C: support weak references

classlib/src/main/java/org/teavm/classlib/java/lang/ref/TReference.java

@@ -17,11 +17,6 @@ package org.teavm.classlib.java.lang.ref;
 
 import org.teavm.classlib.java.lang.TObject;
 
-/**
- *
- * @author Alexey Andreev
- * @param <T> type of an object to which this reference points.
- */
 public abstract class TReference<T> extends TObject {
     public T get() {
         return null;

classlib/src/main/java/org/teavm/classlib/java/lang/ref/TReferenceQueue.java

@@ -15,11 +15,6 @@
  */
 package org.teavm.classlib.java.lang.ref;
 
-/**
- *
- * @author Alexey Andreev
- * @param <T>
- */
 public class TReferenceQueue<T> {
     public TReference<T> poll() {
         return null;

classlib/src/main/java/org/teavm/classlib/java/lang/ref/TWeakReference.java

@@ -35,4 +35,14 @@ public class TWeakReference<T> extends TReference<T> {
     public void clear() {
         value = null;
     }
+
+    @Override
+    public boolean isEnqueued() {
+        return false;
+    }
+
+    @Override
+    public boolean enqueue() {
+        return true;
+    }
 }

core/src/main/java/org/teavm/backend/c/CTarget.java

@@ -51,6 +51,8 @@ import org.teavm.backend.c.generate.StringPoolGenerator;
 import org.teavm.backend.c.generators.ArrayGenerator;
 import org.teavm.backend.c.generators.Generator;
 import org.teavm.backend.c.generators.GeneratorFactory;
+import org.teavm.backend.c.generators.ReferenceQueueGenerator;
+import org.teavm.backend.c.generators.WeakReferenceGenerator;
 import org.teavm.backend.c.intrinsic.AddressIntrinsic;
 import org.teavm.backend.c.intrinsic.AllocatorIntrinsic;
 import org.teavm.backend.c.intrinsic.ConsoleIntrinsic;
@@ -72,7 +74,9 @@ import org.teavm.backend.c.intrinsic.ShadowStackIntrinsic;
 import org.teavm.backend.c.intrinsic.StringsIntrinsic;
 import org.teavm.backend.c.intrinsic.StructureIntrinsic;
 import org.teavm.backend.lowlevel.dependency.ExceptionHandlingDependencyListener;
+import org.teavm.backend.lowlevel.dependency.WeakReferenceDependencyListener;
 import org.teavm.backend.lowlevel.transform.CoroutineTransformation;
+import org.teavm.backend.lowlevel.transform.WeakReferenceTransformation;
 import org.teavm.dependency.ClassDependency;
 import org.teavm.dependency.DependencyAnalyzer;
 import org.teavm.dependency.DependencyListener;
@@ -163,12 +167,14 @@ public class CTarget implements TeaVMTarget, TeaVMCHost {
         List<ClassHolderTransformer> transformers = new ArrayList<>();
         transformers.add(new ClassPatch());
         transformers.add(new CDependencyListener());
+        transformers.add(new WeakReferenceTransformation());
         return transformers;
     }
 
     @Override
     public List<DependencyListener> getDependencyListeners() {
-        return Arrays.asList(new CDependencyListener(), exportDependencyListener, new InteropDependencyListener());
+        return Arrays.asList(new CDependencyListener(), exportDependencyListener, new InteropDependencyListener(),
+                new WeakReferenceDependencyListener());
     }
 
     @Override
@@ -323,6 +329,8 @@ public class CTarget implements TeaVMTarget, TeaVMCHost {
 
         List<Generator> generators = new ArrayList<>();
         generators.add(new ArrayGenerator());
+        generators.add(new WeakReferenceGenerator());
+        generators.add(new ReferenceQueueGenerator());
 
         stringPool = new SimpleStringPool();
         GenerationContext context = new GenerationContext(vtableProvider, characteristics,
@@ -355,10 +363,8 @@ public class CTarget implements TeaVMTarget, TeaVMCHost {
         generateSpecialFunctions(context, runtimeWriter);
         OutputFileUtil.write(runtimeWriter, "runtime.c", buildTarget);
         OutputFileUtil.write(runtimeHeaderWriter, "runtime.h", buildTarget);
-        BufferedCodeWriter stringhashWriter = new BufferedCodeWriter(false);
+        copyResource("stringhash.c", buildTarget);
-        emitResource(stringhashWriter, "stringhash.c");
+        copyResource("references.c", buildTarget);
-        OutputFileUtil.write(stringhashWriter, "stringhash.c", buildTarget);
-
         generateCallSites(buildTarget, context, classes.getClassNames());
         generateStrings(buildTarget, context);
 
@@ -369,6 +375,12 @@ public class CTarget implements TeaVMTarget, TeaVMCHost {
         generateAllFile(classes, types, buildTarget);
     }
 
+    private void copyResource(String name, BuildTarget buildTarget) throws IOException {
+        BufferedCodeWriter writer = new BufferedCodeWriter(false);
+        emitResource(writer, name);
+        OutputFileUtil.write(writer, name, buildTarget);
+    }
+
     private void emitResource(CodeWriter writer, String resourceName) {
         ClassLoader classLoader = CTarget.class.getClassLoader();
         try (BufferedReader reader = new BufferedReader(new InputStreamReader(

core/src/main/java/org/teavm/backend/c/generate/ClassGenerator.java

@@ -16,6 +16,8 @@
 package org.teavm.backend.c.generate;
 
 import java.io.IOException;
+import java.lang.ref.ReferenceQueue;
+import java.lang.ref.WeakReference;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
@@ -64,11 +66,15 @@ import org.teavm.runtime.CallSite;
 import org.teavm.runtime.RuntimeArray;
 import org.teavm.runtime.RuntimeClass;
 import org.teavm.runtime.RuntimeObject;
+import org.teavm.runtime.RuntimeReference;
+import org.teavm.runtime.RuntimeReferenceQueue;
 
 public class ClassGenerator {
     private static final Set<String> classesWithDeclaredStructures = new HashSet<>(Arrays.asList(
             "java.lang.Object", "java.lang.String", "java.lang.Class",
-            RuntimeArray.class.getName(), RuntimeClass.class.getName(), RuntimeObject.class.getName()
+            RuntimeArray.class.getName(), RuntimeClass.class.getName(), RuntimeObject.class.getName(),
+            WeakReference.class.getName(), ReferenceQueue.class.getName(),
+            RuntimeReferenceQueue.class.getName(), RuntimeReference.class.getName()
     ));
 
     private GenerationContext context;
@@ -566,6 +572,15 @@ public class ClassGenerator {
                 superinterfaces = sb.append(" }").toString();
             }
 
+            switch (className) {
+                case "java.lang.ref.WeakReference":
+                    flags |= RuntimeClass.VM_TYPE_WEAKREFERENCE << RuntimeClass.VM_TYPE_SHIFT;
+                    break;
+                case "java.lang.ref.ReferenceQueue":
+                    flags |= RuntimeClass.VM_TYPE_REFERENCEQUEUE << RuntimeClass.VM_TYPE_SHIFT;
+                    break;
+            }
+
         } else if (type instanceof ValueType.Array) {
             includes.includeClass("java.lang.Object");
             parent = "(TeaVM_Class*) &" + context.getNames().forClassInstance(ValueType.object("java.lang.Object"));

core/src/main/java/org/teavm/backend/c/generate/NameProvider.java

@@ -15,6 +15,8 @@
  */
 package org.teavm.backend.c.generate;
 
+import java.lang.ref.ReferenceQueue;
+import java.lang.ref.WeakReference;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashSet;
@@ -25,6 +27,8 @@ import org.teavm.model.FieldReference;
 import org.teavm.runtime.RuntimeArray;
 import org.teavm.runtime.RuntimeClass;
 import org.teavm.runtime.RuntimeObject;
+import org.teavm.runtime.RuntimeReference;
+import org.teavm.runtime.RuntimeReferenceQueue;
 
 public class NameProvider extends LowLevelNameProvider {
     private static final Set<? extends String> keywords = Collections.unmodifiableSet(new HashSet<>(Arrays.asList(
@@ -41,12 +45,18 @@ public class NameProvider extends LowLevelNameProvider {
         occupiedTopLevelNames.add("TeaVM_Array");
         occupiedTopLevelNames.add("TeaVM_String");
         occupiedTopLevelNames.add("TeaVM_Class");
+        occupiedTopLevelNames.add("TeaVM_Reference");
+        occupiedTopLevelNames.add("TeaVM_ReferenceQueue");
 
         classNames.put(RuntimeObject.class.getName(), "TeaVM_Object");
         classNames.put(Object.class.getName(), "TeaVM_Object");
         classNames.put(String.class.getName(), "TeaVM_String");
         classNames.put(RuntimeClass.class.getName(), "TeaVM_Class");
         classNames.put(RuntimeArray.class.getName(), "TeaVM_Array");
+        classNames.put(WeakReference.class.getName(), "TeaVM_Reference");
+        classNames.put(ReferenceQueue.class.getName(), "TeaVM_ReferenceQueue");
+        classNames.put(RuntimeReference.class.getName(), "TeaVM_Reference");
+        classNames.put(RuntimeReferenceQueue.class.getName(), "TeaVM_ReferenceQueue");
 
         memberFieldNames.put(new FieldReference(RuntimeObject.class.getName(), "classReference"), "header");
         memberFieldNames.put(new FieldReference(RuntimeObject.class.getName(), "hashCode"), "hash");
@@ -54,17 +64,23 @@ public class NameProvider extends LowLevelNameProvider {
         memberFieldNames.put(new FieldReference(String.class.getName(), "characters"), "characters");
         memberFieldNames.put(new FieldReference(String.class.getName(), "hashCode"), "hashCode");
 
-        for (String name : new String[] { "size", "flags", "tag", "canary", "name", "itemType", "arrayType",
+        preserveFieldNames(RuntimeClass.class.getName(), "size", "flags", "tag", "canary", "name", "itemType",
-                "isSupertypeOf", "init", "enumValues", "layout", "simpleName", "superinterfaceCount",
+                "arrayType", "isSupertypeOf", "init", "enumValues", "layout", "simpleName", "superinterfaceCount",
-                "superinterfaces" }) {
+                "superinterfaces");
-            memberFieldNames.put(new FieldReference(RuntimeClass.class.getName(), name), name);
-        }
         memberFieldNames.put(new FieldReference(RuntimeClass.class.getName(), "parent"), "superclass");
+        preserveFieldNames(RuntimeReference.class.getName(), "queue", "object", "next");
+        preserveFieldNames(RuntimeReferenceQueue.class.getName(), "first", "last");
 
         occupiedClassNames.put(RuntimeObject.class.getName(), new HashSet<>(Arrays.asList("header")));
         occupiedClassNames.put(RuntimeArray.class.getName(), new HashSet<>(Arrays.asList("length")));
     }
 
+    private void preserveFieldNames(String className, String... fieldNames) {
+        for (String name : fieldNames) {
+            memberFieldNames.put(new FieldReference(className, name), name);
+        }
+    }
+
     @Override
     protected Set<? extends String> getKeywords() {
         return keywords;

core/src/main/java/org/teavm/backend/c/generators/ReferenceQueueGenerator.java

@@ -0,0 +1,47 @@
+/*
+ *  Copyright 2019 Alexey Andreev.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package org.teavm.backend.c.generators;
+
+import java.lang.ref.ReferenceQueue;
+import org.teavm.model.MethodReference;
+
+public class ReferenceQueueGenerator implements Generator {
+    private static final MethodReference OBJECT_INIT = new MethodReference(
+            Object.class, "<init>", void.class);
+
+    @Override
+    public boolean canHandle(MethodReference method) {
+        return method.getClassName().equals(ReferenceQueue.class.getName());
+    }
+
+    @Override
+    public void generate(GeneratorContext context, MethodReference method) {
+        switch (method.getName()) {
+            case "<init>":
+                context.includes().includeClass(OBJECT_INIT.getClassName());
+                context.writer().print(context.names().forMethod(OBJECT_INIT)).print("(")
+                        .print(context.parameterName(0))
+                        .println(");");
+                break;
+
+            case "poll":
+                context.writer().print("return teavm_reference_poll(");
+                context.writer().print("(TeaVM_ReferenceQueue*) ").print(context.parameterName(0));
+                context.writer().println(");");
+                break;
+        }
+    }
+}

core/src/main/java/org/teavm/backend/c/generators/WeakReferenceGenerator.java

@@ -0,0 +1,74 @@
+/*
+ *  Copyright 2019 Alexey Andreev.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package org.teavm.backend.c.generators;
+
+import java.lang.ref.Reference;
+import java.lang.ref.WeakReference;
+import org.teavm.model.MethodReference;
+
+public class WeakReferenceGenerator implements Generator {
+    private static final MethodReference REFERENCE_INIT = new MethodReference(Reference.class, "<init>", void.class);
+    @Override
+    public boolean canHandle(MethodReference method) {
+        return method.getClassName().equals(WeakReference.class.getName());
+    }
+
+    @Override
+    public void generate(GeneratorContext context, MethodReference method) {
+        switch (method.getName()) {
+            case "<init>":
+                context.includes().includeClass(REFERENCE_INIT.getClassName());
+                context.writer().print(context.names().forMethod(REFERENCE_INIT)).print("(")
+                        .print(context.parameterName(0))
+                        .println(");");
+
+                context.writer().print("teavm_reference_init(");
+                context.writer().print("(TeaVM_Reference*) ").print(context.parameterName(0)).print(", ");
+                context.writer().print(context.parameterName(1)).print(", ");
+                if (method.parameterCount() == 2) {
+                    context.writer().print(context.parameterName(2));
+                } else {
+                    context.writer().print("NULL");
+                }
+                context.writer().println(");");
+                break;
+
+            case "get":
+                context.writer().print("return teavm_reference_get(");
+                context.writer().print("(TeaVM_Reference*) ").print(context.parameterName(0));
+                context.writer().println(");");
+                break;
+
+            case "clear":
+                context.writer().print("teavm_reference_clear(");
+                context.writer().print("(TeaVM_Reference*) ").print(context.parameterName(0));
+                context.writer().println(");");
+                break;
+
+            case "isEnqueued":
+                context.writer().print("return teavm_reference_isEnqueued(");
+                context.writer().print("(TeaVM_Reference*) ").print(context.parameterName(0));
+                context.writer().println(");");
+                break;
+
+            case "enqueue":
+                context.writer().print("return teavm_reference_enqueue(");
+                context.writer().print("(TeaVM_Reference*) ").print(context.parameterName(0));
+                context.writer().println(");");
+                break;
+        }
+    }
+}

core/src/main/java/org/teavm/backend/lowlevel/dependency/WeakReferenceDependencyListener.java

@@ -0,0 +1,84 @@
+/*
+ *  Copyright 2019 Alexey Andreev.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package org.teavm.backend.lowlevel.dependency;
+
+import java.lang.ref.Reference;
+import java.lang.ref.ReferenceQueue;
+import java.lang.ref.WeakReference;
+import org.teavm.dependency.AbstractDependencyListener;
+import org.teavm.dependency.DependencyAgent;
+import org.teavm.dependency.DependencyNode;
+import org.teavm.dependency.MethodDependency;
+import org.teavm.model.MethodReference;
+
+public class WeakReferenceDependencyListener extends AbstractDependencyListener {
+    private DependencyNode referentNode;
+    private DependencyNode referenceNode;
+
+    @Override
+    public void started(DependencyAgent agent) {
+        referentNode = agent.createNode();
+        referenceNode = agent.createNode();
+    }
+
+    @Override
+    public void methodReached(DependencyAgent agent, MethodDependency method) {
+        MethodReference methodRef = method.getReference();
+        if (methodRef.getClassName().equals(WeakReference.class.getName())) {
+            switch (methodRef.getName()) {
+                case "<init>":
+                    reachReferenceInit(agent, method);
+                    break;
+                case "get":
+                    reachReferenceGet(method);
+                    break;
+            }
+        } else if (methodRef.getClassName().equals(ReferenceQueue.class.getName())) {
+            switch (methodRef.getName()) {
+                case "<init>":
+                    reachQueueInit(agent, method);
+                    break;
+                case "poll":
+                    reachQueuePoll(method);
+                    break;
+            }
+        }
+        super.methodReached(agent, method);
+    }
+
+    private void reachReferenceInit(DependencyAgent agent, MethodDependency method) {
+        MethodDependency superMethod = agent.linkMethod(new MethodReference(Reference.class, "<init>", void.class));
+        method.getVariable(0).connect(superMethod.getVariable(0));
+        superMethod.use();
+
+        method.getVariable(0).connect(referenceNode);
+        method.getVariable(1).connect(referentNode);
+    }
+
+    private void reachReferenceGet(MethodDependency method) {
+        referentNode.connect(method.getResult());
+    }
+
+    private void reachQueuePoll(MethodDependency method) {
+        referenceNode.connect(method.getResult());
+    }
+
+    private void reachQueueInit(DependencyAgent agent, MethodDependency method) {
+        MethodDependency superMethod = agent.linkMethod(new MethodReference(Object.class, "<init>", void.class));
+        method.getVariable(0).connect(superMethod.getVariable(0));
+        superMethod.use();
+    }
+}

core/src/main/java/org/teavm/backend/lowlevel/transform/WeakReferenceTransformation.java

@@ -0,0 +1,44 @@
+/*
+ *  Copyright 2019 Alexey Andreev.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package org.teavm.backend.lowlevel.transform;
+
+import java.lang.ref.ReferenceQueue;
+import java.lang.ref.WeakReference;
+import org.teavm.model.ClassHolder;
+import org.teavm.model.ClassHolderTransformer;
+import org.teavm.model.ClassHolderTransformerContext;
+import org.teavm.model.ElementModifier;
+import org.teavm.model.FieldHolder;
+import org.teavm.model.MethodHolder;
+
+public class WeakReferenceTransformation implements ClassHolderTransformer {
+    @Override
+    public void transformClass(ClassHolder cls, ClassHolderTransformerContext context) {
+        if (!cls.getName().equals(WeakReference.class.getName())
+                && !cls.getName().equals(ReferenceQueue.class.getName())) {
+            return;
+        }
+
+        for (FieldHolder field : cls.getFields().toArray(new FieldHolder[0])) {
+            cls.removeField(field);
+        }
+
+        for (MethodHolder method : cls.getMethods()) {
+            method.setProgram(null);
+            method.getModifiers().add(ElementModifier.NATIVE);
+        }
+    }
+}

core/src/main/java/org/teavm/runtime/GC.java

@@ -32,6 +32,7 @@ public final class GC {
     static FreeChunkHolder currentChunkPointer;
     static int freeChunks;
     static int freeMemory = (int) availableBytes();
+    static RuntimeReference firstWeakReference;
 
     static native Address gcStorageAddress();
 
@@ -122,12 +123,14 @@ public final class GC {
 
     public static boolean collectGarbage(int size) {
         mark();
+        processReferences();
         sweep();
         updateFreeMemory();
         return true;
     }
 
     private static void mark() {
+        firstWeakReference = null;
         Allocator.fillZero(regionsAddress().toAddress(), regionMaxCount() * Structure.sizeOf(Region.class));
 
         Address staticRoots = Mutator.getStaticGCRoots();
@@ -176,7 +179,55 @@ public final class GC {
 
             RuntimeClass cls = RuntimeClass.getClass(object);
             if (cls.itemType == null) {
+                markObject(cls, object);
+            } else {
+                markArray(cls, (RuntimeArray) object);
+            }
+        }
+    }
+
+    private static void markObject(RuntimeClass cls, RuntimeObject object) {
         while (cls != null) {
+            int type = (cls.flags >> RuntimeClass.VM_TYPE_SHIFT) & RuntimeClass.VM_TYPE_MASK;
+            switch (type) {
+                case RuntimeClass.VM_TYPE_WEAKREFERENCE:
+                    markWeakReference((RuntimeReference) object);
+                    break;
+
+                case RuntimeClass.VM_TYPE_REFERENCEQUEUE:
+                    markReferenceQueue((RuntimeReferenceQueue) object);
+                    break;
+
+                default:
+                    markFields(cls, object);
+                    break;
+            }
+            cls = cls.parent;
+        }
+    }
+
+    private static void markWeakReference(RuntimeReference object) {
+        if (object.queue != null) {
+            mark(object.queue);
+            if (object.next != null && object.object != null) {
+                mark(object.object);
+            }
+        }
+        if (object.next == null && object.object != null) {
+            object.next = firstWeakReference;
+            firstWeakReference = object;
+        }
+    }
+
+    private static void markReferenceQueue(RuntimeReferenceQueue object) {
+        RuntimeReference reference = object.first;
+        while (reference != null) {
+            mark(reference);
+            reference = reference.next;
+        }
+    }
+
+    private static void markFields(RuntimeClass cls, RuntimeObject object) {
         Address layout = cls.layout;
         if (layout != null) {
             short fieldCount = layout.getShort();
@@ -189,11 +240,12 @@ public final class GC {
                 }
             }
         }
-                    cls = cls.parent;
     }
-            } else {
+
-                if ((cls.itemType.flags & RuntimeClass.PRIMITIVE) == 0) {
+    private static void markArray(RuntimeClass cls, RuntimeArray array) {
-                    RuntimeArray array = (RuntimeArray) object;
+        if ((cls.itemType.flags & RuntimeClass.PRIMITIVE) != 0) {
+            return;
+        }
         Address base = Address.align(array.toAddress().add(RuntimeArray.class, 1), Address.sizeOf());
         for (int i = 0; i < array.size; ++i) {
             RuntimeObject reference = base.getAddress().toStructure();
@@ -203,7 +255,25 @@ public final class GC {
             base = base.add(Address.sizeOf());
         }
     }
+
+    private static void processReferences() {
+        RuntimeReference reference = firstWeakReference;
+        while (reference != null) {
+            RuntimeReference next = reference.next;
+            reference.next = null;
+            if ((reference.object.classReference & RuntimeObject.GC_MARKED) == 0) {
+                reference.object = null;
+                RuntimeReferenceQueue queue = reference.queue;
+                if (queue != null) {
+                    if (queue.first == null) {
+                        queue.first = reference;
+                    } else {
+                        queue.last.next = reference;
                     }
+                    queue.last = reference;
+                }
+            }
+            reference = next;
         }
     }
 

core/src/main/java/org/teavm/runtime/RuntimeClass.java

@@ -25,6 +25,9 @@ public class RuntimeClass extends RuntimeObject {
 
     public static final int PRIMITIVE_SHIFT = 3;
     public static final int PRIMITIVE_MASK = 15;
+    public static final int VM_TYPE_SHIFT = 7;
+    public static final int VM_TYPE_MASK = 7;
+
     public static final int BOOLEAN_PRIMITIVE = 0;
     public static final int BYTE_PRIMITIVE = 1;
     public static final int SHORT_PRIMITIVE = 2;
@@ -35,6 +38,10 @@ public class RuntimeClass extends RuntimeObject {
     public static final int DOUBLE_PRIMITIVE = 7;
     public static final int VOID_PRIMITIVE = 8;
 
+    public static final int VM_TYPE_REGULAR = 0;
+    public static final int VM_TYPE_WEAKREFERENCE = 1;
+    public static final int VM_TYPE_REFERENCEQUEUE = 2;
+
     public int size;
     public int flags;
     public int tag;

core/src/main/java/org/teavm/runtime/RuntimeReference.java

@@ -0,0 +1,22 @@
+/*
+ *  Copyright 2019 Alexey Andreev.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package org.teavm.runtime;
+
+public class RuntimeReference extends RuntimeObject {
+    public RuntimeReferenceQueue queue;
+    public RuntimeObject object;
+    public RuntimeReference next;
+}

core/src/main/java/org/teavm/runtime/RuntimeReferenceQueue.java

@@ -0,0 +1,21 @@
+/*
+ *  Copyright 2019 Alexey Andreev.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package org.teavm.runtime;
+
+public class RuntimeReferenceQueue extends RuntimeObject {
+    public RuntimeReference first;
+    public RuntimeReference last;
+}

core/src/main/resources/org/teavm/backend/c/references.c

@@ -0,0 +1,47 @@
+#include "runtime.h"
+
+int32_t teavm_reference_enqueue(TeaVM_Reference* reference) {
+    TeaVM_ReferenceQueue* queue = reference->queue;
+    if (queue == NULL || reference->next != NULL) {
+        return INT32_C(0);
+    }
+
+    if (queue->last == NULL) {
+        queue->first = reference;
+    } else {
+        queue->last->next = reference;
+    }
+    queue->last = reference;
+
+    return INT32_C(1);
+}
+
+int32_t teavm_reference_isEnqueued(TeaVM_Reference* reference) {
+    return reference->queue != NULL && reference->next != NULL ? INT32_C(1) : INT32_C(0);
+}
+
+void teavm_reference_clear(TeaVM_Reference* reference) {
+    reference->object = NULL;
+}
+
+TeaVM_Object* teavm_reference_get(TeaVM_Reference* reference) {
+    return reference->object;
+}
+
+TeaVM_Reference* teavm_reference_poll(TeaVM_ReferenceQueue* queue) {
+    if (queue->first == NULL) {
+        return NULL;
+    }
+
+    TeaVM_Reference* reference = queue->first;
+    queue->first = reference->next;
+    if (queue->first == NULL) {
+        queue->last = NULL;
+    }
+    return reference;
+}
+
+void teavm_reference_init(TeaVM_Reference* reference, TeaVM_Object* object, TeaVM_ReferenceQueue* queue) {
+    reference->object = object;
+    reference->queue = queue;
+}

core/src/main/resources/org/teavm/backend/c/runtime.h

@@ -249,3 +249,30 @@ extern TeaVM_String* teavm_registerString(TeaVM_String*);
 static inline TeaVM_Object* teavm_dereferenceNullable(TeaVM_Object** o) {
     return o != NULL ? *o : NULL;
 }
+
+struct TeaVM_ReferenceQueue;
+
+typedef struct TeaVM_Reference {
+    TeaVM_Object parent;
+    struct TeaVM_ReferenceQueue* queue;
+    TeaVM_Object* object;
+    struct TeaVM_Reference* next;
+} TeaVM_Reference;
+
+typedef struct TeaVM_ReferenceQueue {
+    TeaVM_Object parent;
+    TeaVM_Reference* first;
+    TeaVM_Reference* last;
+} TeaVM_ReferenceQueue;
+
+extern int32_t teavm_reference_enqueue(TeaVM_Reference*);
+
+extern int32_t teavm_reference_isEnqueued(TeaVM_Reference*);
+
+extern void teavm_reference_clear(TeaVM_Reference*);
+
+extern TeaVM_Object* teavm_reference_get(TeaVM_Reference*);
+
+extern TeaVM_Reference* teavm_reference_poll(TeaVM_ReferenceQueue*);
+
+extern void teavm_reference_init(TeaVM_Reference*, TeaVM_Object*, TeaVM_ReferenceQueue*);