WasmSetLocal(tmpVar, new WasmStructGet(objectStruct, instance,
                 WasmGCClassInfoProvider.MONITOR_FIELD_OFFSET)));
 
-        WasmExpression test = new WasmTest(new WasmGetLocal(tmpVar), WasmType.Reference.I31, false);
+        WasmExpression test = new WasmTest(new WasmGetLocal(tmpVar),
+                WasmType.SpecialReferenceKind.I31.asNonNullType());
         test = new WasmIntUnary(WasmIntType.INT32, WasmIntUnaryOperation.EQZ, test);
         var branch = new WasmBranch(test, block);
         branch.setResult(new WasmInt32Constant(-1));
         block.getBody().add(new WasmDrop(branch));
 
-        var i31ref = new WasmCast(new WasmGetLocal(tmpVar), WasmType.Reference.I31);
+        var i31ref = new WasmCast(new WasmGetLocal(tmpVar), WasmType.SpecialReferenceKind.I31.asNonNullType());
         block.getBody().add(new WasmInt31Get(i31ref, WasmSignedType.UNSIGNED));
         context.tempVars().release(tmpVar);
         return block;

core/src/main/java/org/teavm/backend/wasm/model/WasmCompositeType.java

@@ -18,6 +18,7 @@ package org.teavm.backend.wasm.model;
 public abstract class WasmCompositeType extends WasmEntity {
     private String name;
     private WasmType.CompositeReference reference;
+    private WasmType.CompositeReference nonNullReference;
     int recursiveTypeCount = -1;
 
     WasmCompositeType(String name) {
@@ -30,11 +31,18 @@ public abstract class WasmCompositeType extends WasmEntity {
 
     public WasmType.CompositeReference getReference() {
         if (reference == null) {
-            reference = new WasmType.CompositeReference(this);
+            reference = new WasmType.CompositeReference(this, true);
         }
         return reference;
     }
 
+    public WasmType.CompositeReference getNonNullReference() {
+        if (nonNullReference == null) {
+            nonNullReference = new WasmType.CompositeReference(this, false);
+        }
+        return nonNullReference;
+    }
+
     public int getRecursiveTypeCount() {
         return recursiveTypeCount;
     }

core/src/main/java/org/teavm/backend/wasm/model/WasmGlobal.java

@@ -22,6 +22,7 @@ public class WasmGlobal extends WasmEntity {
     private String name;
     private WasmType type;
     private WasmExpression initialValue;
+    private boolean immutable;
 
     public WasmGlobal(String name, WasmType type, WasmExpression initialValue) {
         this.name = name;
@@ -48,4 +49,12 @@ public class WasmGlobal extends WasmEntity {
     public void setInitialValue(WasmExpression initialValue) {
         this.initialValue = Objects.requireNonNull(initialValue);
     }
+
+    public boolean isImmutable() {
+        return immutable;
+    }
+
+    public void setImmutable(boolean immutable) {
+        this.immutable = immutable;
+    }
 }

core/src/main/java/org/teavm/backend/wasm/model/WasmType.java

@@ -63,12 +63,23 @@ public abstract class WasmType {
         public static final SpecialReference STRUCT = SpecialReferenceKind.STRUCT.asType();
         public static final SpecialReference ARRAY = SpecialReferenceKind.ARRAY.asType();
         public static final SpecialReference I31 = SpecialReferenceKind.I31.asType();
+
+        private final boolean nullable;
+
+        public Reference(boolean nullable) {
+            this.nullable = nullable;
+        }
+
+        public boolean isNullable() {
+            return nullable;
+        }
     }
 
     public static final class CompositeReference extends Reference {
         public final WasmCompositeType composite;
 
-        CompositeReference(WasmCompositeType composite) {
+        CompositeReference(WasmCompositeType composite, boolean nullable) {
+            super(nullable);
             this.composite = composite;
         }
     }
@@ -76,7 +87,8 @@ public abstract class WasmType {
     public static final class SpecialReference extends Reference {
         public final SpecialReferenceKind kind;
 
-        private SpecialReference(SpecialReferenceKind kind) {
+        private SpecialReference(SpecialReferenceKind kind, boolean nullable) {
+            super(nullable);
             this.kind = kind;
         }
     }
@@ -90,12 +102,20 @@ public abstract class WasmType {
         I31;
 
         private SpecialReference type;
+        private SpecialReference nonNullType;
 
-        final SpecialReference asType() {
+        public final SpecialReference asType() {
             if (type == null) {
-                type = new SpecialReference(this);
+                type = new SpecialReference(this, true);
             }
             return type;
         }
+
+        public final SpecialReference asNonNullType() {
+            if (nonNullType == null) {
+                nonNullType = new SpecialReference(this, false);
+            }
+            return nonNullType;
+        }
     }
 }

core/src/main/java/org/teavm/backend/wasm/model/expression/WasmCast.java

@@ -21,16 +21,10 @@ import org.teavm.backend.wasm.model.WasmType;
 public class WasmCast extends WasmExpression {
     private WasmExpression value;
     private WasmType.Reference targetType;
-    private boolean nullable;
 
     public WasmCast(WasmExpression value, WasmType.Reference targetType) {
-        this(value, targetType, true);
-    }
-
-    public WasmCast(WasmExpression value, WasmType.Reference targetType, boolean nullable) {
         this.value = Objects.requireNonNull(value);
         this.targetType = Objects.requireNonNull(targetType);
-        this.nullable = nullable;
     }
 
     public WasmExpression getValue() {
@@ -49,14 +43,6 @@ public class WasmCast extends WasmExpression {
         this.targetType = Objects.requireNonNull(targetType);
     }
 
-    public boolean isNullable() {
-        return nullable;
-    }
-
-    public void setNullable(boolean nullable) {
-        this.nullable = nullable;
-    }
-
     @Override
     public void acceptVisitor(WasmExpressionVisitor visitor) {
         visitor.visit(this);

core/src/main/java/org/teavm/backend/wasm/model/expression/WasmTest.java

@@ -21,16 +21,10 @@ import org.teavm.backend.wasm.model.WasmType;
 public class WasmTest extends WasmExpression {
     private WasmExpression value;
     private WasmType.Reference testType;
-    private boolean nullable;
 
     public WasmTest(WasmExpression value, WasmType.Reference testType) {
-        this(value, testType, true);
-    }
-
-    public WasmTest(WasmExpression value, WasmType.Reference testType, boolean nullable) {
         this.value = Objects.requireNonNull(value);
         this.testType = Objects.requireNonNull(testType);
-        this.nullable = nullable;
     }
 
     public WasmExpression getValue() {
@@ -49,13 +43,6 @@ public class WasmTest extends WasmExpression {
         this.testType = Objects.requireNonNull(testType);
     }
 
-    public boolean isNullable() {
-        return nullable;
-    }
-
-    public void setNullable(boolean nullable) {
-        this.nullable = nullable;
-    }
 
     @Override
     public void acceptVisitor(WasmExpressionVisitor visitor) {

core/src/main/java/org/teavm/backend/wasm/parser/CodeListener.java

@@ -135,10 +135,10 @@ public interface CodeListener {
     default void nullConstant(WasmHollowType.Reference type) {
     }
 
-    default void cast(WasmHollowType.Reference type, boolean nullable) {
+    default void cast(WasmHollowType.Reference type) {
     }
 
-    default void test(WasmHollowType.Reference type, boolean nullable) {
+    default void test(WasmHollowType.Reference type) {
     }
 
     default void structNew(int typeIndex) {

core/src/main/java/org/teavm/backend/wasm/parser/CodeParser.java

@@ -589,7 +589,7 @@ public class CodeParser extends BaseSectionParser {
                 break;
 
             case 0xD0:
-                codeListener.nullConstant(reader.readHeapType());
+                codeListener.nullConstant(reader.readHeapType(true));
                 break;
 
             case 0xD2:
@@ -684,19 +684,19 @@ public class CodeParser extends BaseSectionParser {
                 return true;
 
             case 20:
-                codeListener.test(reader.readHeapType(), false);
+                codeListener.test(reader.readHeapType(false));
                 return true;
 
             case 21:
-                codeListener.test(reader.readHeapType(), true);
+                codeListener.test(reader.readHeapType(true));
                 return true;
 
             case 22:
-                codeListener.cast(reader.readHeapType(), false);
+                codeListener.cast(reader.readHeapType(false));
                 return true;
 
             case 23:
-                codeListener.cast(reader.readHeapType(), true);
+                codeListener.cast(reader.readHeapType(true));
                 return true;
 
             case 28:

core/src/main/java/org/teavm/backend/wasm/parser/WasmBinaryReader.java

@@ -16,6 +16,7 @@
 package org.teavm.backend.wasm.parser;
 
 import java.nio.charset.StandardCharsets;
+import org.teavm.backend.wasm.model.WasmType;
 
 public class WasmBinaryReader {
     private final AddressListener addressListener;
@@ -63,43 +64,51 @@ public class WasmBinaryReader {
             case 0x7C:
                 return WasmHollowType.FLOAT64;
             case 0x63:
-                return readHeapType();
+                return readHeapType(true);
+            case 0x64:
+                return readHeapType(false);
             case 0x40:
                 return null;
             default:
-                return readAbsHeapType(typeId);
+                return readAbsHeapType(typeId, true);
         }
     }
 
-    public WasmHollowType.Reference readHeapType() {
+    public WasmHollowType.Reference readHeapType(boolean nullable) {
         var typeId = data[ptr];
         if ((typeId & 0xC0) == 0x40) {
-            var result = readAbsHeapType(typeId);
+            var result = readAbsHeapType(typeId, nullable);
             ++ptr;
             return result;
         }
-        return new WasmHollowType.CompositeReference(readLEB());
+        return new WasmHollowType.CompositeReference(readLEB(), nullable);
     }
 
-    public WasmHollowType.SpecialReference readAbsHeapType(int typeId) {
+    public WasmHollowType.SpecialReference readAbsHeapType(int typeId, boolean nullable) {
         switch (typeId) {
             case 0x70:
-                return WasmHollowType.Reference.FUNC;
+                return special(WasmType.SpecialReferenceKind.FUNC, nullable);
             case 0x6F:
-                return WasmHollowType.Reference.EXTERN;
+                return special(WasmType.SpecialReferenceKind.EXTERN, nullable);
             case 0x6E:
-                return WasmHollowType.Reference.ANY;
+                return special(WasmType.SpecialReferenceKind.ANY, nullable);
             case 0x6C:
-                return WasmHollowType.Reference.I31;
+                return special(WasmType.SpecialReferenceKind.I31, nullable);
             case 0x6B:
-                return WasmHollowType.Reference.STRUCT;
+                return special(WasmType.SpecialReferenceKind.STRUCT, nullable);
             case 0x6A:
-                return WasmHollowType.Reference.ARRAY;
+                return special(WasmType.SpecialReferenceKind.ARRAY, nullable);
             default:
                 throw new ParseException("Unknown type", ptr);
         }
     }
 
+    private static WasmHollowType.SpecialReference special(WasmType.SpecialReferenceKind kind, boolean nullable) {
+        return nullable
+                ? WasmHollowType.Reference.special(kind)
+                : WasmHollowType.SpecialReference.nonNullSpecial(kind);
+    }
+
     public int readSignedLEB() {
         var result = 0;
         var shift = 0;

core/src/main/java/org/teavm/backend/wasm/parser/WasmHollowType.java

@@ -17,7 +17,6 @@ package org.teavm.backend.wasm.parser;
 
 import java.util.Objects;
 import org.teavm.backend.wasm.model.WasmNumType;
-import org.teavm.backend.wasm.model.WasmStorageType;
 import org.teavm.backend.wasm.model.WasmType;
 
 public class WasmHollowType {
@@ -26,8 +25,6 @@ public class WasmHollowType {
     public static final Number FLOAT32 = new Number(WasmNumType.FLOAT32);
     public static final Number FLOAT64 = new Number(WasmNumType.FLOAT64);
 
-    private WasmStorageType.Regular storageType;
-
     private WasmHollowType() {
     }
 
@@ -56,18 +53,45 @@ public class WasmHollowType {
 
 
     public static abstract class Reference extends WasmHollowType {
-        public static final SpecialReference FUNC = new SpecialReference(WasmType.SpecialReferenceKind.FUNC);
+        private static final SpecialReference[] references = new SpecialReference[
-        public static final SpecialReference ANY = new SpecialReference(WasmType.SpecialReferenceKind.ANY);
+                WasmType.SpecialReferenceKind.values().length];
-        public static final SpecialReference EXTERN = new SpecialReference(WasmType.SpecialReferenceKind.EXTERN);
+        private static final SpecialReference[] nonNullReferences = new SpecialReference[
-        public static final SpecialReference STRUCT = new SpecialReference(WasmType.SpecialReferenceKind.STRUCT);
+                WasmType.SpecialReferenceKind.values().length];
-        public static final SpecialReference ARRAY = new SpecialReference(WasmType.SpecialReferenceKind.ARRAY);
+
-        public static final SpecialReference I31 = new SpecialReference(WasmType.SpecialReferenceKind.I31);
+        private final boolean nullable;
+
+        public Reference(boolean nullable) {
+            this.nullable = nullable;
+        }
+
+        public boolean isNullable() {
+            return nullable;
+        }
+
+        public static SpecialReference special(WasmType.SpecialReferenceKind kind) {
+            var result = references[kind.ordinal()];
+            if (result == null) {
+                result = new SpecialReference(kind, true);
+                references[kind.ordinal()] = result;
+            }
+            return result;
+        }
+
+        public static SpecialReference nonNullSpecial(WasmType.SpecialReferenceKind kind) {
+            var result = nonNullReferences[kind.ordinal()];
+            if (result == null) {
+                result = new SpecialReference(kind, false);
+                nonNullReferences[kind.ordinal()] = result;
+            }
+            return result;
+        }
     }
 
     public static final class CompositeReference extends WasmHollowType.Reference {
         public final int index;
 
-        public CompositeReference(int index) {
+        public CompositeReference(int index, boolean nullable) {
+            super(nullable);
             this.index = index;
         }
 
@@ -80,19 +104,20 @@ public class WasmHollowType {
                 return false;
             }
             CompositeReference that = (CompositeReference) o;
-            return index == that.index;
+            return index == that.index && isNullable() == that.isNullable();
         }
 
         @Override
         public int hashCode() {
-            return Objects.hashCode(index);
+            return Objects.hash(index, isNullable());
         }
     }
 
     public static final class SpecialReference extends WasmHollowType.Reference {
         public final WasmType.SpecialReferenceKind kind;
 
-        SpecialReference(WasmType.SpecialReferenceKind kind) {
+        SpecialReference(WasmType.SpecialReferenceKind kind, boolean nullable) {
+            super(nullable);
             this.kind = kind;
         }
     }

core/src/main/java/org/teavm/backend/wasm/render/WasmBinaryRenderer.java

@@ -219,7 +219,7 @@ public class WasmBinaryRenderer {
         section.writeLEB(module.globals.size());
         for (var global : module.globals) {
             section.writeType(global.getType(), module);
-            section.writeByte(1); // mutable
+            section.writeByte(global.isImmutable() ? 0 : 1); // mutable
             global.getInitialValue().acceptVisitor(visitor);
             section.writeByte(0x0b);
         }

core/src/main/java/org/teavm/backend/wasm/render/WasmBinaryRenderingVisitor.java

@@ -1075,7 +1075,7 @@ class WasmBinaryRenderingVisitor implements WasmExpressionVisitor {
         pushLocation(expression);
         expression.getValue().acceptVisitor(this);
         writer.writeByte(0xfb);
-        writer.writeByte(expression.isNullable() ? 23 : 22);
+        writer.writeByte(expression.getTargetType().isNullable() ? 23 : 22);
         writer.writeHeapType(expression.getTargetType(), module);
         popLocation();
     }
@@ -1085,7 +1085,7 @@ class WasmBinaryRenderingVisitor implements WasmExpressionVisitor {
         pushLocation(expression);
         expression.getValue().acceptVisitor(this);
         writer.writeByte(0xfb);
-        writer.writeByte(expression.isNullable() ? 21 : 20);
+        writer.writeByte(expression.getTestType().isNullable() ? 21 : 20);
         writer.writeHeapType(expression.getTestType(), module);
         popLocation();
     }

core/src/main/java/org/teavm/backend/wasm/render/WasmBinaryWriter.java

@@ -37,11 +37,15 @@ public class WasmBinaryWriter {
         if (type instanceof WasmType.Number) {
             writeType(((WasmType.Number) type).number);
         } else if (type instanceof WasmType.SpecialReference) {
-            writeSpecialHeapType(((WasmType.SpecialReference) type).kind);
+            var refType = (WasmType.SpecialReference) type;
+            if (!refType.isNullable()) {
+                writeByte(0x64);
+            }
+            writeSpecialHeapType(refType.kind);
         } else if (type instanceof WasmType.CompositeReference) {
-            writeByte(0x63);
+            var refType = (WasmType.CompositeReference) type;
-            var composite = ((WasmType.CompositeReference) type).composite;
+            writeByte(refType.isNullable() ? 0x63 : 0x64);
-            var index = module.types.indexOf(composite);
+            var index = module.types.indexOf(refType.composite);
             writeSignedLEB(index);
         }
     }