--- a/hotspot/src/share/vm/utilities/array.hpp	Mon Jul 07 12:37:11 2014 +0200
+++ b/hotspot/src/share/vm/utilities/array.hpp	Mon Jul 07 10:12:40 2014 +0200
@@ -305,6 +305,7 @@
   friend class MetadataFactory;
   friend class VMStructs;
   friend class MethodHandleCompiler;           // special case
+  friend class WhiteBox;
 protected:
   int _length;                                 // the number of array elements
   T   _data[1];                                // the array memory
@@ -326,6 +327,29 @@
 
   static size_t byte_sizeof(int length) { return sizeof(Array<T>) + MAX2(length - 1, 0) * sizeof(T); }
 
+  // WhiteBox API helper.
+  static int bytes_to_length(size_t bytes)       {
+    assert(is_size_aligned(bytes, BytesPerWord), "Must be, for now");
+
+    if (sizeof(Array<T>) >= bytes) {
+      return 0;
+    }
+
+    size_t left = bytes - sizeof(Array<T>);
+    assert(is_size_aligned(left, sizeof(T)), "Must be");
+
+    size_t elements = left / sizeof(T);
+    assert(elements <= (size_t)INT_MAX, err_msg("number of elements " SIZE_FORMAT "doesn't fit into an int.", elements));
+
+    int length = (int)elements;
+
+    assert((size_t)size(length) * BytesPerWord == bytes,
+        err_msg("Expected: " SIZE_FORMAT " got: " SIZE_FORMAT,
+                bytes, (size_t)size(length) * BytesPerWord));
+
+    return length;
+  }
+
   explicit Array(int length) : _length(length) {
     assert(length >= 0, "illegal length");
   }