diff --git a/src/de/inetsoftware/jwebassembly/module/DupThis.java b/src/de/inetsoftware/jwebassembly/module/DupThis.java
index d4f5cec..6a0bb8b 100644
--- a/src/de/inetsoftware/jwebassembly/module/DupThis.java
+++ b/src/de/inetsoftware/jwebassembly/module/DupThis.java
@@ -1,5 +1,5 @@
 /*
-   Copyright 2019 Volker Berlin (i-net software)
+   Copyright 2019 - 2021 Volker Berlin (i-net software)
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -82,4 +82,11 @@ class DupThis extends WasmInstruction {
         return 0;
     }
 
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    AnyType[] getPopValueTypes() {
+        return null;
+    }
 }
diff --git a/src/de/inetsoftware/jwebassembly/module/JavaMethodWasmCodeBuilder.java b/src/de/inetsoftware/jwebassembly/module/JavaMethodWasmCodeBuilder.java
index d090018..b48a5c5 100644
--- a/src/de/inetsoftware/jwebassembly/module/JavaMethodWasmCodeBuilder.java
+++ b/src/de/inetsoftware/jwebassembly/module/JavaMethodWasmCodeBuilder.java
@@ -16,6 +16,7 @@
 package de.inetsoftware.jwebassembly.module;
 
 import java.io.IOException;
+import java.util.List;
 
 import javax.annotation.Nonnull;
 
@@ -29,6 +30,8 @@ import de.inetsoftware.classparser.ConstantPool;
 import de.inetsoftware.classparser.ConstantRef;
 import de.inetsoftware.classparser.MethodInfo;
 import de.inetsoftware.jwebassembly.WasmException;
+import de.inetsoftware.jwebassembly.module.TypeManager.StructType;
+import de.inetsoftware.jwebassembly.module.WasmInstruction.Type;
 import de.inetsoftware.jwebassembly.wasm.AnyType;
 import de.inetsoftware.jwebassembly.wasm.ArrayOperator;
 import de.inetsoftware.jwebassembly.wasm.ArrayType;
@@ -36,6 +39,7 @@ import de.inetsoftware.jwebassembly.wasm.NamedStorageType;
 import de.inetsoftware.jwebassembly.wasm.NumericOperator;
 import de.inetsoftware.jwebassembly.wasm.StructOperator;
 import de.inetsoftware.jwebassembly.wasm.ValueType;
+import de.inetsoftware.jwebassembly.wasm.ValueTypeParser;
 import de.inetsoftware.jwebassembly.wasm.WasmBlockOperator;
 
 /**
@@ -73,8 +77,8 @@ class JavaMethodWasmCodeBuilder extends WasmCodeBuilder {
             branchManager.reset( code );
 
             byteCode = code.getByteCode();
-            boolean hasReturn = !method.getType().endsWith( ")V" );
-            writeCode( byteCode, code.getConstantPool(), method.getDeclaringClassFile(), hasReturn );
+            AnyType returnType = new ValueTypeParser( method.getType().substring( method.getType().lastIndexOf( ')' ) + 1), getTypeManager() ).next();
+            writeCode( byteCode, code.getConstantPool(), method.getDeclaringClassFile(), returnType );
             calculateVariables();
         } catch( Exception ioex ) {
             int lineNumber = byteCode == null ? -1 : byteCode.getLineNumber();
@@ -91,12 +95,13 @@ class JavaMethodWasmCodeBuilder extends WasmCodeBuilder {
      *            the constant pool of the the current class
      * @param classFile
      *            the declaring class file
-     * @param hasReturn
-     *            if the method has a return value
+     * @param returnType
+     *            the return type of the method
      * @throws WasmException
      *             if some Java code can't converted
      */
-    private void writeCode( CodeInputStream byteCode, ConstantPool constantPool, ClassFile classFile, boolean hasReturn ) throws WasmException {
+    private void writeCode( CodeInputStream byteCode, ConstantPool constantPool, ClassFile classFile, AnyType returnType ) throws WasmException {
+        boolean nullConstants = false;
         int lineNumber = -1;
         try {
             boolean wide = false;
@@ -109,6 +114,7 @@ class JavaMethodWasmCodeBuilder extends WasmCodeBuilder {
                         break;
                     case 1: // aconst_null
                         addStructInstruction( StructOperator.NULL, "java/lang/Object", null, codePos, lineNumber );
+                        nullConstants = true;
                         break;
                     case 2: // iconst_m1
                     case 3: // iconst_0
@@ -580,7 +586,7 @@ class JavaMethodWasmCodeBuilder extends WasmCodeBuilder {
                                 type = ValueType.f64;
                                 break;
                             case 176: // areturn
-                                type = ValueType.externref;
+                                type = getOptions().useGC() ? returnType : ValueType.externref;
                                 break;
                         }
                         addBlockInstruction( WasmBlockOperator.RETURN, type, codePos, lineNumber );
@@ -723,12 +729,17 @@ class JavaMethodWasmCodeBuilder extends WasmCodeBuilder {
             }
             branchManager.calculate();
             branchManager.handle( byteCode ); // add branch operations
-            if( hasReturn && !isEndsWithReturn() ) {
+            if( returnType != null && !isEndsWithReturn() ) {
                 // if a method ends with a loop or block without a break then code after the loop is no reachable
                 // Java does not need a return byte code in this case
                 // But WebAssembly need the dead code to validate
                 addBlockInstruction( WasmBlockOperator.UNREACHABLE, null, byteCode.getCodePosition(), byteCode.getLineNumber() );
             }
+
+            if( nullConstants && getOptions().useGC() ) {
+                patchTypeOfNullConst();
+            }
+
         } catch( Exception ex ) {
             throw WasmException.create( ex, lineNumber );
         }
@@ -925,4 +936,33 @@ class JavaMethodWasmCodeBuilder extends WasmCodeBuilder {
         branchManager.addIfOperator( codePos, offset, byteCode.getLineNumber(), compare );
     }
 
+    /**
+     * NULL const has no type in Java. In WebAssembly currently.
+     */
+    private void patchTypeOfNullConst() {
+        List<WasmInstruction> instructions = getInstructions();
+        int size = instructions.size();
+        for( int i = 0; i < size; i++ ) {
+            WasmInstruction instr = instructions.get( i );
+            if( instr.getType() == Type.Struct ) {
+                WasmStructInstruction structInst = (WasmStructInstruction)instr;
+                if( structInst.getOperator() == StructOperator.NULL ) {
+                    int count = 0;
+                    for( int s = i + 1; s < size; s++ ) {
+                        WasmInstruction nextInstr = instructions.get( s );
+                        count -= nextInstr.getPopCount();
+                        if( count < 0 ) {
+                            AnyType[] popValueTypes = nextInstr.getPopValueTypes();
+                            structInst.setStructType( (StructType)popValueTypes[-1 - count] );
+                            break;
+                        }
+                        if( nextInstr.getPushValueType() != null ) {
+                            count++;
+                        }
+                    }
+                }
+            }
+        }
+    }
+
 }
diff --git a/src/de/inetsoftware/jwebassembly/module/JumpInstruction.java b/src/de/inetsoftware/jwebassembly/module/JumpInstruction.java
index 1b1574d..90c23a4 100644
--- a/src/de/inetsoftware/jwebassembly/module/JumpInstruction.java
+++ b/src/de/inetsoftware/jwebassembly/module/JumpInstruction.java
@@ -1,5 +1,5 @@
 /*
-   Copyright 2020 Volker Berlin (i-net software)
+   Copyright 2020 - 2021 Volker Berlin (i-net software)
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -95,4 +95,13 @@ class JumpInstruction extends WasmInstruction {
     int getJumpPosition() {
         return jumpPos;
     }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    AnyType[] getPopValueTypes() {
+        // TODO
+        return null;
+    }
 }
diff --git a/src/de/inetsoftware/jwebassembly/module/WasmArrayInstruction.java b/src/de/inetsoftware/jwebassembly/module/WasmArrayInstruction.java
index d7d312c..7adb76a 100644
--- a/src/de/inetsoftware/jwebassembly/module/WasmArrayInstruction.java
+++ b/src/de/inetsoftware/jwebassembly/module/WasmArrayInstruction.java
@@ -26,7 +26,6 @@ import de.inetsoftware.jwebassembly.module.TypeManager.StructType;
 import de.inetsoftware.jwebassembly.wasm.AnyType;
 import de.inetsoftware.jwebassembly.wasm.ArrayOperator;
 import de.inetsoftware.jwebassembly.wasm.ArrayType;
-import de.inetsoftware.jwebassembly.wasm.StructOperator;
 import de.inetsoftware.jwebassembly.wasm.ValueType;
 
 /**
@@ -228,4 +227,27 @@ class WasmArrayInstruction extends WasmInstruction {
                 throw new WasmException( "Unknown array operation: " + op, -1 );
         }
     }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    AnyType[] getPopValueTypes() {
+        switch( op ) {
+            case GET:
+            case GET_S:
+            case GET_U:
+                return new AnyType[] { arrayType, ValueType.i32 };
+            case NEW_ARRAY_WITH_RTT:
+                return new AnyType[] { ValueType.i32, ValueType.i32 }; // size, rtt type
+            case NEW:
+                return new AnyType[] { ValueType.i32 };
+            case LEN:
+                return new AnyType[] { arrayType };
+            case SET:
+                return new AnyType[] { arrayType, ValueType.i32, type };
+            default:
+                throw new WasmException( "Unknown array operation: " + op, -1 );
+        }
+    }
 }
diff --git a/src/de/inetsoftware/jwebassembly/module/WasmBlockInstruction.java b/src/de/inetsoftware/jwebassembly/module/WasmBlockInstruction.java
index ca3a132..e477016 100644
--- a/src/de/inetsoftware/jwebassembly/module/WasmBlockInstruction.java
+++ b/src/de/inetsoftware/jwebassembly/module/WasmBlockInstruction.java
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018 - 2020 Volker Berlin (i-net software)
+   Copyright 2018 - 2021 Volker Berlin (i-net software)
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -123,8 +123,31 @@ class WasmBlockInstruction extends WasmInstruction {
             case THROW:
             case RETHROW:
                 return 1;
+            case RETURN:
+                return data == null ? 0 : 1;
             default:
                 return 0;
         }
     }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    AnyType[] getPopValueTypes() {
+        switch( op ) {
+            case IF:
+            case BR_IF:
+                return new AnyType[] { ValueType.i32 };
+            case DROP:
+                return new AnyType[] { ValueType.anyref };
+            case THROW:
+            case RETHROW:
+                return new AnyType[] { ValueType.exnref };
+            case RETURN:
+                return data == null ? null : new AnyType[] { (AnyType)data };
+            default:
+                return null;
+        }
+    }
 }
diff --git a/src/de/inetsoftware/jwebassembly/module/WasmCallInstruction.java b/src/de/inetsoftware/jwebassembly/module/WasmCallInstruction.java
index 29e3916..10ae130 100644
--- a/src/de/inetsoftware/jwebassembly/module/WasmCallInstruction.java
+++ b/src/de/inetsoftware/jwebassembly/module/WasmCallInstruction.java
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018 - 2020 Volker Berlin (i-net software)
+   Copyright 2018 - 2021 Volker Berlin (i-net software)
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -136,6 +136,29 @@ class WasmCallInstruction extends WasmInstruction {
         return paramCount;
     }
 
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    AnyType[] getPopValueTypes() {
+        countParams();
+        if( paramCount == 0 ) {
+            return null;
+        }
+        AnyType[] valueTypes = new AnyType[paramCount];
+        int idx = 0;
+        if( needThisParameter ) {
+            valueTypes[ idx++ ] = types.valueOf( name.className );
+        }
+        if( idx < paramCount ) {
+            Iterator<AnyType> parser = name.getSignature( types );
+            while( idx < paramCount ) {
+                valueTypes[ idx++ ] = parser.next();
+            }
+        }
+        return valueTypes;
+    }
+
     /**
      * Count the parameters in the signature
      */
diff --git a/src/de/inetsoftware/jwebassembly/module/WasmConstInstruction.java b/src/de/inetsoftware/jwebassembly/module/WasmConstInstruction.java
index 0a177b3..68513c1 100644
--- a/src/de/inetsoftware/jwebassembly/module/WasmConstInstruction.java
+++ b/src/de/inetsoftware/jwebassembly/module/WasmConstInstruction.java
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018 - 2019 Volker Berlin (i-net software)
+   Copyright 2018 - 2021 Volker Berlin (i-net software)
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -121,4 +121,12 @@ class WasmConstInstruction extends WasmInstruction {
     int getPopCount() {
         return 0;
     }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    AnyType[] getPopValueTypes() {
+        return null;
+    }
 }
diff --git a/src/de/inetsoftware/jwebassembly/module/WasmConvertInstruction.java b/src/de/inetsoftware/jwebassembly/module/WasmConvertInstruction.java
index af90fd3..7535e68 100644
--- a/src/de/inetsoftware/jwebassembly/module/WasmConvertInstruction.java
+++ b/src/de/inetsoftware/jwebassembly/module/WasmConvertInstruction.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2018 - 2019 Volker Berlin (i-net software)
+ * Copyright 2018 - 2021 Volker Berlin (i-net software)
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -97,4 +97,34 @@ class WasmConvertInstruction extends WasmInstruction {
     int getPopCount() {
         return 1;
     }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    AnyType[] getPopValueTypes() {
+        switch( conversion ) {
+            case i2b:
+            case i2c:
+            case i2s:
+            case i2l:
+            case i2f:
+            case i2d:
+                return new AnyType[] { ValueType.i32 };
+            case l2i:
+            case l2f:
+            case l2d:
+                return new AnyType[] { ValueType.i64 };
+            case f2i:
+            case f2l:
+            case f2d:
+                return new AnyType[] { ValueType.f32 };
+            case d2i:
+            case d2l:
+            case d2f:
+                return new AnyType[] { ValueType.f64 };
+            default:
+                throw new Error( conversion.toString() );
+        }
+    }
 }
diff --git a/src/de/inetsoftware/jwebassembly/module/WasmGlobalInstruction.java b/src/de/inetsoftware/jwebassembly/module/WasmGlobalInstruction.java
index 0e9aabb..bcacd10 100644
--- a/src/de/inetsoftware/jwebassembly/module/WasmGlobalInstruction.java
+++ b/src/de/inetsoftware/jwebassembly/module/WasmGlobalInstruction.java
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018 - 2019 Volker Berlin (i-net software)
+   Copyright 2018 - 2021 Volker Berlin (i-net software)
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -98,4 +98,12 @@ class WasmGlobalInstruction extends WasmInstruction {
     int getPopCount() {
         return load ? 0 : 1;
     }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    AnyType[] getPopValueTypes() {
+        return load ? null : new AnyType[] { type };
+    }
 }
diff --git a/src/de/inetsoftware/jwebassembly/module/WasmInstruction.java b/src/de/inetsoftware/jwebassembly/module/WasmInstruction.java
index dcf1728..e3f7784 100644
--- a/src/de/inetsoftware/jwebassembly/module/WasmInstruction.java
+++ b/src/de/inetsoftware/jwebassembly/module/WasmInstruction.java
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018 - 2020 Volker Berlin (i-net software)
+   Copyright 2018 - 2021 Volker Berlin (i-net software)
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -115,4 +115,6 @@ abstract class WasmInstruction {
      * @return the count
      */
     abstract int getPopCount();
+
+    abstract AnyType[] getPopValueTypes();
 }
diff --git a/src/de/inetsoftware/jwebassembly/module/WasmLocalInstruction.java b/src/de/inetsoftware/jwebassembly/module/WasmLocalInstruction.java
index ae52f68..a82216a 100644
--- a/src/de/inetsoftware/jwebassembly/module/WasmLocalInstruction.java
+++ b/src/de/inetsoftware/jwebassembly/module/WasmLocalInstruction.java
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018 - 2020 Volker Berlin (i-net software)
+   Copyright 2018 - 2021 Volker Berlin (i-net software)
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -122,4 +122,12 @@ class WasmLocalInstruction extends WasmInstruction {
     int getPopCount() {
         return op == get  ? 0 : 1;
     }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    AnyType[] getPopValueTypes() {
+        return op == get  ? null : new AnyType[] { localVariables.getValueType( getIndex() ) };
+    }
 }
diff --git a/src/de/inetsoftware/jwebassembly/module/WasmMemoryInstruction.java b/src/de/inetsoftware/jwebassembly/module/WasmMemoryInstruction.java
index 6bbd640..114b80c 100644
--- a/src/de/inetsoftware/jwebassembly/module/WasmMemoryInstruction.java
+++ b/src/de/inetsoftware/jwebassembly/module/WasmMemoryInstruction.java
@@ -1,5 +1,5 @@
 /*
-   Copyright 2019 Volker Berlin (i-net software)
+   Copyright 2019 - 2021 Volker Berlin (i-net software)
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -95,4 +95,12 @@ class WasmMemoryInstruction extends WasmInstruction {
     int getPopCount() {
         return op.name().startsWith( "load" ) ? 0 : 1;
     }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    AnyType[] getPopValueTypes() {
+        return op.name().startsWith( "load" ) ? null : new AnyType[] { type };
+    }
 }
diff --git a/src/de/inetsoftware/jwebassembly/module/WasmNopInstruction.java b/src/de/inetsoftware/jwebassembly/module/WasmNopInstruction.java
index 042e058..ce5e00a 100644
--- a/src/de/inetsoftware/jwebassembly/module/WasmNopInstruction.java
+++ b/src/de/inetsoftware/jwebassembly/module/WasmNopInstruction.java
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018 - 2019 Volker Berlin (i-net software)
+   Copyright 2018 - 2021 Volker Berlin (i-net software)
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -71,4 +71,12 @@ class WasmNopInstruction extends WasmInstruction {
     int getPopCount() {
         return 0;
     }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    AnyType[] getPopValueTypes() {
+        return null;
+    }
 }
diff --git a/src/de/inetsoftware/jwebassembly/module/WasmNumericInstruction.java b/src/de/inetsoftware/jwebassembly/module/WasmNumericInstruction.java
index 2cfa645..04a951a 100644
--- a/src/de/inetsoftware/jwebassembly/module/WasmNumericInstruction.java
+++ b/src/de/inetsoftware/jwebassembly/module/WasmNumericInstruction.java
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018 - 2019 Volker Berlin (i-net software)
+   Copyright 2018 - 2021 Volker Berlin (i-net software)
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -111,4 +111,26 @@ class WasmNumericInstruction extends WasmInstruction {
                 return 2;
         }
     }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    AnyType[] getPopValueTypes() {
+        switch( numOp ) {
+            case eqz:
+            case ifnull:
+            case ifnonnull:
+            case neg:
+            case sqrt:
+            case abs:
+            case ceil:
+            case floor:
+            case trunc:
+            case nearest:
+                return new AnyType[] { ValueType.i32 };
+            default:
+                return new AnyType[] { valueType, valueType };
+        }
+    }
 }
diff --git a/src/de/inetsoftware/jwebassembly/module/WasmStructInstruction.java b/src/de/inetsoftware/jwebassembly/module/WasmStructInstruction.java
index a782431..3767e8d 100644
--- a/src/de/inetsoftware/jwebassembly/module/WasmStructInstruction.java
+++ b/src/de/inetsoftware/jwebassembly/module/WasmStructInstruction.java
@@ -18,6 +18,7 @@ package de.inetsoftware.jwebassembly.module;
 
 import java.io.IOException;
 import java.util.List;
+import java.util.Objects;
 
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
@@ -41,7 +42,7 @@ class WasmStructInstruction extends WasmInstruction {
     @Nonnull
     private final StructOperator   op;
 
-    private final StructType       type;
+    private       StructType       type;
 
     private final NamedStorageType fieldName;
 
@@ -176,6 +177,14 @@ class WasmStructInstruction extends WasmInstruction {
         return type;
     }
 
+    /**
+     * Set a new type for NULL const. 
+     * @param type the type
+     */
+    void setStructType( @Nonnull StructType type ) {
+        this.type = Objects.requireNonNull( type );
+    }
+
     /**
      * {@inheritDoc}
      */
@@ -285,4 +294,29 @@ class WasmStructInstruction extends WasmInstruction {
                 throw new WasmException( "Unknown array operation: " + op, -1 );
         }
     }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    AnyType[] getPopValueTypes() {
+        switch( op ) {
+            case GET:
+                return new AnyType[] { type };
+            case INSTANCEOF:
+            case CAST:
+                return new AnyType[] { options.types.valueOf( "java/lang/Object" ) };
+            case NEW_WITH_RTT:
+                return new AnyType[] { ValueType.i32 };// rtt type
+            case SET:
+                return new AnyType[] { type, fieldName.getType() };
+            case NEW:
+            case NEW_DEFAULT:
+            case NULL:
+            case RTT_CANON:
+                return null;
+            default:
+                throw new WasmException( "Unknown array operation: " + op, -1 );
+        }
+    }
 }
diff --git a/src/de/inetsoftware/jwebassembly/module/WasmTableInstruction.java b/src/de/inetsoftware/jwebassembly/module/WasmTableInstruction.java
index 0fd7452..151b500 100644
--- a/src/de/inetsoftware/jwebassembly/module/WasmTableInstruction.java
+++ b/src/de/inetsoftware/jwebassembly/module/WasmTableInstruction.java
@@ -1,5 +1,5 @@
 /*
-   Copyright 2019 Volker Berlin (i-net software)
+   Copyright 2019 - 2021 Volker Berlin (i-net software)
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -85,4 +85,12 @@ class WasmTableInstruction extends WasmInstruction {
     int getPopCount() {
         return load  ? 0 : 1;
     }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    AnyType[] getPopValueTypes() {
+        return load  ? null : new AnyType[] { ValueType.externref };
+    }
 }