Small optimization for Heap.getAllClasses().

Author: christianhaeubl

substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/HeapImpl.java

@@ -84,6 +84,7 @@
 import com.oracle.svm.core.thread.VMThreads.SafepointBehavior;
 import com.oracle.svm.core.util.UnsignedUtils;
 import com.oracle.svm.core.util.UserError;
+import com.oracle.svm.core.util.VMError;
 
 import jdk.graal.compiler.api.directives.GraalDirectives;
 import jdk.graal.compiler.api.replacements.Fold;
@@ -340,31 +341,39 @@ public int getClassCount() {
     protected List<Class<?>> getAllClasses() {
         /* Two threads might race to set classList, but they compute the same result. */
         if (classList == null) {
-            ArrayList<Class<?>> list = new ArrayList<>(1024);
-            for (ImageHeapInfo info = firstImageHeapInfo; info != null; info = info.next) {
-                ImageHeapWalker.walkRegions(info, new ClassListBuilderVisitor(list));
-            }
-            list.trimToSize();
-
+            ArrayList<Class<?>> list = findAllDynamicHubs();
             /* Ensure that other threads see consistent values once the list is published. */
             MembarNode.memoryBarrier(MembarNode.FenceKind.STORE_STORE);
             classList = list;
         }
-        assert classList.size() == getClassCount();
         return classList;
     }
 
+    private ArrayList<Class<?>> findAllDynamicHubs() {
+        int dynamicHubCount = getClassCount();
+
+        ArrayList<Class<?>> list = new ArrayList<>(dynamicHubCount);
+        for (ImageHeapInfo info = firstImageHeapInfo; info != null; info = info.next) {
+            ImageHeapWalker.walkRegions(info, new ClassListBuilderVisitor(list.size() + info.dynamicHubCount, list));
+        }
+
+        VMError.guarantee(dynamicHubCount == list.size(), "Found fewer DynamicHubs in the image heap than expected.");
+        return list;
+    }
+
     private static class ClassListBuilderVisitor implements MemoryWalker.ImageHeapRegionVisitor, ObjectVisitor {
+        private final int dynamicHubCount;
         private final List<Class<?>> list;
 
-        ClassListBuilderVisitor(List<Class<?>> list) {
+        ClassListBuilderVisitor(int dynamicHubCount, List<Class<?>> list) {
+            this.dynamicHubCount = dynamicHubCount;
             this.list = list;
         }
 
         @Override
         public <T> boolean visitNativeImageHeapRegion(T region, MemoryWalker.NativeImageHeapRegionAccess<T> access) {
             if (!access.isWritable(region) && !access.consistsOfHugeObjects(region)) {
-                access.visitObjects(region, this);
+                return access.visitObjects(region, this);
             }
             return true;
         }
@@ -374,6 +383,7 @@ public <T> boolean visitNativeImageHeapRegion(T region, MemoryWalker.NativeImage
         public boolean visitObject(Object o) {
             if (o instanceof Class<?>) {
                 list.add((Class<?>) o);
+                return list.size() != dynamicHubCount;
             }
             return true;
         }