diff --git a/base/src/main/java/proguard/ProGuard.java b/base/src/main/java/proguard/ProGuard.java
index 859bf439..27f5888c 100644
--- a/base/src/main/java/proguard/ProGuard.java
+++ b/base/src/main/java/proguard/ProGuard.java
@@ -28,6 +28,7 @@ import proguard.configuration.ConfigurationLoggingAdder;
 import proguard.evaluation.IncompleteClassHierarchyException;
 import proguard.configuration.InitialStateInfo;
 import proguard.mark.Marker;
+import proguard.obfuscate.ObfuscationPreparation;
 import proguard.obfuscate.Obfuscator;
 import proguard.optimize.LineNumberTrimmer;
 import proguard.optimize.Optimizer;
@@ -504,6 +505,8 @@ public class ProGuard
             System.out.println("Obfuscating...");
         }
 
+        new ObfuscationPreparation().execute(appView);
+
         // Perform the actual obfuscation.
         new Obfuscator().execute(appView);
     }
diff --git a/base/src/main/java/proguard/obfuscate/ObfuscationPreparation.java b/base/src/main/java/proguard/obfuscate/ObfuscationPreparation.java
new file mode 100644
index 00000000..da8ed522
--- /dev/null
+++ b/base/src/main/java/proguard/obfuscate/ObfuscationPreparation.java
@@ -0,0 +1,41 @@
+/*
+ * ProGuard -- shrinking, optimization, obfuscation, and preverification
+ *             of Java bytecode.
+ *
+ * Copyright (c) 2002-2021 Guardsquare NV
+ */
+
+package proguard.obfuscate;
+
+import proguard.AppView;
+import proguard.classfile.visitor.ClassCleaner;
+import proguard.pass.Pass;
+import proguard.util.PrintWriterUtil;
+
+import java.io.*;
+
+public class ObfuscationPreparation implements Pass
+{
+    @Override
+    public void execute(AppView appView) throws IOException
+    {
+        // We'll apply a mapping, if requested.
+        if (appView.configuration.verbose &&
+            appView.configuration.applyMapping != null)
+        {
+            System.out.println("Applying mapping from [" + PrintWriterUtil.fileName(appView.configuration.applyMapping) + "]...");
+        }
+
+        // Check if we have at least some keep commands.
+        if (appView.configuration.keep         == null &&
+            appView.configuration.applyMapping == null &&
+            appView.configuration.printMapping == null)
+        {
+            throw new IOException("You have to specify '-keep' options for the obfuscation step.");
+        }
+
+        // Clean up any old processing info.
+        appView.programClassPool.classesAccept(new ClassCleaner());
+        appView.libraryClassPool.classesAccept(new ClassCleaner());
+    }
+}
diff --git a/base/src/main/java/proguard/obfuscate/Obfuscator.java b/base/src/main/java/proguard/obfuscate/Obfuscator.java
index 7a92085a..c236d175 100644
--- a/base/src/main/java/proguard/obfuscate/Obfuscator.java
+++ b/base/src/main/java/proguard/obfuscate/Obfuscator.java
@@ -56,23 +56,11 @@ public class Obfuscator implements Pass
     @Override
     public void execute(AppView appView) throws IOException
     {
-        // Check if we have at least some keep commands.
-        if (appView.configuration.keep         == null &&
-            appView.configuration.applyMapping == null &&
-            appView.configuration.printMapping == null)
-        {
-            throw new IOException("You have to specify '-keep' options for the obfuscation step.");
-        }
-
         // We're using the system's default character encoding for writing to
         // the standard output and error output.
         PrintWriter out = new PrintWriter(System.out, true);
         PrintWriter err = new PrintWriter(System.err, true);
 
-        // Clean up any old processing info.
-        appView.programClassPool.classesAccept(new ClassCleaner());
-        appView.libraryClassPool.classesAccept(new ClassCleaner());
-
         // Link all non-private, non-static methods in all class hierarchies.
         ClassVisitor memberInfoLinker =
             new BottomClassFilter(new MethodLinker());
@@ -211,13 +199,6 @@ public class Obfuscator implements Pass
         // override the names of library classes and of library class members.
         if (appView.configuration.applyMapping != null)
         {
-            if (appView.configuration.verbose)
-            {
-                out.println("Applying mapping from [" +
-                            PrintWriterUtil.fileName(appView.configuration.applyMapping) +
-                            "]...");
-            }
-
             WarningPrinter warningPrinter = new WarningPrinter(err, appView.configuration.warn);
 
             MappingReader reader = new MappingReader(appView.configuration.applyMapping);
diff --git a/base/src/main/java/proguard/optimize/Optimizer.java b/base/src/main/java/proguard/optimize/Optimizer.java
index 08d41eef..d1bf23fe 100644
--- a/base/src/main/java/proguard/optimize/Optimizer.java
+++ b/base/src/main/java/proguard/optimize/Optimizer.java
@@ -46,6 +46,9 @@ import java.util.*;
 /**
  * This pass optimizes class pools according to a given configuration.
  *
+ * This pass is stateful. It tracks when no more optimizations are
+ * possible, and then all further runs of this pass will have no effect.
+ *
  * @author Eric Lafortune
  */
 public class Optimizer implements Pass