--- old/src/jdk.internal.vm.compiler/share/classes/org.graalvm.compiler.options/src/org/graalvm/compiler/options/ModifiableOptionValues.java	2017-08-03 22:42:53.129535510 -0700
+++ new/src/jdk.internal.vm.compiler/share/classes/org.graalvm.compiler.options/src/org/graalvm/compiler/options/ModifiableOptionValues.java	2017-08-03 22:42:53.030531072 -0700
@@ -48,7 +48,16 @@
     }
 
     /**
+     * Value that can be used in {@link #update(UnmodifiableEconomicMap)} and
+     * {@link #update(OptionKey, Object)} to remove an explicitly set value for a key such that
+     * {@link OptionKey#hasBeenSet(OptionValues)} will return {@code false} for the key.
+     */
+    public static final Object UNSET_KEY = new Object();
+
+    /**
      * Updates this object with the given key/value pair.
+     *
+     * @see #UNSET_KEY
      */
     public void update(OptionKey<?> key, Object value) {
         UnmodifiableEconomicMap<OptionKey<?>, Object> expect;
@@ -56,14 +65,20 @@
         do {
             expect = v.get();
             newMap = EconomicMap.create(Equivalence.IDENTITY, expect);
-            key.update(newMap, value);
-            // Need to do the null encoding here as `key.update()` doesn't do it
-            newMap.put(key, encodeNull(value));
+            if (value == UNSET_KEY) {
+                newMap.removeKey(key);
+            } else {
+                key.update(newMap, value);
+                // Need to do the null encoding here as `key.update()` doesn't do it
+                newMap.put(key, encodeNull(value));
+            }
         } while (!v.compareAndSet(expect, newMap));
     }
 
     /**
      * Updates this object with the key/value pairs in {@code values}.
+     *
+     * @see #UNSET_KEY
      */
     public void update(UnmodifiableEconomicMap<OptionKey<?>, Object> values) {
         if (values.isEmpty()) {
@@ -78,9 +93,13 @@
             while (cursor.advance()) {
                 OptionKey<?> key = cursor.getKey();
                 Object value = cursor.getValue();
-                key.update(newMap, value);
-                // Need to do the null encoding here as `key.update()` doesn't do it
-                newMap.put(key, encodeNull(value));
+                if (value == UNSET_KEY) {
+                    newMap.removeKey(key);
+                } else {
+                    key.update(newMap, value);
+                    // Need to do the null encoding here as `key.update()` doesn't do it
+                    newMap.put(key, encodeNull(value));
+                }
             }
         } while (!v.compareAndSet(expect, newMap));
     }