Fix object identity hash.

The object identity hash is now stored in the monitor word after
being computed. Hashes are computed by a pseudo random number
generator.

When we write the image, we eagerly compute object hashes to
prevent pages getting dirtied.

Bug: 8981901

Change-Id: Ic8edacbacb0afc7055fd740a52444929f88ed564

1 files changed, 48 insertions, 0 deletions

diff --git a/runtime/mirror/object.cc b/runtime/mirror/object.cc
index 92c05b2..49bad4c 100644
--- a/runtime/mirror/object.cc
+++ b/runtime/mirror/object.cc
@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+#include <ctime>
+
 #include "object.h"
 
 #include "art_field.h"
@@ -82,6 +84,52 @@ Object* Object::Clone(Thread* self) {
   return copy.get();
 }
 
+uint32_t Object::GenerateIdentityHashCode() {
+  static AtomicInteger seed(987654321 + std::time(nullptr));
+  uint32_t expected_value, new_value;
+  do {
+    expected_value = static_cast<uint32_t>(seed.load());
+    new_value = expected_value * 1103515245 + 12345;
+  } while (!seed.compare_and_swap(static_cast<int32_t>(expected_value),
+                                  static_cast<int32_t>(new_value)));
+  return expected_value & LockWord::kHashMask;
+}
+
+int32_t Object::IdentityHashCode() const {
+  while (true) {
+    LockWord lw = GetLockWord();
+    switch (lw.GetState()) {
+      case LockWord::kUnlocked: {
+        // Try to compare and swap in a new hash, if we succeed we will return the hash on the next
+        // loop iteration.
+        LockWord hash_word(LockWord::FromHashCode(GenerateIdentityHashCode()));
+        DCHECK_EQ(hash_word.GetState(), LockWord::kHashCode);
+        if (const_cast<Object*>(this)->CasLockWord(lw, hash_word)) {
+          return hash_word.GetHashCode();
+        }
+        break;
+      }
+      case LockWord::kThinLocked: {
+        // Inflate the thin lock to a monitor and stick the hash code inside of the monitor.
+        Thread* self = Thread::Current();
+        Monitor::InflateThinLocked(self, const_cast<Object*>(this), lw, GenerateIdentityHashCode());
+        break;
+      }
+      case LockWord::kFatLocked: {
+        // Already inflated, return the has stored in the monitor.
+        Monitor* monitor = lw.FatLockMonitor();
+        DCHECK(monitor != nullptr);
+        return monitor->GetHashCode();
+      }
+      case LockWord::kHashCode: {
+        return lw.GetHashCode();
+      }
+    }
+  }
+  LOG(FATAL) << "Unreachable";
+  return 0;
+}
+
 void Object::CheckFieldAssignmentImpl(MemberOffset field_offset, const Object* new_value) {
   const Class* c = GetClass();
   if (Runtime::Current()->GetClassLinker() == NULL ||