Fixed a bug in the HDL model const merger

src/main/java/de/neemann/digital/hdl/model2/optimizations/MergeConstants.java

@@ -7,6 +7,7 @@ package de.neemann.digital.hdl.model2.optimizations;
 
 import de.neemann.digital.hdl.model2.*;
 import de.neemann.digital.hdl.model2.expression.ExprConstant;
+import de.neemann.digital.hdl.model2.expression.ExprVar;
 
 import java.util.ArrayList;
 
@@ -54,6 +55,8 @@ public class MergeConstants implements Optimization {
         for (HDLPort p : ins.toArray(new HDLPort[ins.size()]))
             p.setNet(usedNet);
 
+        circuit.replaceNetByExpression(obsoleteNet, new ExprVar(usedNet));
+
         circuit.removeNet(obsoleteNet);
     }
 

src/main/java/de/neemann/digital/hdl/printer/CodePrinter.java

@@ -102,6 +102,9 @@ public class CodePrinter implements Closeable {
      * @throws IOException IOException
      */
     public CodePrinter print(String str) throws IOException {
+        if (str == null)
+            str = "null";
+
         for (int i = 0; i < str.length(); i++)
             print(str.charAt(i));
         return this;

src/test/java/de/neemann/digital/hdl/vhdl2/VHDLSimulatorTest.java

@@ -88,6 +88,18 @@ public class VHDLSimulatorTest extends TestCase {
         }
     }
 
+    public void testMultiplierInSimulator() throws Exception {
+        File file = new File(Resources.getRoot(), "../../main/dig/combinatorial/Multiply8Bit.dig");
+        try {
+            checkVHDLExport(file);
+        } catch (FileScanner.SkipAllException e) {
+            // if ghdl is not installed its also ok
+        } catch (Exception e) {
+            System.out.println(ExceptionWithOrigin.getOriginOf(e));
+            throw e;
+        }
+    }
+
     public void testGHDLInSimulator() throws Exception {
         try {
             ProcessStarter.start(null, GHDL, "--help");