Compacting collector.

The compacting collector is currently similar to semispace. It works by
copying objects back and forth between two bump pointer spaces. There
are types of objects which are "non-movable" due to current runtime
limitations. These are Classes, Methods, and Fields.

Bump pointer spaces are a new type of continuous alloc space which have
no lock in the allocation code path. When you allocate from these it uses
atomic operations to increase an index. Traversing the objects in the bump
pointer space relies on Object::SizeOf matching the allocated size exactly.

Runtime changes:
JNI::GetArrayElements returns copies objects if you attempt to get the
backing data of a movable array. For GetArrayElementsCritical, we return
direct backing storage for any types of arrays, but temporarily disable
the GC until the critical region is completed.

Added a new runtime call called VisitObjects, this is used in place of
the old pattern which was flushing the allocation stack and walking
the bitmaps.

Changed image writer to be compaction safe and use object monitor word
for forwarding addresses.

Added a bunch of added SIRTs to ClassLinker, MethodLinker, etc..

TODO: Enable switching allocators, compacting on background, etc..

Bug: 8981901

Change-Id: I3c886fd322a6eef2b99388d19a765042ec26ab99

9 files changed, 292 insertions, 266 deletions

diff --git a/compiler/driver/compiler_driver.cc b/compiler/driver/compiler_driver.cc
index 9cc94e8c0d..4af492bf6e 100644
--- a/compiler/driver/compiler_driver.cc
+++ b/compiler/driver/compiler_driver.cc
@@ -513,10 +513,9 @@ void CompilerDriver::CompileAll(jobject class_loader,
   }
 }
 
-static DexToDexCompilationLevel GetDexToDexCompilationlevel(mirror::ClassLoader* class_loader,
-                                                            const DexFile& dex_file,
-                                                            const DexFile::ClassDef& class_def)
-    SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
+static DexToDexCompilationLevel GetDexToDexCompilationlevel(
+    SirtRef<mirror::ClassLoader>& class_loader, const DexFile& dex_file,
+    const DexFile::ClassDef& class_def) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
   const char* descriptor = dex_file.GetClassDescriptor(class_def);
   ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
   mirror::Class* klass = class_linker->FindClass(descriptor, class_loader);
@@ -531,7 +530,7 @@ static DexToDexCompilationLevel GetDexToDexCompilationlevel(mirror::ClassLoader*
   // function). Since image classes can be verified again while compiling an application,
   // we must prevent the DEX-to-DEX compiler from introducing them.
   // TODO: find a way to enable "quick" instructions for image classes and remove this check.
-  bool compiling_image_classes = (class_loader == NULL);
+  bool compiling_image_classes = class_loader.get() == nullptr;
   if (compiling_image_classes) {
     return kRequired;
   } else if (klass->IsVerified()) {
@@ -579,7 +578,8 @@ void CompilerDriver::CompileOne(const mirror::ArtMethod* method, base::TimingLog
   {
     ScopedObjectAccess soa(Thread::Current());
     const DexFile::ClassDef& class_def = dex_file->GetClassDef(class_def_idx);
-    mirror::ClassLoader* class_loader = soa.Decode<mirror::ClassLoader*>(jclass_loader);
+    SirtRef<mirror::ClassLoader> class_loader(soa.Self(),
+                                              soa.Decode<mirror::ClassLoader*>(jclass_loader));
     dex_to_dex_compilation_level = GetDexToDexCompilationlevel(class_loader, *dex_file, class_def);
   }
   CompileMethod(code_item, method->GetAccessFlags(), method->GetInvokeType(),
@@ -721,8 +721,8 @@ void CompilerDriver::LoadImageClasses(base::TimingLogger& timings)
     for (const std::pair<uint16_t, const DexFile*>& exception_type : unresolved_exception_types) {
       uint16_t exception_type_idx = exception_type.first;
       const DexFile* dex_file = exception_type.second;
-      mirror::DexCache* dex_cache = class_linker->FindDexCache(*dex_file);
-      mirror:: ClassLoader* class_loader = NULL;
+      SirtRef<mirror::DexCache> dex_cache(self, class_linker->FindDexCache(*dex_file));
+      SirtRef<mirror::ClassLoader> class_loader(self, nullptr);
       SirtRef<mirror::Class> klass(self, class_linker->ResolveType(*dex_file, exception_type_idx,
                                                                    dex_cache, class_loader));
       if (klass.get() == NULL) {
@@ -782,15 +782,14 @@ void CompilerDriver::UpdateImageClasses(base::TimingLogger& timings) {
     const char* old_cause = self->StartAssertNoThreadSuspension("ImageWriter");
     gc::Heap* heap = Runtime::Current()->GetHeap();
     // TODO: Image spaces only?
+    ScopedObjectAccess soa(Thread::Current());
     WriterMutexLock mu(self, *Locks::heap_bitmap_lock_);
-    heap->FlushAllocStack();
-    heap->GetLiveBitmap()->Walk(FindClinitImageClassesCallback, this);
+    heap->VisitObjects(FindClinitImageClassesCallback, this);
     self->EndAssertNoThreadSuspension(old_cause);
   }
 }
 
-bool CompilerDriver::CanAssumeTypeIsPresentInDexCache(const DexFile& dex_file,
-                                                      uint32_t type_idx) {
+bool CompilerDriver::CanAssumeTypeIsPresentInDexCache(const DexFile& dex_file, uint32_t type_idx) {
   if (IsImage() &&
       IsImageClass(dex_file.StringDataByIdx(dex_file.GetTypeId(type_idx).descriptor_idx_))) {
     if (kIsDebugBuild) {
@@ -815,7 +814,7 @@ bool CompilerDriver::CanAssumeStringIsPresentInDexCache(const DexFile& dex_file,
   if (IsImage()) {
     // We resolve all const-string strings when building for the image.
     ScopedObjectAccess soa(Thread::Current());
-    mirror::DexCache* dex_cache = Runtime::Current()->GetClassLinker()->FindDexCache(dex_file);
+    SirtRef<mirror::DexCache> dex_cache(soa.Self(), Runtime::Current()->GetClassLinker()->FindDexCache(dex_file));
     Runtime::Current()->GetClassLinker()->ResolveString(dex_file, string_idx, dex_cache);
     result = true;
   }
@@ -903,26 +902,27 @@ bool CompilerDriver::CanAccessInstantiableTypeWithoutChecks(uint32_t referrer_id
 }
 
 static mirror::Class* ComputeCompilingMethodsClass(ScopedObjectAccess& soa,
-                                                   mirror::DexCache* dex_cache,
+                                                   SirtRef<mirror::DexCache>& dex_cache,
                                                    const DexCompilationUnit* mUnit)
     SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
   // The passed dex_cache is a hint, sanity check before asking the class linker that will take a
   // lock.
   if (dex_cache->GetDexFile() != mUnit->GetDexFile()) {
-    dex_cache = mUnit->GetClassLinker()->FindDexCache(*mUnit->GetDexFile());
+    dex_cache.reset(mUnit->GetClassLinker()->FindDexCache(*mUnit->GetDexFile()));
   }
-  mirror::ClassLoader* class_loader = soa.Decode<mirror::ClassLoader*>(mUnit->GetClassLoader());
-  const DexFile::MethodId& referrer_method_id = mUnit->GetDexFile()->GetMethodId(mUnit->GetDexMethodIndex());
+  SirtRef<mirror::ClassLoader>
+      class_loader(soa.Self(), soa.Decode<mirror::ClassLoader*>(mUnit->GetClassLoader()));
+  const DexFile::MethodId& referrer_method_id =
+      mUnit->GetDexFile()->GetMethodId(mUnit->GetDexMethodIndex());
   return mUnit->GetClassLinker()->ResolveType(*mUnit->GetDexFile(), referrer_method_id.class_idx_,
                                               dex_cache, class_loader);
 }
 
-static mirror::ArtField* ComputeFieldReferencedFromCompilingMethod(ScopedObjectAccess& soa,
-                                                                const DexCompilationUnit* mUnit,
-                                                                uint32_t field_idx)
+static mirror::ArtField* ComputeFieldReferencedFromCompilingMethod(
+    ScopedObjectAccess& soa, const DexCompilationUnit* mUnit, uint32_t field_idx)
     SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
-  mirror::DexCache* dex_cache = mUnit->GetClassLinker()->FindDexCache(*mUnit->GetDexFile());
-  mirror::ClassLoader* class_loader = soa.Decode<mirror::ClassLoader*>(mUnit->GetClassLoader());
+  SirtRef<mirror::DexCache> dex_cache(soa.Self(), mUnit->GetClassLinker()->FindDexCache(*mUnit->GetDexFile()));
+  SirtRef<mirror::ClassLoader> class_loader(soa.Self(), soa.Decode<mirror::ClassLoader*>(mUnit->GetClassLoader()));
   return mUnit->GetClassLinker()->ResolveField(*mUnit->GetDexFile(), field_idx, dex_cache,
                                                class_loader, false);
 }
@@ -932,8 +932,8 @@ static mirror::ArtMethod* ComputeMethodReferencedFromCompilingMethod(ScopedObjec
                                                                      uint32_t method_idx,
                                                                      InvokeType type)
     SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
-  mirror::DexCache* dex_cache = mUnit->GetClassLinker()->FindDexCache(*mUnit->GetDexFile());
-  mirror::ClassLoader* class_loader = soa.Decode<mirror::ClassLoader*>(mUnit->GetClassLoader());
+  SirtRef<mirror::DexCache> dex_cache(soa.Self(), mUnit->GetClassLinker()->FindDexCache(*mUnit->GetDexFile()));
+  SirtRef<mirror::ClassLoader> class_loader(soa.Self(), soa.Decode<mirror::ClassLoader*>(mUnit->GetClassLoader()));
   return mUnit->GetClassLinker()->ResolveMethod(*mUnit->GetDexFile(), method_idx, dex_cache,
                                                 class_loader, NULL, type);
 }
@@ -947,9 +947,10 @@ bool CompilerDriver::ComputeInstanceFieldInfo(uint32_t field_idx, const DexCompi
   // Try to resolve field and ignore if an Incompatible Class Change Error (ie is static).
   mirror::ArtField* resolved_field = ComputeFieldReferencedFromCompilingMethod(soa, mUnit, field_idx);
   if (resolved_field != NULL && !resolved_field->IsStatic()) {
+    SirtRef<mirror::DexCache> dex_cache(soa.Self(),
+                                        resolved_field->GetDeclaringClass()->GetDexCache());
     mirror::Class* referrer_class =
-        ComputeCompilingMethodsClass(soa, resolved_field->GetDeclaringClass()->GetDexCache(),
-                                     mUnit);
+        ComputeCompilingMethodsClass(soa, dex_cache, mUnit);
     if (referrer_class != NULL) {
       mirror::Class* fields_class = resolved_field->GetDeclaringClass();
       bool access_ok = referrer_class->CanAccess(fields_class) &&
@@ -997,9 +998,9 @@ bool CompilerDriver::ComputeStaticFieldInfo(uint32_t field_idx, const DexCompila
   // Try to resolve field and ignore if an Incompatible Class Change Error (ie isn't static).
   mirror::ArtField* resolved_field = ComputeFieldReferencedFromCompilingMethod(soa, mUnit, field_idx);
   if (resolved_field != NULL && resolved_field->IsStatic()) {
+    SirtRef<mirror::DexCache> dex_cache(soa.Self(), resolved_field->GetDeclaringClass()->GetDexCache());
     mirror::Class* referrer_class =
-        ComputeCompilingMethodsClass(soa, resolved_field->GetDeclaringClass()->GetDexCache(),
-                                     mUnit);
+        ComputeCompilingMethodsClass(soa, dex_cache, mUnit);
     if (referrer_class != NULL) {
       mirror::Class* fields_class = resolved_field->GetDeclaringClass();
       if (fields_class == referrer_class) {
@@ -1085,7 +1086,7 @@ void CompilerDriver::GetCodeAndMethodForDirectCall(InvokeType* type, InvokeType
   *direct_code = 0;
   *direct_method = 0;
   bool use_dex_cache = false;
-  bool compiling_boot = Runtime::Current()->GetHeap()->GetContinuousSpaces().size() == 1;
+  const bool compiling_boot = Runtime::Current()->GetHeap()->IsCompilingBoot();
   if (compiler_backend_ == kPortable) {
     if (sharp_type != kStatic && sharp_type != kDirect) {
       return;
@@ -1198,9 +1199,9 @@ bool CompilerDriver::ComputeInvokeInfo(const DexCompilationUnit* mUnit, const ui
     }
     // Don't try to fast-path if we don't understand the caller's class or this appears to be an
     // Incompatible Class Change Error.
+    SirtRef<mirror::DexCache> dex_cache(soa.Self(), resolved_method->GetDeclaringClass()->GetDexCache());
     mirror::Class* referrer_class =
-        ComputeCompilingMethodsClass(soa, resolved_method->GetDeclaringClass()->GetDexCache(),
-                                     mUnit);
+        ComputeCompilingMethodsClass(soa, dex_cache, mUnit);
     bool icce = resolved_method->CheckIncompatibleClassChange(*invoke_type);
     if (referrer_class != NULL && !icce) {
       mirror::Class* methods_class = resolved_method->GetDeclaringClass();
@@ -1254,10 +1255,8 @@ bool CompilerDriver::ComputeInvokeInfo(const DexCompilationUnit* mUnit, const ui
           const MethodReference* devirt_map_target =
               verifier::MethodVerifier::GetDevirtMap(caller_method, dex_pc);
           if (devirt_map_target != NULL) {
-            mirror::DexCache* target_dex_cache =
-                mUnit->GetClassLinker()->FindDexCache(*devirt_map_target->dex_file);
-            mirror::ClassLoader* class_loader =
-                soa.Decode<mirror::ClassLoader*>(mUnit->GetClassLoader());
+            SirtRef<mirror::DexCache> target_dex_cache(soa.Self(), mUnit->GetClassLinker()->FindDexCache(*devirt_map_target->dex_file));
+            SirtRef<mirror::ClassLoader> class_loader(soa.Self(), soa.Decode<mirror::ClassLoader*>(mUnit->GetClassLoader()));
             mirror::ArtMethod* called_method =
                 mUnit->GetClassLinker()->ResolveMethod(*devirt_map_target->dex_file,
                                                        devirt_map_target->dex_method_index,
@@ -1509,13 +1508,11 @@ static void ResolveClassFieldsAndMethods(const ParallelCompilationManager* manag
   const DexFile::ClassDef& class_def = dex_file.GetClassDef(class_def_index);
   if (!SkipClass(class_linker, jclass_loader, dex_file, class_def)) {
     ScopedObjectAccess soa(self);
-    mirror::ClassLoader* class_loader = soa.Decode<mirror::ClassLoader*>(jclass_loader);
-    mirror::DexCache* dex_cache = class_linker->FindDexCache(dex_file);
-
+    SirtRef<mirror::ClassLoader> class_loader(soa.Self(), soa.Decode<mirror::ClassLoader*>(jclass_loader));
+    SirtRef<mirror::DexCache> dex_cache(soa.Self(), class_linker->FindDexCache(dex_file));
     // Resolve the class.
     mirror::Class* klass = class_linker->ResolveType(dex_file, class_def.class_idx_, dex_cache,
                                                      class_loader);
-
     bool resolve_fields_and_methods;
     if (klass == NULL) {
       // Class couldn't be resolved, for example, super-class is in a different dex file. Don't
@@ -1598,8 +1595,8 @@ static void ResolveType(const ParallelCompilationManager* manager, size_t type_i
   ScopedObjectAccess soa(Thread::Current());
   ClassLinker* class_linker = manager->GetClassLinker();
   const DexFile& dex_file = *manager->GetDexFile();
-  mirror::DexCache* dex_cache = class_linker->FindDexCache(dex_file);
-  mirror::ClassLoader* class_loader = soa.Decode<mirror::ClassLoader*>(manager->GetClassLoader());
+  SirtRef<mirror::DexCache> dex_cache(soa.Self(), class_linker->FindDexCache(dex_file));
+  SirtRef<mirror::ClassLoader> class_loader(soa.Self(), soa.Decode<mirror::ClassLoader*>(manager->GetClassLoader()));
   mirror::Class* klass = class_linker->ResolveType(dex_file, type_idx, dex_cache, class_loader);
 
   if (klass == NULL) {
@@ -1652,8 +1649,9 @@ static void VerifyClass(const ParallelCompilationManager* manager, size_t class_
   const char* descriptor = dex_file.GetClassDescriptor(class_def);
   ClassLinker* class_linker = manager->GetClassLinker();
   jobject jclass_loader = manager->GetClassLoader();
-  mirror::Class* klass = class_linker->FindClass(descriptor,
-                                                 soa.Decode<mirror::ClassLoader*>(jclass_loader));
+  SirtRef<mirror::ClassLoader> class_loader(
+      soa.Self(), soa.Decode<mirror::ClassLoader*>(jclass_loader));
+  mirror::Class* klass = class_linker->FindClass(descriptor, class_loader);
   if (klass == NULL) {
     CHECK(soa.Self()->IsExceptionPending());
     soa.Self()->ClearException();
@@ -1663,11 +1661,10 @@ static void VerifyClass(const ParallelCompilationManager* manager, size_t class_
      * This is to ensure the class is structurally sound for compilation. An unsound class
      * will be rejected by the verifier and later skipped during compilation in the compiler.
      */
-    mirror::DexCache* dex_cache = class_linker->FindDexCache(dex_file);
+    SirtRef<mirror::DexCache> dex_cache(soa.Self(), class_linker->FindDexCache(dex_file));
     std::string error_msg;
-    if (verifier::MethodVerifier::VerifyClass(&dex_file, dex_cache,
-                                              soa.Decode<mirror::ClassLoader*>(jclass_loader),
-                                              &class_def, true, &error_msg) ==
+    if (verifier::MethodVerifier::VerifyClass(&dex_file, dex_cache, class_loader, &class_def, true,
+                                              &error_msg) ==
                                                   verifier::MethodVerifier::kHardFailure) {
       LOG(ERROR) << "Verification failed on class " << PrettyDescriptor(descriptor)
                  << " because: " << error_msg;
@@ -2124,7 +2121,8 @@ static void InitializeClass(const ParallelCompilationManager* manager, size_t cl
   const char* descriptor = dex_file.StringDataByIdx(class_type_id.descriptor_idx_);
 
   ScopedObjectAccess soa(Thread::Current());
-  mirror::ClassLoader* class_loader = soa.Decode<mirror::ClassLoader*>(jclass_loader);
+  SirtRef<mirror::ClassLoader> class_loader(soa.Self(),
+                                            soa.Decode<mirror::ClassLoader*>(jclass_loader));
   mirror::Class* klass = manager->GetClassLinker()->FindClass(descriptor, class_loader);
 
   if (klass != NULL && !SkipClass(jclass_loader, dex_file, klass)) {
@@ -2253,7 +2251,8 @@ void CompilerDriver::CompileClass(const ParallelCompilationManager* manager, siz
   DexToDexCompilationLevel dex_to_dex_compilation_level = kDontDexToDexCompile;
   {
     ScopedObjectAccess soa(Thread::Current());
-    mirror::ClassLoader* class_loader = soa.Decode<mirror::ClassLoader*>(jclass_loader);
+    SirtRef<mirror::ClassLoader> class_loader(soa.Self(),
+                                              soa.Decode<mirror::ClassLoader*>(jclass_loader));
     dex_to_dex_compilation_level = GetDexToDexCompilationlevel(class_loader, dex_file, class_def);
   }
   ClassDataItemIterator it(dex_file, class_data);
diff --git a/compiler/driver/compiler_driver.h b/compiler/driver/compiler_driver.h
index 9321f06526..9bfea6ff0a 100644
--- a/compiler/driver/compiler_driver.h
+++ b/compiler/driver/compiler_driver.h
@@ -368,7 +368,8 @@ class CompilerDriver {
                          ThreadPool& thread_pool, base::TimingLogger& timings)
       LOCKS_EXCLUDED(Locks::mutator_lock_, compiled_classes_lock_);
 
-  void UpdateImageClasses(base::TimingLogger& timings);
+  void UpdateImageClasses(base::TimingLogger& timings)
+      LOCKS_EXCLUDED(Locks::mutator_lock_);
   static void FindClinitImageClassesCallback(mirror::Object* object, void* arg)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
diff --git a/compiler/driver/compiler_driver_test.cc b/compiler/driver/compiler_driver_test.cc
index c6687bb4aa..bfc93b3c8f 100644
--- a/compiler/driver/compiler_driver_test.cc
+++ b/compiler/driver/compiler_driver_test.cc
@@ -78,7 +78,9 @@ class CompilerDriverTest : public CommonTest {
       const DexFile::ClassDef& class_def = dex_file.GetClassDef(i);
       const char* descriptor = dex_file.GetClassDescriptor(class_def);
       ScopedObjectAccess soa(Thread::Current());
-      mirror::Class* c = class_linker->FindClass(descriptor, soa.Decode<mirror::ClassLoader*>(class_loader));
+      Thread* self = Thread::Current();
+      SirtRef<mirror::ClassLoader> loader(self, soa.Decode<mirror::ClassLoader*>(class_loader));
+      mirror::Class* c = class_linker->FindClass(descriptor, loader);
       CHECK(c != NULL);
       for (size_t i = 0; i < c->NumDirectMethods(); i++) {
         MakeExecutable(c->GetDirectMethod(i));
@@ -142,8 +144,9 @@ TEST_F(CompilerDriverTest, AbstractMethodErrorStub) {
   jobject class_loader;
   {
     ScopedObjectAccess soa(Thread::Current());
-    CompileVirtualMethod(NULL, "java.lang.Class", "isFinalizable", "()Z");
-    CompileDirectMethod(NULL, "java.lang.Object", "<init>", "()V");
+    SirtRef<mirror::ClassLoader> null_loader(soa.Self(), nullptr);
+    CompileVirtualMethod(null_loader, "java.lang.Class", "isFinalizable", "()Z");
+    CompileDirectMethod(null_loader, "java.lang.Object", "<init>", "()V");
     class_loader = LoadDex("AbstractMethod");
   }
   ASSERT_TRUE(class_loader != NULL);
diff --git a/compiler/image_test.cc b/compiler/image_test.cc
index a8b7c881f4..9d9c06401e 100644
--- a/compiler/image_test.cc
+++ b/compiler/image_test.cc
@@ -94,8 +94,8 @@ TEST_F(ImageTest, WriteRead) {
     ASSERT_NE(0U, image_header.GetImageBitmapSize());
 
     gc::Heap* heap = Runtime::Current()->GetHeap();
-    ASSERT_EQ(1U, heap->GetContinuousSpaces().size());
-    gc::space::ContinuousSpace* space = heap->GetContinuousSpaces().front();
+    ASSERT_TRUE(!heap->GetContinuousSpaces().empty());
+    gc::space::ContinuousSpace* space = heap->GetNonMovingSpace();
     ASSERT_FALSE(space->IsImageSpace());
     ASSERT_TRUE(space != NULL);
     ASSERT_TRUE(space->IsDlMallocSpace());
@@ -139,11 +139,8 @@ TEST_F(ImageTest, WriteRead) {
   class_linker_ = runtime_->GetClassLinker();
 
   gc::Heap* heap = Runtime::Current()->GetHeap();
-  ASSERT_EQ(2U, heap->GetContinuousSpaces().size());
-  ASSERT_TRUE(heap->GetContinuousSpaces()[0]->IsImageSpace());
-  ASSERT_FALSE(heap->GetContinuousSpaces()[0]->IsDlMallocSpace());
-  ASSERT_FALSE(heap->GetContinuousSpaces()[1]->IsImageSpace());
-  ASSERT_TRUE(heap->GetContinuousSpaces()[1]->IsDlMallocSpace());
+  ASSERT_TRUE(heap->HasImageSpace());
+  ASSERT_TRUE(heap->GetNonMovingSpace()->IsDlMallocSpace());
 
   gc::space::ImageSpace* image_space = heap->GetImageSpace();
   image_space->VerifyImageAllocations();
diff --git a/compiler/image_writer.cc b/compiler/image_writer.cc
index 75be2c9c43..c22f8d6c01 100644
--- a/compiler/image_writer.cc
+++ b/compiler/image_writer.cc
@@ -75,8 +75,6 @@ bool ImageWriter::Write(const std::string& image_filename,
   image_begin_ = reinterpret_cast<byte*>(image_begin);
 
   ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
-  const std::vector<DexCache*>& all_dex_caches = class_linker->GetDexCaches();
-  dex_caches_.insert(all_dex_caches.begin(), all_dex_caches.end());
 
   UniquePtr<File> oat_file(OS::OpenFileReadWrite(oat_filename.c_str()));
   if (oat_file.get() == NULL) {
@@ -121,22 +119,16 @@ bool ImageWriter::Write(const std::string& image_filename,
   }
   gc::Heap* heap = Runtime::Current()->GetHeap();
   heap->CollectGarbage(false);  // Remove garbage.
-  // Trim size of alloc spaces.
-  for (const auto& space : heap->GetContinuousSpaces()) {
-    if (space->IsDlMallocSpace()) {
-      space->AsDlMallocSpace()->Trim();
-    }
-  }
 
   if (!AllocMemory()) {
     return false;
   }
-#ifndef NDEBUG
-  {  // NOLINT(whitespace/braces)
+
+  if (kIsDebugBuild) {
     ScopedObjectAccess soa(Thread::Current());
     CheckNonImageClassesRemoved();
   }
-#endif
+
   Thread::Current()->TransitionFromSuspendedToRunnable();
   size_t oat_loaded_size = 0;
   size_t oat_data_offset = 0;
@@ -144,8 +136,6 @@ bool ImageWriter::Write(const std::string& image_filename,
   CalculateNewObjectOffsets(oat_loaded_size, oat_data_offset);
   CopyAndFixupObjects();
   PatchOatCodeAndMethods();
-  // Record allocations into the image bitmap.
-  RecordImageAllocations();
   Thread::Current()->TransitionFromRunnableToSuspended(kNative);
 
   UniquePtr<File> image_file(OS::CreateEmptyFile(image_filename.c_str()));
@@ -178,39 +168,82 @@ bool ImageWriter::Write(const std::string& image_filename,
   return true;
 }
 
-void ImageWriter::RecordImageAllocations() {
-  uint64_t start_time = NanoTime();
-  CHECK(image_bitmap_.get() != nullptr);
-  for (const auto& it : offsets_) {
-    mirror::Object* obj = reinterpret_cast<mirror::Object*>(image_->Begin() + it.second);
-    DCHECK_ALIGNED(obj, kObjectAlignment);
-    image_bitmap_->Set(obj);
+void ImageWriter::SetImageOffset(mirror::Object* object, size_t offset) {
+  DCHECK(object != nullptr);
+  DCHECK_NE(offset, 0U);
+  DCHECK(!IsImageOffsetAssigned(object));
+  mirror::Object* obj = reinterpret_cast<mirror::Object*>(image_->Begin() + offset);
+  DCHECK_ALIGNED(obj, kObjectAlignment);
+  image_bitmap_->Set(obj);
+  // Before we stomp over the lock word, save the hash code for later.
+  Monitor::Deflate(Thread::Current(), object);;
+  LockWord lw(object->GetLockWord());
+  switch (lw.GetState()) {
+    case LockWord::kFatLocked: {
+      LOG(FATAL) << "Fat locked object " << obj << " found during object copy";
+      break;
+    }
+    case LockWord::kThinLocked: {
+      LOG(FATAL) << "Thin locked object " << obj << " found during object copy";
+      break;
+    }
+    case LockWord::kUnlocked:
+      // No hash, don't need to save it.
+      break;
+    case LockWord::kHashCode:
+      saved_hashes_.push_back(std::make_pair(obj, lw.GetHashCode()));
+      break;
+    default:
+      LOG(FATAL) << "Unreachable.";
+      break;
   }
-  LOG(INFO) << "RecordImageAllocations took " << PrettyDuration(NanoTime() - start_time);
+  object->SetLockWord(LockWord::FromForwardingAddress(offset));
+  DCHECK(IsImageOffsetAssigned(object));
 }
 
-bool ImageWriter::AllocMemory() {
-  size_t size = 0;
-  for (const auto& space : Runtime::Current()->GetHeap()->GetContinuousSpaces()) {
-    if (space->IsDlMallocSpace()) {
-      size += space->Size();
-    }
-  }
+void ImageWriter::AssignImageOffset(mirror::Object* object) {
+  DCHECK(object != nullptr);
+  SetImageOffset(object, image_end_);
+  image_end_ += RoundUp(object->SizeOf(), 8);  // 64-bit alignment
+  DCHECK_LT(image_end_, image_->Size());
+}
 
-  int prot = PROT_READ | PROT_WRITE;
-  size_t length = RoundUp(size, kPageSize);
+bool ImageWriter::IsImageOffsetAssigned(const mirror::Object* object) const {
+  DCHECK(object != nullptr);
+  return object->GetLockWord().GetState() == LockWord::kForwardingAddress;
+}
+
+size_t ImageWriter::GetImageOffset(const mirror::Object* object) const {
+  DCHECK(object != nullptr);
+  DCHECK(IsImageOffsetAssigned(object));
+  LockWord lock_word = object->GetLockWord();
+  size_t offset = lock_word.ForwardingAddress();
+  DCHECK_LT(offset, image_end_);
+  return offset;
+}
+
+bool ImageWriter::AllocMemory() {
+  size_t length = RoundUp(Runtime::Current()->GetHeap()->GetTotalMemory(), kPageSize);
   std::string error_msg;
-  image_.reset(MemMap::MapAnonymous("image writer image", NULL, length, prot, &error_msg));
+  image_.reset(MemMap::MapAnonymous("image writer image", NULL, length, PROT_READ | PROT_WRITE,
+                                    &error_msg));
   if (UNLIKELY(image_.get() == nullptr)) {
     LOG(ERROR) << "Failed to allocate memory for image file generation: " << error_msg;
     return false;
   }
+
+  // Create the image bitmap.
+  image_bitmap_.reset(gc::accounting::SpaceBitmap::Create("image bitmap", image_->Begin(),
+                                                          length));
+  if (image_bitmap_.get() == nullptr) {
+    LOG(ERROR) << "Failed to allocate memory for image bitmap";
+    return false;
+  }
   return true;
 }
 
 void ImageWriter::ComputeLazyFieldsForImageClasses() {
-  Runtime* runtime = Runtime::Current();
-  ClassLinker* class_linker = runtime->GetClassLinker();
+  ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
   class_linker->VisitClassesWithoutClassesLock(ComputeLazyFieldsForClassesVisitor, NULL);
 }
 
@@ -223,13 +256,12 @@ void ImageWriter::ComputeEagerResolvedStringsCallback(Object* obj, void* arg) {
   if (!obj->GetClass()->IsStringClass()) {
     return;
   }
-  String* string = obj->AsString();
+  mirror::String* string = obj->AsString();
   const uint16_t* utf16_string = string->GetCharArray()->GetData() + string->GetOffset();
-  ImageWriter* writer = reinterpret_cast<ImageWriter*>(arg);
-  for (DexCache* dex_cache : writer->dex_caches_) {
+  for (DexCache* dex_cache : Runtime::Current()->GetClassLinker()->GetDexCaches()) {
     const DexFile& dex_file = *dex_cache->GetDexFile();
     const DexFile::StringId* string_id = dex_file.FindStringId(utf16_string);
-    if (string_id != NULL) {
+    if (string_id != nullptr) {
       // This string occurs in this dex file, assign the dex cache entry.
       uint32_t string_idx = dex_file.GetIndexForStringId(*string_id);
       if (dex_cache->GetResolvedString(string_idx) == NULL) {
@@ -239,13 +271,9 @@ void ImageWriter::ComputeEagerResolvedStringsCallback(Object* obj, void* arg) {
   }
 }
 
-void ImageWriter::ComputeEagerResolvedStrings()
-    SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
-  // TODO: Check image spaces only?
-  gc::Heap* heap = Runtime::Current()->GetHeap();
-  WriterMutexLock mu(Thread::Current(), *Locks::heap_bitmap_lock_);
-  heap->FlushAllocStack();
-  heap->GetLiveBitmap()->Walk(ComputeEagerResolvedStringsCallback, this);
+void ImageWriter::ComputeEagerResolvedStrings() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
+  ReaderMutexLock mu(Thread::Current(), *Locks::heap_bitmap_lock_);
+  Runtime::Current()->GetHeap()->VisitObjects(ComputeEagerResolvedStringsCallback, this);
 }
 
 bool ImageWriter::IsImageClass(const Class* klass) {
@@ -278,7 +306,7 @@ void ImageWriter::PruneNonImageClasses() {
 
   // Clear references to removed classes from the DexCaches.
   ArtMethod* resolution_method = runtime->GetResolutionMethod();
-  for (DexCache* dex_cache : dex_caches_) {
+  for (DexCache* dex_cache : class_linker->GetDexCaches()) {
     for (size_t i = 0; i < dex_cache->NumResolvedTypes(); i++) {
       Class* klass = dex_cache->GetResolvedType(i);
       if (klass != NULL && !IsImageClass(klass)) {
@@ -311,31 +339,22 @@ bool ImageWriter::NonImageClassesVisitor(Class* klass, void* arg) {
 
 void ImageWriter::CheckNonImageClassesRemoved()
     SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
-  if (compiler_driver_.GetImageClasses() == NULL) {
-    return;
-  }
-
-  gc::Heap* heap = Runtime::Current()->GetHeap();
-  Thread* self = Thread::Current();
-  {
-    WriterMutexLock mu(self, *Locks::heap_bitmap_lock_);
-    heap->FlushAllocStack();
+  if (compiler_driver_.GetImageClasses() != nullptr) {
+    gc::Heap* heap = Runtime::Current()->GetHeap();
+    ReaderMutexLock mu(Thread::Current(), *Locks::heap_bitmap_lock_);
+    heap->VisitObjects(CheckNonImageClassesRemovedCallback, this);
   }
-
-  ReaderMutexLock mu(self, *Locks::heap_bitmap_lock_);
-  heap->GetLiveBitmap()->Walk(CheckNonImageClassesRemovedCallback, this);
 }
 
 void ImageWriter::CheckNonImageClassesRemovedCallback(Object* obj, void* arg) {
   ImageWriter* image_writer = reinterpret_cast<ImageWriter*>(arg);
-  if (!obj->IsClass()) {
-    return;
-  }
-  Class* klass = obj->AsClass();
-  if (!image_writer->IsImageClass(klass)) {
-    image_writer->DumpImageClasses();
-    CHECK(image_writer->IsImageClass(klass)) << ClassHelper(klass).GetDescriptor()
-                                             << " " << PrettyDescriptor(klass);
+  if (obj->IsClass()) {
+    Class* klass = obj->AsClass();
+    if (!image_writer->IsImageClass(klass)) {
+      image_writer->DumpImageClasses();
+      CHECK(image_writer->IsImageClass(klass)) << ClassHelper(klass).GetDescriptor()
+                                               << " " << PrettyDescriptor(klass);
+    }
   }
 }
 
@@ -347,53 +366,50 @@ void ImageWriter::DumpImageClasses() {
   }
 }
 
-void ImageWriter::CalculateNewObjectOffsetsCallback(Object* obj, void* arg) {
+void ImageWriter::CalculateObjectOffsets(Object* obj) {
   DCHECK(obj != NULL);
-  DCHECK(arg != NULL);
-  ImageWriter* image_writer = reinterpret_cast<ImageWriter*>(arg);
-
   // if it is a string, we want to intern it if its not interned.
   if (obj->GetClass()->IsStringClass()) {
     // we must be an interned string that was forward referenced and already assigned
-    if (image_writer->IsImageOffsetAssigned(obj)) {
+    if (IsImageOffsetAssigned(obj)) {
       DCHECK_EQ(obj, obj->AsString()->Intern());
       return;
     }
-    SirtRef<String> interned(Thread::Current(), obj->AsString()->Intern());
-    if (obj != interned.get()) {
-      if (!image_writer->IsImageOffsetAssigned(interned.get())) {
+    Thread* self = Thread::Current();
+    SirtRef<Object> sirt_obj(self, obj);
+    mirror::String* interned = obj->AsString()->Intern();
+    if (sirt_obj.get() != interned) {
+      if (!IsImageOffsetAssigned(interned)) {
         // interned obj is after us, allocate its location early
-        image_writer->AssignImageOffset(interned.get());
+        AssignImageOffset(interned);
       }
       // point those looking for this object to the interned version.
-      image_writer->SetImageOffset(obj, image_writer->GetImageOffset(interned.get()));
+      SetImageOffset(sirt_obj.get(), GetImageOffset(interned));
       return;
     }
     // else (obj == interned), nothing to do but fall through to the normal case
   }
 
-  image_writer->AssignImageOffset(obj);
+  AssignImageOffset(obj);
 }
 
 ObjectArray<Object>* ImageWriter::CreateImageRoots() const {
   Runtime* runtime = Runtime::Current();
   ClassLinker* class_linker = runtime->GetClassLinker();
-  Class* object_array_class = class_linker->FindSystemClass("[Ljava/lang/Object;");
   Thread* self = Thread::Current();
+  SirtRef<Class> object_array_class(self, class_linker->FindSystemClass("[Ljava/lang/Object;"));
 
   // build an Object[] of all the DexCaches used in the source_space_
-  ObjectArray<Object>* dex_caches = ObjectArray<Object>::Alloc(self, object_array_class,
-                                                               dex_caches_.size());
+  ObjectArray<Object>* dex_caches = ObjectArray<Object>::Alloc(self, object_array_class.get(),
+                                                               class_linker->GetDexCaches().size());
   int i = 0;
-  for (DexCache* dex_cache : dex_caches_) {
+  for (DexCache* dex_cache : class_linker->GetDexCaches()) {
     dex_caches->Set(i++, dex_cache);
   }
 
   // build an Object[] of the roots needed to restore the runtime
-  SirtRef<ObjectArray<Object> >
-      image_roots(self,
-                  ObjectArray<Object>::Alloc(self, object_array_class,
-                                             ImageHeader::kImageRootsMax));
+  SirtRef<ObjectArray<Object> > image_roots(
+      self, ObjectArray<Object>::Alloc(self, object_array_class.get(), ImageHeader::kImageRootsMax));
   image_roots->Set(ImageHeader::kResolutionMethod, runtime->GetResolutionMethod());
   image_roots->Set(ImageHeader::kImtConflictMethod, runtime->GetImtConflictMethod());
   image_roots->Set(ImageHeader::kDefaultImt, runtime->GetDefaultImt());
@@ -405,24 +421,82 @@ ObjectArray<Object>* ImageWriter::CreateImageRoots() const {
                    runtime->GetCalleeSaveMethod(Runtime::kRefsAndArgs));
   image_roots->Set(ImageHeader::kOatLocation,
                    String::AllocFromModifiedUtf8(self, oat_file_->GetLocation().c_str()));
-  image_roots->Set(ImageHeader::kDexCaches,
-                   dex_caches);
-  image_roots->Set(ImageHeader::kClassRoots,
-                   class_linker->GetClassRoots());
+  image_roots->Set(ImageHeader::kDexCaches, dex_caches);
+  image_roots->Set(ImageHeader::kClassRoots, class_linker->GetClassRoots());
   for (int i = 0; i < ImageHeader::kImageRootsMax; i++) {
     CHECK(image_roots->Get(i) != NULL);
   }
   return image_roots.get();
 }
 
+// Walk instance fields of the given Class. Separate function to allow recursion on the super
+// class.
+void ImageWriter::WalkInstanceFields(mirror::Object* obj, mirror::Class* klass) {
+  // Visit fields of parent classes first.
+  SirtRef<mirror::Class> sirt_class(Thread::Current(), klass);
+  mirror::Class* super = sirt_class->GetSuperClass();
+  if (super != nullptr) {
+    WalkInstanceFields(obj, super);
+  }
+  //
+  size_t num_reference_fields = sirt_class->NumReferenceInstanceFields();
+  for (size_t i = 0; i < num_reference_fields; ++i) {
+    mirror::ArtField* field = sirt_class->GetInstanceField(i);
+    MemberOffset field_offset = field->GetOffset();
+    mirror::Object* value = obj->GetFieldObject<mirror::Object*>(field_offset, false);
+    if (value != nullptr) {
+      WalkFieldsInOrder(value);
+    }
+  }
+}
+
+// For an unvisited object, visit it then all its children found via fields.
+void ImageWriter::WalkFieldsInOrder(mirror::Object* obj) {
+  if (!IsImageOffsetAssigned(obj)) {
+    // Walk instance fields of all objects
+    Thread* self = Thread::Current();
+    SirtRef<mirror::Object> sirt_obj(self, obj);
+    SirtRef<mirror::Class> klass(self, obj->GetClass());
+    // visit the object itself.
+    CalculateObjectOffsets(sirt_obj.get());
+    WalkInstanceFields(sirt_obj.get(), klass.get());
+    // Walk static fields of a Class.
+    if (sirt_obj->IsClass()) {
+      size_t num_static_fields = klass->NumReferenceStaticFields();
+      for (size_t i = 0; i < num_static_fields; ++i) {
+        mirror::ArtField* field = klass->GetStaticField(i);
+        MemberOffset field_offset = field->GetOffset();
+        mirror::Object* value = sirt_obj->GetFieldObject<mirror::Object*>(field_offset, false);
+        if (value != nullptr) {
+          WalkFieldsInOrder(value);
+        }
+      }
+    } else if (sirt_obj->IsObjectArray()) {
+      // Walk elements of an object array.
+      int32_t length = sirt_obj->AsObjectArray<mirror::Object>()->GetLength();
+      for (int32_t i = 0; i < length; i++) {
+        mirror::ObjectArray<mirror::Object>* obj_array = sirt_obj->AsObjectArray<mirror::Object>();
+        mirror::Object* value = obj_array->Get(i);
+        if (value != nullptr) {
+          WalkFieldsInOrder(value);
+        }
+      }
+    }
+  }
+}
+
+void ImageWriter::WalkFieldsCallback(mirror::Object* obj, void* arg) {
+  ImageWriter* writer = reinterpret_cast<ImageWriter*>(arg);
+  DCHECK(writer != nullptr);
+  writer->WalkFieldsInOrder(obj);
+}
+
 void ImageWriter::CalculateNewObjectOffsets(size_t oat_loaded_size, size_t oat_data_offset) {
   CHECK_NE(0U, oat_loaded_size);
   Thread* self = Thread::Current();
   SirtRef<ObjectArray<Object> > image_roots(self, CreateImageRoots());
 
   gc::Heap* heap = Runtime::Current()->GetHeap();
-  const auto& spaces = heap->GetContinuousSpaces();
-  DCHECK(!spaces.empty());
   DCHECK_EQ(0U, image_end_);
 
   // Leave space for the header, but do not write it yet, we need to
@@ -431,21 +505,14 @@ void ImageWriter::CalculateNewObjectOffsets(size_t oat_loaded_size, size_t oat_d
 
   {
     WriterMutexLock mu(self, *Locks::heap_bitmap_lock_);
-    heap->FlushAllocStack();
     // TODO: Image spaces only?
-    // TODO: Add InOrderWalk to heap bitmap.
     const char* old = self->StartAssertNoThreadSuspension("ImageWriter");
-    DCHECK(heap->GetLargeObjectsSpace()->GetLiveObjects()->IsEmpty());
-    for (const auto& space : spaces) {
-      space->GetLiveBitmap()->InOrderWalk(CalculateNewObjectOffsetsCallback, this);
-      DCHECK_LT(image_end_, image_->Size());
-    }
+    DCHECK_LT(image_end_, image_->Size());
+    // Clear any pre-existing monitors which may have been in the monitor words.
+    heap->VisitObjects(WalkFieldsCallback, this);
     self->EndAssertNoThreadSuspension(old);
   }
 
-  // Create the image bitmap.
-  image_bitmap_.reset(gc::accounting::SpaceBitmap::Create("image bitmap", image_->Begin(),
-                                                          image_end_));
   const byte* oat_file_begin = image_begin_ + RoundUp(image_end_, kPageSize);
   const byte* oat_file_end = oat_file_begin + oat_loaded_size;
   oat_data_begin_ = oat_file_begin + oat_data_offset;
@@ -456,7 +523,8 @@ void ImageWriter::CalculateNewObjectOffsets(size_t oat_loaded_size, size_t oat_d
   ImageHeader image_header(reinterpret_cast<uint32_t>(image_begin_),
                            static_cast<uint32_t>(image_end_),
                            RoundUp(image_end_, kPageSize),
-                           image_bitmap_->Size(),
+                           RoundUp(image_end_ / gc::accounting::SpaceBitmap::kAlignment,
+                                   sizeof(size_t)),
                            reinterpret_cast<uint32_t>(GetImageAddress(image_roots.get())),
                            oat_file_->GetOatHeader().GetChecksum(),
                            reinterpret_cast<uint32_t>(oat_file_begin),
@@ -477,17 +545,19 @@ void ImageWriter::CopyAndFixupObjects()
   heap->DisableObjectValidation();
   // TODO: Image spaces only?
   WriterMutexLock mu(self, *Locks::heap_bitmap_lock_);
-  heap->FlushAllocStack();
-  heap->GetLiveBitmap()->Walk(CopyAndFixupObjectsCallback, this);
+  heap->VisitObjects(CopyAndFixupObjectsCallback, this);
+  // Fix up the object previously had hash codes.
+  for (const std::pair<mirror::Object*, uint32_t>& hash_pair : saved_hashes_) {
+    hash_pair.first->SetLockWord(LockWord::FromHashCode(hash_pair.second));
+  }
+  saved_hashes_.clear();
   self->EndAssertNoThreadSuspension(old_cause);
 }
 
-void ImageWriter::CopyAndFixupObjectsCallback(Object* object, void* arg) {
-  DCHECK(object != NULL);
+void ImageWriter::CopyAndFixupObjectsCallback(Object* obj, void* arg) {
+  DCHECK(obj != NULL);
   DCHECK(arg != NULL);
-  const Object* obj = object;
   ImageWriter* image_writer = reinterpret_cast<ImageWriter*>(arg);
-
   // see GetLocalAddress for similar computation
   size_t offset = image_writer->GetImageOffset(obj);
   byte* dst = image_writer->image_->Begin() + offset;
@@ -498,33 +568,7 @@ void ImageWriter::CopyAndFixupObjectsCallback(Object* object, void* arg) {
   Object* copy = reinterpret_cast<Object*>(dst);
   // Write in a hash code of objects which have inflated monitors or a hash code in their monitor
   // word.
-  LockWord lw(copy->GetLockWord());
-  switch (lw.GetState()) {
-    case LockWord::kFatLocked: {
-      Monitor* monitor = lw.FatLockMonitor();
-      CHECK(monitor != nullptr);
-      CHECK(!monitor->IsLocked());
-      if (monitor->HasHashCode()) {
-        copy->SetLockWord(LockWord::FromHashCode(monitor->GetHashCode()));
-      } else {
-        copy->SetLockWord(LockWord());
-      }
-      break;
-    }
-    case LockWord::kThinLocked: {
-      LOG(FATAL) << "Thin locked object " << obj << " found during object copy";
-      break;
-    }
-    case LockWord::kUnlocked:
-      break;
-    case LockWord::kHashCode:
-      // Do nothing since we can just keep the same hash code.
-      CHECK_NE(lw.GetHashCode(), 0);
-      break;
-    default:
-      LOG(FATAL) << "Unreachable.";
-      break;
-  }
+  copy->SetLockWord(LockWord());
   image_writer->FixupObject(obj, copy);
 }
 
@@ -629,19 +673,13 @@ void ImageWriter::FixupInstanceFields(const Object* orig, Object* copy) {
   DCHECK(copy != NULL);
   Class* klass = orig->GetClass();
   DCHECK(klass != NULL);
-  FixupFields(orig,
-              copy,
-              klass->GetReferenceInstanceOffsets(),
-              false);
+  FixupFields(orig, copy, klass->GetReferenceInstanceOffsets(), false);
 }
 
 void ImageWriter::FixupStaticFields(const Class* orig, Class* copy) {
   DCHECK(orig != NULL);
   DCHECK(copy != NULL);
-  FixupFields(orig,
-              copy,
-              orig->GetReferenceStaticOffsets(),
-              true);
+  FixupFields(orig, copy, orig->GetReferenceStaticOffsets(), true);
 }
 
 void ImageWriter::FixupFields(const Object* orig,
@@ -693,11 +731,13 @@ void ImageWriter::FixupFields(const Object* orig,
 static ArtMethod* GetTargetMethod(const CompilerDriver::PatchInformation* patch)
     SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
   ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
-  DexCache* dex_cache = class_linker->FindDexCache(patch->GetDexFile());
+  Thread* self = Thread::Current();
+  SirtRef<mirror::DexCache> dex_cache(self, class_linker->FindDexCache(patch->GetDexFile()));
+  SirtRef<mirror::ClassLoader> class_loader(self, nullptr);
   ArtMethod* method = class_linker->ResolveMethod(patch->GetDexFile(),
                                                   patch->GetTargetMethodIdx(),
                                                   dex_cache,
-                                                  NULL,
+                                                  class_loader,
                                                   NULL,
                                                   patch->GetTargetInvokeType());
   CHECK(method != NULL)
@@ -749,15 +789,15 @@ void ImageWriter::SetPatchLocation(const CompilerDriver::PatchInformation* patch
   // TODO: make this Thumb2 specific
   uint8_t* base = reinterpret_cast<uint8_t*>(reinterpret_cast<uint32_t>(oat_code) & ~0x1);
   uint32_t* patch_location = reinterpret_cast<uint32_t*>(base + patch->GetLiteralOffset());
-#ifndef NDEBUG
-  const DexFile::MethodId& id = patch->GetDexFile().GetMethodId(patch->GetTargetMethodIdx());
-  uint32_t expected = reinterpret_cast<uint32_t>(&id);
-  uint32_t actual = *patch_location;
-  CHECK(actual == expected || actual == value) << std::hex
-    << "actual=" << actual
-    << "expected=" << expected
-    << "value=" << value;
-#endif
+  if (kIsDebugBuild) {
+    const DexFile::MethodId& id = patch->GetDexFile().GetMethodId(patch->GetTargetMethodIdx());
+    uint32_t expected = reinterpret_cast<uint32_t>(&id);
+    uint32_t actual = *patch_location;
+    CHECK(actual == expected || actual == value) << std::hex
+      << "actual=" << actual
+      << "expected=" << expected
+      << "value=" << value;
+  }
   *patch_location = value;
   oat_header.UpdateChecksum(patch_location, sizeof(value));
 }
diff --git a/compiler/image_writer.h b/compiler/image_writer.h
index 0b408e85cc..695f59b40e 100644
--- a/compiler/image_writer.h
+++ b/compiler/image_writer.h
@@ -63,31 +63,11 @@ class ImageWriter {
   void RecordImageAllocations() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
   // We use the lock word to store the offset of the object in the image.
-  void AssignImageOffset(mirror::Object* object)
-      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
-    DCHECK(object != NULL);
-    SetImageOffset(object, image_end_);
-    image_end_ += RoundUp(object->SizeOf(), 8);  // 64-bit alignment
-    DCHECK_LT(image_end_, image_->Size());
-  }
-
-  void SetImageOffset(mirror::Object* object, size_t offset) {
-    DCHECK(object != NULL);
-    DCHECK_NE(offset, 0U);
-    DCHECK(!IsImageOffsetAssigned(object));
-    offsets_.Put(object, offset);
-  }
-
-  size_t IsImageOffsetAssigned(const mirror::Object* object) const {
-    DCHECK(object != NULL);
-    return offsets_.find(object) != offsets_.end();
-  }
-
-  size_t GetImageOffset(const mirror::Object* object) const {
-    DCHECK(object != NULL);
-    DCHECK(IsImageOffsetAssigned(object));
-    return offsets_.find(object)->second;
-  }
+  void AssignImageOffset(mirror::Object* object) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+  void SetImageOffset(mirror::Object* object, size_t offset)
+      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+  bool IsImageOffsetAssigned(const mirror::Object* object) const;
+  size_t GetImageOffset(const mirror::Object* object) const;
 
   mirror::Object* GetImageAddress(const mirror::Object* object) const {
     if (object == NULL) {
@@ -147,7 +127,14 @@ class ImageWriter {
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
   mirror::ObjectArray<mirror::Object>* CreateImageRoots() const
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
-  static void CalculateNewObjectOffsetsCallback(mirror::Object* obj, void* arg)
+  void CalculateObjectOffsets(mirror::Object* obj)
+      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+
+  void WalkInstanceFields(mirror::Object* obj, mirror::Class* klass)
+      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+  void WalkFieldsInOrder(mirror::Object* obj)
+      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+  static void WalkFieldsCallback(mirror::Object* obj, void* arg)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
   // Creates the contiguous image in memory and adjusts pointers.
@@ -180,9 +167,6 @@ class ImageWriter {
 
   const CompilerDriver& compiler_driver_;
 
-  // Map of Object to where it will be at runtime.
-  SafeMap<const mirror::Object*, size_t> offsets_;
-
   // oat file with code for this image
   OatFile* oat_file_;
 
@@ -195,6 +179,9 @@ class ImageWriter {
   // Beginning target image address for the output image.
   byte* image_begin_;
 
+  // Saved hashes (objects are inside of the image so that they don't move).
+  std::vector<std::pair<mirror::Object*, uint32_t> > saved_hashes_;
+
   // Beginning target oat address for the pointers from the output image to its oat file.
   const byte* oat_data_begin_;
 
@@ -211,9 +198,6 @@ class ImageWriter {
   uint32_t quick_imt_conflict_trampoline_offset_;
   uint32_t quick_resolution_trampoline_offset_;
   uint32_t quick_to_interpreter_bridge_offset_;
-
-  // DexCaches seen while scanning for fixing up CodeAndDirectMethods
-  std::set<mirror::DexCache*> dex_caches_;
 };
 
 }  // namespace art
diff --git a/compiler/jni/jni_compiler_test.cc b/compiler/jni/jni_compiler_test.cc
index 667b913039..21dd11ef20 100644
--- a/compiler/jni/jni_compiler_test.cc
+++ b/compiler/jni/jni_compiler_test.cc
@@ -48,9 +48,9 @@ class JniCompilerTest : public CommonTest {
   void CompileForTest(jobject class_loader, bool direct,
                       const char* method_name, const char* method_sig) {
     ScopedObjectAccess soa(Thread::Current());
+    SirtRef<mirror::ClassLoader> loader(soa.Self(), soa.Decode<mirror::ClassLoader*>(class_loader));
     // Compile the native method before starting the runtime
-    mirror::Class* c = class_linker_->FindClass("LMyClassNatives;",
-                                                soa.Decode<mirror::ClassLoader*>(class_loader));
+    mirror::Class* c = class_linker_->FindClass("LMyClassNatives;", loader);
     mirror::ArtMethod* method;
     if (direct) {
       method = c->FindDirectMethod(method_name, method_sig);
diff --git a/compiler/oat_test.cc b/compiler/oat_test.cc
index 6213b45c41..c423f34f7f 100644
--- a/compiler/oat_test.cc
+++ b/compiler/oat_test.cc
@@ -130,7 +130,8 @@ TEST_F(OatTest, WriteRead) {
       num_virtual_methods = it.NumVirtualMethods();
     }
     const char* descriptor = dex_file->GetClassDescriptor(class_def);
-    mirror::Class* klass = class_linker->FindClass(descriptor, NULL);
+    SirtRef<mirror::ClassLoader> loader(Thread::Current(), nullptr);
+    mirror::Class* klass = class_linker->FindClass(descriptor, loader);
 
     UniquePtr<const OatFile::OatClass> oat_class(oat_dex_file->GetOatClass(i));
     CHECK_EQ(mirror::Class::Status::kStatusNotReady, oat_class->GetStatus()) << descriptor;
diff --git a/compiler/oat_writer.cc b/compiler/oat_writer.cc
index f3bb11272e..28fb1479d7 100644
--- a/compiler/oat_writer.cc
+++ b/compiler/oat_writer.cc
@@ -405,23 +405,23 @@ size_t OatWriter::InitOatCodeMethod(size_t offset, size_t oat_class_index,
     size_t gc_map_size = gc_map.size() * sizeof(gc_map[0]);
     gc_map_offset = (gc_map_size == 0) ? 0 : offset;
 
-#if !defined(NDEBUG)
-    // We expect GC maps except when the class hasn't been verified or the method is native
-    ClassReference class_ref(&dex_file, class_def_index);
-    CompiledClass* compiled_class = compiler_driver_->GetCompiledClass(class_ref);
-    mirror::Class::Status status;
-    if (compiled_class != NULL) {
-      status = compiled_class->GetStatus();
-    } else if (verifier::MethodVerifier::IsClassRejected(class_ref)) {
-      status = mirror::Class::kStatusError;
-    } else {
-      status = mirror::Class::kStatusNotReady;
+    if (kIsDebugBuild) {
+      // We expect GC maps except when the class hasn't been verified or the method is native
+      ClassReference class_ref(&dex_file, class_def_index);
+      CompiledClass* compiled_class = compiler_driver_->GetCompiledClass(class_ref);
+      mirror::Class::Status status;
+      if (compiled_class != NULL) {
+        status = compiled_class->GetStatus();
+      } else if (verifier::MethodVerifier::IsClassRejected(class_ref)) {
+        status = mirror::Class::kStatusError;
+      } else {
+        status = mirror::Class::kStatusNotReady;
+      }
+      CHECK(gc_map_size != 0 || is_native || status < mirror::Class::kStatusVerified)
+          << &gc_map << " " << gc_map_size << " " << (is_native ? "true" : "false") << " "
+          << (status < mirror::Class::kStatusVerified) << " " << status << " "
+          << PrettyMethod(method_idx, dex_file);
     }
-    CHECK(gc_map_size != 0 || is_native || status < mirror::Class::kStatusVerified)
-        << &gc_map << " " << gc_map_size << " " << (is_native ? "true" : "false") << " "
-        << (status < mirror::Class::kStatusVerified) << " " << status << " "
-        << PrettyMethod(method_idx, dex_file);
-#endif
 
     // Deduplicate GC maps
     SafeMap<const std::vector<uint8_t>*, uint32_t>::iterator gc_map_iter =
@@ -448,11 +448,12 @@ size_t OatWriter::InitOatCodeMethod(size_t offset, size_t oat_class_index,
 
   if (compiler_driver_->IsImage()) {
     ClassLinker* linker = Runtime::Current()->GetClassLinker();
-    mirror::DexCache* dex_cache = linker->FindDexCache(dex_file);
     // Unchecked as we hold mutator_lock_ on entry.
     ScopedObjectAccessUnchecked soa(Thread::Current());
+    SirtRef<mirror::DexCache> dex_cache(soa.Self(), linker->FindDexCache(dex_file));
+    SirtRef<mirror::ClassLoader> class_loader(soa.Self(), nullptr);
     mirror::ArtMethod* method = linker->ResolveMethod(dex_file, method_idx, dex_cache,
-                                                      NULL, NULL, invoke_type);
+                                                      class_loader, nullptr, invoke_type);
     CHECK(method != NULL);
     method->SetFrameSizeInBytes(frame_size_in_bytes);
     method->SetCoreSpillMask(core_spill_mask);