Change dex caches to be weak roots

Changed dex caches to be weak roots. This is necessary for class
unloading since the resolved types arrays would keep classes live
when they should be unloaded. Currently the dex caches still don't
get freed due to the class loader roots.

Also deleted some unused functionality in image writer.

Bug: 22720414
Change-Id: If22cb3cad7e3baabc8158a77d7f20799faf4c341
diff --git a/compiler/driver/compiler_driver-inl.h b/compiler/driver/compiler_driver-inl.h
index 80387f2..83f391d 100644
--- a/compiler/driver/compiler_driver-inl.h
+++ b/compiler/driver/compiler_driver-inl.h
@@ -31,7 +31,7 @@
 namespace art {
 
 inline mirror::DexCache* CompilerDriver::GetDexCache(const DexCompilationUnit* mUnit) {
-  return mUnit->GetClassLinker()->FindDexCache(*mUnit->GetDexFile());
+  return mUnit->GetClassLinker()->FindDexCache(*mUnit->GetDexFile(), false);
 }
 
 inline mirror::ClassLoader* CompilerDriver::GetClassLoader(ScopedObjectAccess& soa,
@@ -87,7 +87,7 @@
 }
 
 inline mirror::DexCache* CompilerDriver::FindDexCache(const DexFile* dex_file) {
-  return Runtime::Current()->GetClassLinker()->FindDexCache(*dex_file);
+  return Runtime::Current()->GetClassLinker()->FindDexCache(*dex_file, false);
 }
 
 inline ArtField* CompilerDriver::ResolveField(
@@ -339,7 +339,7 @@
     // Sharpen a virtual call into a direct call. The method_idx is into referrer's
     // dex cache, check that this resolved method is where we expect it.
     CHECK_EQ(target_method->dex_file, mUnit->GetDexFile());
-    DCHECK_EQ(dex_cache.Get(), mUnit->GetClassLinker()->FindDexCache(*mUnit->GetDexFile()));
+    DCHECK_EQ(dex_cache.Get(), mUnit->GetClassLinker()->FindDexCache(*mUnit->GetDexFile(), false));
     CHECK_EQ(referrer_class->GetDexCache()->GetResolvedMethod(
         target_method->dex_method_index, pointer_size),
              resolved_method) << PrettyMethod(resolved_method);
@@ -369,7 +369,7 @@
           nullptr, kVirtual);
     } else {
       StackHandleScope<1> hs(soa.Self());
-      auto target_dex_cache(hs.NewHandle(class_linker->FindDexCache(*devirt_target->dex_file)));
+      auto target_dex_cache(hs.NewHandle(class_linker->RegisterDexFile(*devirt_target->dex_file)));
       called_method = class_linker->ResolveMethod(
           *devirt_target->dex_file, devirt_target->dex_method_index, target_dex_cache,
           class_loader, nullptr, kVirtual);
diff --git a/compiler/driver/compiler_driver.cc b/compiler/driver/compiler_driver.cc
index fa25a17..d38677e 100644
--- a/compiler/driver/compiler_driver.cc
+++ b/compiler/driver/compiler_driver.cc
@@ -936,7 +936,7 @@
       uint16_t exception_type_idx = exception_type.first;
       const DexFile* dex_file = exception_type.second;
       StackHandleScope<2> hs2(self);
-      Handle<mirror::DexCache> dex_cache(hs2.NewHandle(class_linker->FindDexCache(*dex_file)));
+      Handle<mirror::DexCache> dex_cache(hs2.NewHandle(class_linker->RegisterDexFile(*dex_file)));
       Handle<mirror::Class> klass(hs2.NewHandle(
           class_linker->ResolveType(*dex_file, exception_type_idx, dex_cache,
                                     NullHandle<mirror::ClassLoader>())));
@@ -1170,7 +1170,8 @@
       IsImageClass(dex_file.StringDataByIdx(dex_file.GetTypeId(type_idx).descriptor_idx_))) {
     {
       ScopedObjectAccess soa(Thread::Current());
-      mirror::DexCache* dex_cache = Runtime::Current()->GetClassLinker()->FindDexCache(dex_file);
+      mirror::DexCache* dex_cache = Runtime::Current()->GetClassLinker()->FindDexCache(
+          dex_file, false);
       mirror::Class* resolved_class = dex_cache->GetResolvedType(type_idx);
       if (resolved_class == nullptr) {
         // Erroneous class.
@@ -1195,9 +1196,9 @@
     // We resolve all const-string strings when building for the image.
     ScopedObjectAccess soa(Thread::Current());
     StackHandleScope<1> hs(soa.Self());
-    Handle<mirror::DexCache> dex_cache(
-        hs.NewHandle(Runtime::Current()->GetClassLinker()->FindDexCache(dex_file)));
-    Runtime::Current()->GetClassLinker()->ResolveString(dex_file, string_idx, dex_cache);
+    ClassLinker* const class_linker = Runtime::Current()->GetClassLinker();
+    Handle<mirror::DexCache> dex_cache(hs.NewHandle(class_linker->FindDexCache(dex_file, false)));
+    class_linker->ResolveString(dex_file, string_idx, dex_cache);
     result = true;
   }
   if (result) {
@@ -1222,7 +1223,7 @@
     *equals_referrers_class = false;
   }
   ScopedObjectAccess soa(Thread::Current());
-  mirror::DexCache* dex_cache = Runtime::Current()->GetClassLinker()->FindDexCache(dex_file);
+  mirror::DexCache* dex_cache = Runtime::Current()->GetClassLinker()->FindDexCache(dex_file, false);
   // Get type from dex cache assuming it was populated by the verifier
   mirror::Class* resolved_class = dex_cache->GetResolvedType(type_idx);
   if (resolved_class == nullptr) {
@@ -1259,7 +1260,8 @@
                                                             const DexFile& dex_file,
                                                             uint32_t type_idx) {
   ScopedObjectAccess soa(Thread::Current());
-  mirror::DexCache* dex_cache = Runtime::Current()->GetClassLinker()->FindDexCache(dex_file);
+  mirror::DexCache* dex_cache = Runtime::Current()->GetClassLinker()->FindDexCache(
+      dex_file, false);
   // Get type from dex cache assuming it was populated by the verifier.
   mirror::Class* resolved_class = dex_cache->GetResolvedType(type_idx);
   if (resolved_class == nullptr) {
@@ -1288,7 +1290,7 @@
                                         uintptr_t* direct_type_ptr, bool* out_is_finalizable) {
   ScopedObjectAccess soa(Thread::Current());
   Runtime* runtime = Runtime::Current();
-  mirror::DexCache* dex_cache = runtime->GetClassLinker()->FindDexCache(dex_file);
+  mirror::DexCache* dex_cache = runtime->GetClassLinker()->FindDexCache(dex_file, false);
   mirror::Class* resolved_class = dex_cache->GetResolvedType(type_idx);
   if (resolved_class == nullptr) {
     return false;
@@ -1417,7 +1419,7 @@
   {
     StackHandleScope<2> hs(soa.Self());
     Handle<mirror::DexCache> dex_cache_handle(
-        hs.NewHandle(mUnit->GetClassLinker()->FindDexCache(*mUnit->GetDexFile())));
+        hs.NewHandle(mUnit->GetClassLinker()->FindDexCache(*mUnit->GetDexFile(), false)));
     Handle<mirror::ClassLoader> class_loader_handle(
         hs.NewHandle(soa.Decode<mirror::ClassLoader*>(mUnit->GetClassLoader())));
     resolved_field =
@@ -1467,7 +1469,7 @@
   {
     StackHandleScope<2> hs(soa.Self());
     Handle<mirror::DexCache> dex_cache_handle(
-        hs.NewHandle(mUnit->GetClassLinker()->FindDexCache(*mUnit->GetDexFile())));
+        hs.NewHandle(mUnit->GetClassLinker()->FindDexCache(*mUnit->GetDexFile(), false)));
     Handle<mirror::ClassLoader> class_loader_handle(
         hs.NewHandle(soa.Decode<mirror::ClassLoader*>(mUnit->GetClassLoader())));
     resolved_field =
@@ -1653,7 +1655,7 @@
   // Try to resolve the method and compiling method's class.
   StackHandleScope<3> hs(soa.Self());
   Handle<mirror::DexCache> dex_cache(
-      hs.NewHandle(mUnit->GetClassLinker()->FindDexCache(*mUnit->GetDexFile())));
+      hs.NewHandle(mUnit->GetClassLinker()->FindDexCache(*mUnit->GetDexFile(), false)));
   Handle<mirror::ClassLoader> class_loader(hs.NewHandle(
       soa.Decode<mirror::ClassLoader*>(mUnit->GetClassLoader())));
   uint32_t method_idx = target_method->dex_method_index;
@@ -1905,7 +1907,7 @@
     StackHandleScope<2> hs(soa.Self());
     Handle<mirror::ClassLoader> class_loader(
         hs.NewHandle(soa.Decode<mirror::ClassLoader*>(jclass_loader)));
-    Handle<mirror::DexCache> dex_cache(hs.NewHandle(class_linker->FindDexCache(dex_file)));
+    Handle<mirror::DexCache> dex_cache(hs.NewHandle(class_linker->FindDexCache(dex_file, false)));
     // Resolve the class.
     mirror::Class* klass = class_linker->ResolveType(dex_file, class_def.class_idx_, dex_cache,
                                                      class_loader);
@@ -1998,7 +2000,7 @@
     ClassLinker* class_linker = manager_->GetClassLinker();
     const DexFile& dex_file = *manager_->GetDexFile();
     StackHandleScope<2> hs(soa.Self());
-    Handle<mirror::DexCache> dex_cache(hs.NewHandle(class_linker->FindDexCache(dex_file)));
+    Handle<mirror::DexCache> dex_cache(hs.NewHandle(class_linker->RegisterDexFile(dex_file)));
     Handle<mirror::ClassLoader> class_loader(
         hs.NewHandle(soa.Decode<mirror::ClassLoader*>(manager_->GetClassLoader())));
     mirror::Class* klass = class_linker->ResolveType(dex_file, type_idx, dex_cache, class_loader);
@@ -2084,7 +2086,7 @@
        * 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.
        */
-      Handle<mirror::DexCache> dex_cache(hs.NewHandle(class_linker->FindDexCache(dex_file)));
+      Handle<mirror::DexCache> dex_cache(hs.NewHandle(class_linker->FindDexCache(dex_file, false)));
       std::string error_msg;
       if (verifier::MethodVerifier::VerifyClass(soa.Self(), &dex_file, dex_cache, class_loader,
                                                 &class_def, true, &error_msg) ==
diff --git a/compiler/image_writer.cc b/compiler/image_writer.cc
index 93897aa..dbd3366 100644
--- a/compiler/image_writer.cc
+++ b/compiler/image_writer.cc
@@ -70,7 +70,6 @@
 
 // Separate objects into multiple bins to optimize dirty memory use.
 static constexpr bool kBinObjects = true;
-static constexpr bool kComputeEagerResolvedStrings = false;
 
 static void CheckNoDexObjectsCallback(Object* obj, void* arg ATTRIBUTE_UNUSED)
     SHARED_REQUIRES(Locks::mutator_lock_) {
@@ -90,11 +89,6 @@
     PruneNonImageClasses();  // Remove junk
     ComputeLazyFieldsForImageClasses();  // Add useful information
 
-    // Calling this can in theory fill in some resolved strings. However, in practice it seems to
-    // never resolve any.
-    if (kComputeEagerResolvedStrings) {
-      ComputeEagerResolvedStrings();
-    }
     Thread::Current()->TransitionFromRunnableToSuspended(kNative);
   }
   gc::Heap* heap = Runtime::Current()->GetHeap();
@@ -302,11 +296,15 @@
 
 void ImageWriter::PrepareDexCacheArraySlots() {
   ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
-  ReaderMutexLock mu(Thread::Current(), *class_linker->DexLock());
-  size_t dex_cache_count = class_linker->GetDexCacheCount();
+  Thread* const self = Thread::Current();
+  ReaderMutexLock mu(self, *class_linker->DexLock());
   uint32_t size = 0u;
-  for (size_t idx = 0; idx < dex_cache_count; ++idx) {
-    DexCache* dex_cache = class_linker->GetDexCache(idx);
+  for (jobject weak_root : class_linker->GetDexCaches()) {
+    mirror::DexCache* dex_cache =
+        down_cast<mirror::DexCache*>(self->DecodeJObject(weak_root));
+    if (dex_cache == nullptr) {
+      continue;
+    }
     const DexFile* dex_file = dex_cache->GetDexFile();
     dex_cache_array_starts_.Put(dex_file, size);
     DexCacheArraysLayout layout(target_ptr_size_, dex_file);
@@ -554,39 +552,6 @@
   class_linker->VisitClassesWithoutClassesLock(&visitor);
 }
 
-void ImageWriter::ComputeEagerResolvedStringsCallback(Object* obj, void* arg ATTRIBUTE_UNUSED) {
-  if (!obj->GetClass()->IsStringClass()) {
-    return;
-  }
-  mirror::String* string = obj->AsString();
-  const uint16_t* utf16_string = string->GetValue();
-  size_t utf16_length = static_cast<size_t>(string->GetLength());
-  ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
-  ReaderMutexLock mu(Thread::Current(), *class_linker->DexLock());
-  size_t dex_cache_count = class_linker->GetDexCacheCount();
-  for (size_t i = 0; i < dex_cache_count; ++i) {
-    DexCache* dex_cache = class_linker->GetDexCache(i);
-    const DexFile& dex_file = *dex_cache->GetDexFile();
-    const DexFile::StringId* string_id;
-    if (UNLIKELY(utf16_length == 0)) {
-      string_id = dex_file.FindStringId("");
-    } else {
-      string_id = dex_file.FindStringId(utf16_string, utf16_length);
-    }
-    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) == nullptr) {
-        dex_cache->SetResolvedString(string_idx, string);
-      }
-    }
-  }
-}
-
-void ImageWriter::ComputeEagerResolvedStrings() {
-  Runtime::Current()->GetHeap()->VisitObjects(ComputeEagerResolvedStringsCallback, this);
-}
-
 bool ImageWriter::IsImageClass(Class* klass) {
   if (klass == nullptr) {
     return false;
@@ -631,16 +596,14 @@
 
   // Clear references to removed classes from the DexCaches.
   const ArtMethod* resolution_method = runtime->GetResolutionMethod();
-  size_t dex_cache_count;
-  {
-    ReaderMutexLock mu(self, *class_linker->DexLock());
-    dex_cache_count = class_linker->GetDexCacheCount();
-  }
-  for (size_t idx = 0; idx < dex_cache_count; ++idx) {
-    DexCache* dex_cache;
-    {
-      ReaderMutexLock mu(self, *class_linker->DexLock());
-      dex_cache = class_linker->GetDexCache(idx);
+
+  ScopedAssertNoThreadSuspension sa(self, __FUNCTION__);
+  ReaderMutexLock mu(self, *Locks::classlinker_classes_lock_);  // For ClassInClassTable
+  ReaderMutexLock mu2(self, *class_linker->DexLock());
+  for (jobject weak_root : class_linker->GetDexCaches()) {
+    mirror::DexCache* dex_cache = down_cast<mirror::DexCache*>(self->DecodeJObject(weak_root));
+    if (dex_cache == nullptr) {
+      continue;
     }
     for (size_t i = 0; i < dex_cache->NumResolvedTypes(); i++) {
       Class* klass = dex_cache->GetResolvedType(i);
@@ -762,8 +725,12 @@
     ReaderMutexLock mu(self, *class_linker->DexLock());
     CHECK_EQ(dex_cache_count, class_linker->GetDexCacheCount())
         << "The number of dex caches changed.";
-    for (size_t i = 0; i < dex_cache_count; ++i) {
-      dex_caches->Set<false>(i, class_linker->GetDexCache(i));
+    size_t i = 0;
+    for (jobject weak_root : class_linker->GetDexCaches()) {
+      mirror::DexCache* dex_cache =
+          down_cast<mirror::DexCache*>(self->DecodeJObject(weak_root));
+      dex_caches->Set<false>(i, dex_cache);
+      ++i;
     }
   }
 
diff --git a/compiler/image_writer.h b/compiler/image_writer.h
index c8aa82d..778521c 100644
--- a/compiler/image_writer.h
+++ b/compiler/image_writer.h
@@ -225,11 +225,6 @@
   void ComputeLazyFieldsForImageClasses()
       SHARED_REQUIRES(Locks::mutator_lock_);
 
-  // Wire dex cache resolved strings to strings in the image to avoid runtime resolution.
-  void ComputeEagerResolvedStrings() SHARED_REQUIRES(Locks::mutator_lock_);
-  static void ComputeEagerResolvedStringsCallback(mirror::Object* obj, void* arg)
-      SHARED_REQUIRES(Locks::mutator_lock_);
-
   // Remove unwanted classes from various roots.
   void PruneNonImageClasses() SHARED_REQUIRES(Locks::mutator_lock_);
 
diff --git a/compiler/oat_writer.cc b/compiler/oat_writer.cc
index 64e7487..0e0b224 100644
--- a/compiler/oat_writer.cc
+++ b/compiler/oat_writer.cc
@@ -691,6 +691,8 @@
     OatClass* oat_class = writer_->oat_classes_[oat_class_index_];
     const CompiledMethod* compiled_method = oat_class->GetCompiledMethod(class_def_method_index);
 
+    // No thread suspension since dex_cache_ that may get invalidated if that occurs.
+    ScopedAssertNoThreadSuspension tsc(Thread::Current(), __FUNCTION__);
     if (compiled_method != nullptr) {  // ie. not an abstract method
       size_t file_offset = file_offset_;
       OutputStream* out = out_;
diff --git a/compiler/optimizing/reference_type_propagation.cc b/compiler/optimizing/reference_type_propagation.cc
index 97b9972..824f28e 100644
--- a/compiler/optimizing/reference_type_propagation.cc
+++ b/compiler/optimizing/reference_type_propagation.cc
@@ -362,7 +362,7 @@
     if (kIsDebugBuild) {
       ScopedObjectAccess soa(Thread::Current());
       ClassLinker* cl = Runtime::Current()->GetClassLinker();
-      mirror::DexCache* dex_cache = cl->FindDexCache(instr->AsInvoke()->GetDexFile());
+      mirror::DexCache* dex_cache = cl->FindDexCache(instr->AsInvoke()->GetDexFile(), false);
       ArtMethod* method = dex_cache->GetResolvedMethod(
           instr->AsInvoke()->GetDexMethodIndex(), cl->GetImagePointerSize());
       DCHECK(method != nullptr);
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index 99736e9..07cf88c 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -1207,6 +1207,14 @@
     oat_file_.reset();
   }
 
+  void Shutdown() {
+    ScopedObjectAccess soa(Thread::Current());
+    for (jobject dex_cache : dex_caches_) {
+      soa.Env()->DeleteLocalRef(dex_cache);
+    }
+    dex_caches_.clear();
+  }
+
   // Set up the environment for compilation. Includes starting the runtime and loading/opening the
   // boot class path.
   bool Setup() {
@@ -1320,8 +1328,9 @@
       compiled_methods_.reset(nullptr);  // By default compile everything.
     }
 
+    ClassLinker* const class_linker = Runtime::Current()->GetClassLinker();
     if (boot_image_option_.empty()) {
-      dex_files_ = Runtime::Current()->GetClassLinker()->GetBootClassPath();
+      dex_files_ = class_linker->GetBootClassPath();
     } else {
       if (dex_filenames_.empty()) {
         ATRACE_BEGIN("Opening zip archive from file descriptor");
@@ -1374,11 +1383,15 @@
         }
       }
     }
-    // Ensure opened dex files are writable for dex-to-dex transformations.
+    // Ensure opened dex files are writable for dex-to-dex transformations. Also ensure that
+    // the dex caches stay live since we don't want class unloading to occur during compilation.
     for (const auto& dex_file : dex_files_) {
       if (!dex_file->EnableWrite()) {
         PLOG(ERROR) << "Failed to make .dex file writeable '" << dex_file->GetLocation() << "'\n";
       }
+      ScopedObjectAccess soa(self);
+      dex_caches_.push_back(soa.AddLocalReference<jobject>(
+          class_linker->RegisterDexFile(*dex_file)));
     }
 
     // If we use a swap file, ensure we are above the threshold to make it necessary.
@@ -1423,6 +1436,7 @@
     // Handle and ClassLoader creation needs to come after Runtime::Create
     jobject class_loader = nullptr;
     Thread* self = Thread::Current();
+
     if (!boot_image_option_.empty()) {
       ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
       OpenClassPathFiles(runtime_->GetClassPathString(), dex_files_, &class_path_files_);
@@ -1957,6 +1971,7 @@
   bool is_host_;
   std::string android_root_;
   std::vector<const DexFile*> dex_files_;
+  std::vector<jobject> dex_caches_;
   std::vector<std::unique_ptr<const DexFile>> opened_dex_files_;
   std::unique_ptr<CompilerDriver> driver_;
   std::vector<std::string> verbose_methods_;
@@ -2107,11 +2122,15 @@
     return EXIT_FAILURE;
   }
 
+  bool result;
   if (dex2oat.IsImage()) {
-    return CompileImage(dex2oat);
+    result = CompileImage(dex2oat);
   } else {
-    return CompileApp(dex2oat);
+    result = CompileApp(dex2oat);
   }
+
+  dex2oat.Shutdown();
+  return result;
 }
 }  // namespace art
 
diff --git a/oatdump/oatdump.cc b/oatdump/oatdump.cc
index 44b78ff..1950d56 100644
--- a/oatdump/oatdump.cc
+++ b/oatdump/oatdump.cc
@@ -812,11 +812,15 @@
       DumpDexCode(vios->Stream(), dex_file, code_item);
     }
 
+    std::unique_ptr<StackHandleScope<1>> hs;
     std::unique_ptr<verifier::MethodVerifier> verifier;
     if (Runtime::Current() != nullptr) {
+      // We need to have the handle scope stay live until after the verifier since the verifier has
+      // a handle to the dex cache from hs.
+      hs.reset(new StackHandleScope<1>(Thread::Current()));
       vios->Stream() << "VERIFIER TYPE ANALYSIS:\n";
       ScopedIndentation indent2(vios);
-      verifier.reset(DumpVerifier(vios,
+      verifier.reset(DumpVerifier(vios, hs.get(),
                                   dex_method_idx, &dex_file, class_def, code_item,
                                   method_access_flags));
     }
@@ -1389,6 +1393,7 @@
   }
 
   verifier::MethodVerifier* DumpVerifier(VariableIndentationOutputStream* vios,
+                                         StackHandleScope<1>* hs,
                                          uint32_t dex_method_idx,
                                          const DexFile* dex_file,
                                          const DexFile::ClassDef& class_def,
@@ -1396,9 +1401,8 @@
                                          uint32_t method_access_flags) {
     if ((method_access_flags & kAccNative) == 0) {
       ScopedObjectAccess soa(Thread::Current());
-      StackHandleScope<1> hs(soa.Self());
       Handle<mirror::DexCache> dex_cache(
-          hs.NewHandle(Runtime::Current()->GetClassLinker()->FindDexCache(*dex_file)));
+          hs->NewHandle(Runtime::Current()->GetClassLinker()->RegisterDexFile(*dex_file)));
       DCHECK(options_.class_loader_ != nullptr);
       return verifier::MethodVerifier::VerifyMethodAndDump(
           soa.Self(), vios, dex_method_idx, dex_file, dex_cache, *options_.class_loader_,
@@ -1599,10 +1603,13 @@
       dex_cache_arrays_.clear();
       {
         ReaderMutexLock mu(self, *class_linker->DexLock());
-        for (size_t i = 0; i < class_linker->GetDexCacheCount(); ++i) {
-          auto* dex_cache = class_linker->GetDexCache(i);
-          dex_cache_arrays_.insert(dex_cache->GetResolvedFields());
-          dex_cache_arrays_.insert(dex_cache->GetResolvedMethods());
+        for (jobject weak_root : class_linker->GetDexCaches()) {
+          mirror::DexCache* dex_cache =
+              down_cast<mirror::DexCache*>(self->DecodeJObject(weak_root));
+          if (dex_cache != nullptr) {
+            dex_cache_arrays_.insert(dex_cache->GetResolvedFields());
+            dex_cache_arrays_.insert(dex_cache->GetResolvedMethods());
+          }
         }
       }
       ReaderMutexLock mu(self, *Locks::heap_bitmap_lock_);
diff --git a/runtime/base/mutex.h b/runtime/base/mutex.h
index 848c904..5f2caef 100644
--- a/runtime/base/mutex.h
+++ b/runtime/base/mutex.h
@@ -64,6 +64,7 @@
   kJdwpSocketLock,
   kRegionSpaceRegionLock,
   kTransactionLogLock,
+  kJniWeakGlobalsLock,
   kReferenceQueueSoftReferencesLock,
   kReferenceQueuePhantomReferencesLock,
   kReferenceQueueFinalizerReferencesLock,
diff --git a/runtime/class_linker-inl.h b/runtime/class_linker-inl.h
index 11901b3..d2dbff6 100644
--- a/runtime/class_linker-inl.h
+++ b/runtime/class_linker-inl.h
@@ -195,12 +195,6 @@
   return klass;
 }
 
-inline mirror::DexCache* ClassLinker::GetDexCache(size_t idx) {
-  dex_lock_.AssertSharedHeld(Thread::Current());
-  DCHECK(idx < dex_caches_.size());
-  return dex_caches_[idx].Read();
-}
-
 }  // namespace art
 
 #endif  // ART_RUNTIME_CLASS_LINKER_INL_H_
diff --git a/runtime/class_linker.cc b/runtime/class_linker.cc
index dc273d8..667dae2 100644
--- a/runtime/class_linker.cc
+++ b/runtime/class_linker.cc
@@ -273,7 +273,6 @@
       array_iftable_(nullptr),
       find_array_class_cache_next_victim_(0),
       init_done_(false),
-      log_new_dex_caches_roots_(false),
       log_new_class_table_roots_(false),
       intern_table_(intern_table),
       quick_resolution_trampoline_(nullptr),
@@ -332,6 +331,12 @@
   java_lang_Class->SetSuperClass(java_lang_Object.Get());
   mirror::Class::SetStatus(java_lang_Object, mirror::Class::kStatusLoaded, self);
 
+  java_lang_Object->SetObjectSize(sizeof(mirror::Object));
+  runtime->SetSentinel(heap->AllocObject<true>(self,
+                                               java_lang_Object.Get(),
+                                               java_lang_Object->GetObjectSize(),
+                                               VoidFunctor()));
+
   // Object[] next to hold class roots.
   Handle<mirror::Class> object_array_class(hs.NewHandle(
       AllocClass(self, java_lang_Class.Get(),
@@ -1139,11 +1144,11 @@
   quick_imt_conflict_trampoline_ = oat_file.GetOatHeader().GetQuickImtConflictTrampoline();
   quick_generic_jni_trampoline_ = oat_file.GetOatHeader().GetQuickGenericJniTrampoline();
   quick_to_interpreter_bridge_trampoline_ = oat_file.GetOatHeader().GetQuickToInterpreterBridge();
+  StackHandleScope<2> hs(self);
   mirror::Object* dex_caches_object = space->GetImageHeader().GetImageRoot(ImageHeader::kDexCaches);
-  mirror::ObjectArray<mirror::DexCache>* dex_caches =
-      dex_caches_object->AsObjectArray<mirror::DexCache>();
+  Handle<mirror::ObjectArray<mirror::DexCache>> dex_caches(
+      hs.NewHandle(dex_caches_object->AsObjectArray<mirror::DexCache>()));
 
-  StackHandleScope<1> hs(self);
   Handle<mirror::ObjectArray<mirror::Class>> class_roots(hs.NewHandle(
           space->GetImageHeader().GetImageRoot(ImageHeader::kClassRoots)->
           AsObjectArray<mirror::Class>()));
@@ -1153,6 +1158,13 @@
   // as being Strings or not
   mirror::String::SetClass(GetClassRoot(kJavaLangString));
 
+  mirror::Class* java_lang_Object = GetClassRoot(kJavaLangObject);
+  java_lang_Object->SetObjectSize(sizeof(mirror::Object));
+  Runtime::Current()->SetSentinel(Runtime::Current()->GetHeap()->AllocObject<true>(self,
+                                                          java_lang_Object,
+                                                          java_lang_Object->GetObjectSize(),
+                                                          VoidFunctor()));
+
   CHECK_EQ(oat_file.GetOatHeader().GetDexFileCount(),
            static_cast<uint32_t>(dex_caches->GetLength()));
   for (int32_t i = 0; i < dex_caches->GetLength(); i++) {
@@ -1246,7 +1258,6 @@
 }
 
 bool ClassLinker::ClassInClassTable(mirror::Class* klass) {
-  ReaderMutexLock mu(Thread::Current(), *Locks::classlinker_classes_lock_);
   ClassTable* const class_table = ClassTableForClassLoader(klass->GetClassLoader());
   return class_table != nullptr && class_table->Contains(klass);
 }
@@ -1303,27 +1314,6 @@
 // mapped image.
 void ClassLinker::VisitRoots(RootVisitor* visitor, VisitRootFlags flags) {
   class_roots_.VisitRootIfNonNull(visitor, RootInfo(kRootVMInternal));
-  Thread* const self = Thread::Current();
-  {
-    ReaderMutexLock mu(self, dex_lock_);
-    if ((flags & kVisitRootFlagAllRoots) != 0) {
-      for (GcRoot<mirror::DexCache>& dex_cache : dex_caches_) {
-        dex_cache.VisitRoot(visitor, RootInfo(kRootVMInternal));
-      }
-    } else if ((flags & kVisitRootFlagNewRoots) != 0) {
-      for (size_t index : new_dex_cache_roots_) {
-        dex_caches_[index].VisitRoot(visitor, RootInfo(kRootVMInternal));
-      }
-    }
-    if ((flags & kVisitRootFlagClearRootLog) != 0) {
-      new_dex_cache_roots_.clear();
-    }
-    if ((flags & kVisitRootFlagStartLoggingNewRoots) != 0) {
-      log_new_dex_caches_roots_ = true;
-    } else if ((flags & kVisitRootFlagStopLoggingNewRoots) != 0) {
-      log_new_dex_caches_roots_ = false;
-    }
-  }
   VisitClassRoots(visitor, flags);
   array_iftable_.VisitRootIfNonNull(visitor, RootInfo(kRootVMInternal));
   for (GcRoot<mirror::Class>& root : find_array_class_cache_) {
@@ -1698,7 +1688,6 @@
                 long_array->GetWithoutChecks(j)));
             const DexFile::ClassDef* dex_class_def = cp_dex_file->FindClassDef(descriptor, hash);
             if (dex_class_def != nullptr) {
-              RegisterDexFile(*cp_dex_file);
               mirror::Class* klass = DefineClass(self, descriptor, hash, class_loader,
                                                  *cp_dex_file, *dex_class_def);
               if (klass == nullptr) {
@@ -1844,7 +1833,7 @@
     CHECK(self->IsExceptionPending());  // Expect an OOME.
     return nullptr;
   }
-  klass->SetDexCache(FindDexCache(dex_file));
+  klass->SetDexCache(RegisterDexFile(dex_file));
 
   SetupClass(dex_file, dex_class_def, klass, class_loader.Get());
 
@@ -2478,58 +2467,52 @@
   RegisterDexFile(dex_file, dex_cache);
 }
 
-bool ClassLinker::IsDexFileRegisteredLocked(const DexFile& dex_file) {
-  dex_lock_.AssertSharedHeld(Thread::Current());
-  for (GcRoot<mirror::DexCache>& root : dex_caches_) {
-    mirror::DexCache* dex_cache = root.Read();
-    if (dex_cache->GetDexFile() == &dex_file) {
-      return true;
-    }
-  }
-  return false;
-}
-
-bool ClassLinker::IsDexFileRegistered(const DexFile& dex_file) {
-  ReaderMutexLock mu(Thread::Current(), dex_lock_);
-  return IsDexFileRegisteredLocked(dex_file);
-}
-
 void ClassLinker::RegisterDexFileLocked(const DexFile& dex_file,
                                         Handle<mirror::DexCache> dex_cache) {
-  dex_lock_.AssertExclusiveHeld(Thread::Current());
+  Thread* const self = Thread::Current();
+  dex_lock_.AssertExclusiveHeld(self);
   CHECK(dex_cache.Get() != nullptr) << dex_file.GetLocation();
   CHECK(dex_cache->GetLocation()->Equals(dex_file.GetLocation()))
       << dex_cache->GetLocation()->ToModifiedUtf8() << " " << dex_file.GetLocation();
-  dex_caches_.push_back(GcRoot<mirror::DexCache>(dex_cache.Get()));
-  dex_cache->SetDexFile(&dex_file);
-  if (log_new_dex_caches_roots_) {
-    // TODO: This is not safe if we can remove dex caches.
-    new_dex_cache_roots_.push_back(dex_caches_.size() - 1);
+  // Clean up pass to remove null dex caches.
+  // Null dex caches can occur due to class unloading and we are lazily removing null entries.
+  JavaVMExt* const vm = self->GetJniEnv()->vm;
+  for (auto it = dex_caches_.begin(); it != dex_caches_.end();) {
+    mirror::Object* dex_cache_root = self->DecodeJObject(*it);
+    if (dex_cache_root == nullptr) {
+      vm->DeleteWeakGlobalRef(self, *it);
+      it = dex_caches_.erase(it);
+    } else {
+      ++it;
+    }
   }
+  dex_caches_.push_back(vm->AddWeakGlobalRef(self, dex_cache.Get()));
+  dex_cache->SetDexFile(&dex_file);
 }
 
-void ClassLinker::RegisterDexFile(const DexFile& dex_file) {
+mirror::DexCache* ClassLinker::RegisterDexFile(const DexFile& dex_file) {
   Thread* self = Thread::Current();
   {
     ReaderMutexLock mu(self, dex_lock_);
-    if (IsDexFileRegisteredLocked(dex_file)) {
-      return;
+    mirror::DexCache* dex_cache = FindDexCacheLocked(dex_file, true);
+    if (dex_cache != nullptr) {
+      return dex_cache;
     }
   }
   // Don't alloc while holding the lock, since allocation may need to
   // suspend all threads and another thread may need the dex_lock_ to
   // get to a suspend point.
   StackHandleScope<1> hs(self);
-  Handle<mirror::DexCache> dex_cache(hs.NewHandle(AllocDexCache(self, dex_file)));
-  CHECK(dex_cache.Get() != nullptr) << "Failed to allocate dex cache for "
-                                    << dex_file.GetLocation();
-  {
-    WriterMutexLock mu(self, dex_lock_);
-    if (IsDexFileRegisteredLocked(dex_file)) {
-      return;
-    }
-    RegisterDexFileLocked(dex_file, dex_cache);
+  Handle<mirror::DexCache> h_dex_cache(hs.NewHandle(AllocDexCache(self, dex_file)));
+  CHECK(h_dex_cache.Get() != nullptr) << "Failed to allocate dex cache for "
+                                      << dex_file.GetLocation();
+  WriterMutexLock mu(self, dex_lock_);
+  mirror::DexCache* dex_cache = FindDexCacheLocked(dex_file, true);
+  if (dex_cache != nullptr) {
+    return dex_cache;
   }
+  RegisterDexFileLocked(dex_file, h_dex_cache);
+  return h_dex_cache.Get();
 }
 
 void ClassLinker::RegisterDexFile(const DexFile& dex_file,
@@ -2538,36 +2521,44 @@
   RegisterDexFileLocked(dex_file, dex_cache);
 }
 
-mirror::DexCache* ClassLinker::FindDexCache(const DexFile& dex_file) {
-  ReaderMutexLock mu(Thread::Current(), dex_lock_);
+mirror::DexCache* ClassLinker::FindDexCache(const DexFile& dex_file, bool allow_failure) {
+  Thread* const self = Thread::Current();
+  ReaderMutexLock mu(self, dex_lock_);
+  return FindDexCacheLocked(dex_file, allow_failure);
+}
+
+mirror::DexCache* ClassLinker::FindDexCacheLocked(const DexFile& dex_file, bool allow_failure) {
+  Thread* const self = Thread::Current();
   // Search assuming unique-ness of dex file.
-  for (size_t i = 0; i != dex_caches_.size(); ++i) {
-    mirror::DexCache* dex_cache = GetDexCache(i);
-    if (dex_cache->GetDexFile() == &dex_file) {
+  for (jobject weak_root : dex_caches_) {
+    mirror::DexCache* dex_cache = down_cast<mirror::DexCache*>(self->DecodeJObject(weak_root));
+    if (dex_cache != nullptr && dex_cache->GetDexFile() == &dex_file) {
       return dex_cache;
     }
   }
-  // Search matching by location name.
+  if (allow_failure) {
+    return nullptr;
+  }
   std::string location(dex_file.GetLocation());
-  for (size_t i = 0; i != dex_caches_.size(); ++i) {
-    mirror::DexCache* dex_cache = GetDexCache(i);
-    if (dex_cache->GetDexFile()->GetLocation() == location) {
-      return dex_cache;
-    }
-  }
   // Failure, dump diagnostic and abort.
-  for (size_t i = 0; i != dex_caches_.size(); ++i) {
-    mirror::DexCache* dex_cache = GetDexCache(i);
-    LOG(ERROR) << "Registered dex file " << i << " = " << dex_cache->GetDexFile()->GetLocation();
+  for (jobject weak_root : dex_caches_) {
+    mirror::DexCache* dex_cache = down_cast<mirror::DexCache*>(self->DecodeJObject(weak_root));
+    if (dex_cache != nullptr) {
+      LOG(ERROR) << "Registered dex file " << dex_cache->GetDexFile()->GetLocation();
+    }
   }
   LOG(FATAL) << "Failed to find DexCache for DexFile " << location;
   UNREACHABLE();
 }
 
 void ClassLinker::FixupDexCaches(ArtMethod* resolution_method) {
-  ReaderMutexLock mu(Thread::Current(), dex_lock_);
-  for (auto& dex_cache : dex_caches_) {
-    dex_cache.Read()->Fixup(resolution_method, image_pointer_size_);
+  Thread* const self = Thread::Current();
+  ReaderMutexLock mu(self, dex_lock_);
+  for (jobject weak_root : dex_caches_) {
+    mirror::DexCache* dex_cache = down_cast<mirror::DexCache*>(self->DecodeJObject(weak_root));
+    if (dex_cache != nullptr) {
+      dex_cache->Fixup(resolution_method, image_pointer_size_);
+    }
   }
 }
 
@@ -3403,11 +3394,13 @@
   DCHECK(proxy_class->IsProxyClass());
   DCHECK(proxy_method->IsProxyMethod());
   {
-    ReaderMutexLock mu(Thread::Current(), dex_lock_);
+    Thread* const self = Thread::Current();
+    ReaderMutexLock mu(self, dex_lock_);
     // Locate the dex cache of the original interface/Object
-    for (const GcRoot<mirror::DexCache>& root : dex_caches_) {
-      auto* dex_cache = root.Read();
-      if (proxy_method->HasSameDexCacheResolvedTypes(dex_cache->GetResolvedTypes())) {
+    for (jobject weak_root : dex_caches_) {
+      mirror::DexCache* dex_cache = down_cast<mirror::DexCache*>(self->DecodeJObject(weak_root));
+      if (dex_cache != nullptr &&
+          proxy_method->HasSameDexCacheResolvedTypes(dex_cache->GetResolvedTypes())) {
         ArtMethod* resolved_method = dex_cache->GetResolvedMethod(
             proxy_method->GetDexMethodIndex(), image_pointer_size_);
         CHECK(resolved_method != nullptr);
@@ -5864,11 +5857,6 @@
   // We could move the jobject to the callers, but all call-sites do this...
   ScopedObjectAccessUnchecked soa(self);
 
-  // Register the dex files.
-  for (const DexFile* dex_file : dex_files) {
-    RegisterDexFile(*dex_file);
-  }
-
   // For now, create a libcore-level DexFile for each ART DexFile. This "explodes" multidex.
   StackHandleScope<10> hs(self);
 
diff --git a/runtime/class_linker.h b/runtime/class_linker.h
index fbf4035..cc56e8b 100644
--- a/runtime/class_linker.h
+++ b/runtime/class_linker.h
@@ -278,7 +278,7 @@
   void RunRootClinits() SHARED_REQUIRES(Locks::mutator_lock_)
       REQUIRES(!dex_lock_, !Roles::uninterruptible_);
 
-  void RegisterDexFile(const DexFile& dex_file)
+  mirror::DexCache* RegisterDexFile(const DexFile& dex_file)
       REQUIRES(!dex_lock_) SHARED_REQUIRES(Locks::mutator_lock_);
   void RegisterDexFile(const DexFile& dex_file, Handle<mirror::DexCache> dex_cache)
       REQUIRES(!dex_lock_) SHARED_REQUIRES(Locks::mutator_lock_);
@@ -309,9 +309,7 @@
   void VisitRoots(RootVisitor* visitor, VisitRootFlags flags)
       REQUIRES(!dex_lock_) SHARED_REQUIRES(Locks::mutator_lock_);
 
-  mirror::DexCache* FindDexCache(const DexFile& dex_file)
-      REQUIRES(!dex_lock_) SHARED_REQUIRES(Locks::mutator_lock_);
-  bool IsDexFileRegistered(const DexFile& dex_file)
+  mirror::DexCache* FindDexCache(const DexFile& dex_file, bool allow_failure = false)
       REQUIRES(!dex_lock_) SHARED_REQUIRES(Locks::mutator_lock_);
   void FixupDexCaches(ArtMethod* resolution_method)
       REQUIRES(!dex_lock_) SHARED_REQUIRES(Locks::mutator_lock_);
@@ -471,7 +469,7 @@
 
   // Used by image writer for checking.
   bool ClassInClassTable(mirror::Class* klass)
-      REQUIRES(!Locks::classlinker_classes_lock_)
+      REQUIRES(Locks::classlinker_classes_lock_)
       SHARED_REQUIRES(Locks::mutator_lock_);
 
   ArtMethod* CreateRuntimeMethod();
@@ -561,8 +559,9 @@
 
   void RegisterDexFileLocked(const DexFile& dex_file, Handle<mirror::DexCache> dex_cache)
       REQUIRES(dex_lock_) SHARED_REQUIRES(Locks::mutator_lock_);
-  bool IsDexFileRegisteredLocked(const DexFile& dex_file)
-      SHARED_REQUIRES(dex_lock_, Locks::mutator_lock_);
+  mirror::DexCache* FindDexCacheLocked(const DexFile& dex_file, bool allow_failure)
+      REQUIRES(dex_lock_)
+      SHARED_REQUIRES(Locks::mutator_lock_);
 
   bool InitializeClass(Thread* self, Handle<mirror::Class> klass, bool can_run_clinit,
                        bool can_init_parents)
@@ -631,7 +630,9 @@
   size_t GetDexCacheCount() SHARED_REQUIRES(Locks::mutator_lock_, dex_lock_) {
     return dex_caches_.size();
   }
-  mirror::DexCache* GetDexCache(size_t idx) SHARED_REQUIRES(Locks::mutator_lock_, dex_lock_);
+  const std::list<jobject>& GetDexCaches() SHARED_REQUIRES(Locks::mutator_lock_, dex_lock_) {
+    return dex_caches_;
+  }
 
   const OatFile* FindOpenedOatFileFromOatLocation(const std::string& oat_location)
       REQUIRES(!dex_lock_);
@@ -702,8 +703,9 @@
   std::vector<std::unique_ptr<const DexFile>> opened_dex_files_;
 
   mutable ReaderWriterMutex dex_lock_ DEFAULT_MUTEX_ACQUIRED_AFTER;
-  std::vector<size_t> new_dex_cache_roots_ GUARDED_BY(dex_lock_);
-  std::vector<GcRoot<mirror::DexCache>> dex_caches_ GUARDED_BY(dex_lock_);
+  // JNI weak globals to allow dex caches to get unloaded. We lazily delete weak globals when we
+  // register new dex files.
+  std::list<jobject> dex_caches_ GUARDED_BY(dex_lock_);
   std::vector<const OatFile*> oat_files_ GUARDED_BY(dex_lock_);
 
   // This contains the class laoders which have class tables. It is populated by
@@ -736,7 +738,6 @@
   size_t find_array_class_cache_next_victim_;
 
   bool init_done_;
-  bool log_new_dex_caches_roots_ GUARDED_BY(dex_lock_);
   bool log_new_class_table_roots_ GUARDED_BY(Locks::classlinker_classes_lock_);
 
   InternTable* intern_table_;
diff --git a/runtime/common_runtime_test.cc b/runtime/common_runtime_test.cc
index 5f9e413..56c5d1a 100644
--- a/runtime/common_runtime_test.cc
+++ b/runtime/common_runtime_test.cc
@@ -551,7 +551,8 @@
   }
 
   Thread* self = Thread::Current();
-  jobject class_loader = Runtime::Current()->GetClassLinker()->CreatePathClassLoader(self,                                                                                   class_path);
+  jobject class_loader = Runtime::Current()->GetClassLinker()->CreatePathClassLoader(self,
+                                                                                     class_path);
   self->SetClassLoaderOverride(class_loader);
   return class_loader;
 }
diff --git a/runtime/java_vm_ext.cc b/runtime/java_vm_ext.cc
index 9d41018..2fd0517 100644
--- a/runtime/java_vm_ext.cc
+++ b/runtime/java_vm_ext.cc
@@ -373,7 +373,7 @@
       globals_(gGlobalsInitial, gGlobalsMax, kGlobal),
       libraries_(new Libraries),
       unchecked_functions_(&gJniInvokeInterface),
-      weak_globals_lock_("JNI weak global reference table lock"),
+      weak_globals_lock_("JNI weak global reference table lock", kJniWeakGlobalsLock),
       weak_globals_(kWeakGlobalsInitial, kWeakGlobalsMax, kWeakGlobal),
       allow_new_weak_globals_(true),
       weak_globals_add_condition_("weak globals add condition", weak_globals_lock_) {
diff --git a/runtime/native/dalvik_system_DexFile.cc b/runtime/native/dalvik_system_DexFile.cc
index 4f97d20..9bd320c 100644
--- a/runtime/native/dalvik_system_DexFile.cc
+++ b/runtime/native/dalvik_system_DexFile.cc
@@ -171,7 +171,7 @@
     if (array == nullptr) {
       ScopedObjectAccess soa(env);
       for (auto& dex_file : dex_files) {
-        if (Runtime::Current()->GetClassLinker()->IsDexFileRegistered(*dex_file)) {
+        if (Runtime::Current()->GetClassLinker()->FindDexCache(*dex_file, true) != nullptr) {
           dex_file.release();
         }
       }
@@ -209,7 +209,7 @@
   // TODO: The Runtime should support unloading of classes and freeing of the
   // dex files for those unloaded classes rather than leaking dex files here.
   for (auto& dex_file : *dex_files) {
-    if (!Runtime::Current()->GetClassLinker()->IsDexFileRegistered(*dex_file)) {
+    if (Runtime::Current()->GetClassLinker()->FindDexCache(*dex_file, true) == nullptr) {
       delete dex_file;
     }
   }
diff --git a/runtime/native/dalvik_system_VMRuntime.cc b/runtime/native/dalvik_system_VMRuntime.cc
index 9ea339a..5a9c43b 100644
--- a/runtime/native/dalvik_system_VMRuntime.cc
+++ b/runtime/native/dalvik_system_VMRuntime.cc
@@ -425,14 +425,16 @@
 static void PreloadDexCachesStatsFilled(DexCacheStats* filled)
     SHARED_REQUIRES(Locks::mutator_lock_) {
   if (!kPreloadDexCachesCollectStats) {
-      return;
+    return;
   }
-  ClassLinker* linker = Runtime::Current()->GetClassLinker();
-  const std::vector<const DexFile*>& boot_class_path = linker->GetBootClassPath();
-  for (size_t i = 0; i< boot_class_path.size(); i++) {
-    const DexFile* dex_file = boot_class_path[i];
+  ClassLinker* const class_linker = Runtime::Current()->GetClassLinker();
+  for (const DexFile* dex_file : class_linker->GetBootClassPath()) {
     CHECK(dex_file != nullptr);
-    mirror::DexCache* dex_cache = linker->FindDexCache(*dex_file);
+    mirror::DexCache* const dex_cache = class_linker->FindDexCache(*dex_file, true);
+    // If dex cache was deallocated, just continue.
+    if (dex_cache == nullptr) {
+      continue;
+    }
     for (size_t j = 0; j < dex_cache->NumStrings(); j++) {
       mirror::String* string = dex_cache->GetResolvedString(j);
       if (string != nullptr) {
@@ -446,7 +448,7 @@
       }
     }
     for (size_t j = 0; j < dex_cache->NumResolvedFields(); j++) {
-      ArtField* field = linker->GetResolvedField(j, dex_cache);
+      ArtField* field = class_linker->GetResolvedField(j, dex_cache);
       if (field != nullptr) {
         filled->num_fields++;
       }
@@ -490,11 +492,11 @@
   }
 
   const std::vector<const DexFile*>& boot_class_path = linker->GetBootClassPath();
-  for (size_t i = 0; i< boot_class_path.size(); i++) {
+  for (size_t i = 0; i < boot_class_path.size(); i++) {
     const DexFile* dex_file = boot_class_path[i];
     CHECK(dex_file != nullptr);
     StackHandleScope<1> hs(soa.Self());
-    Handle<mirror::DexCache> dex_cache(hs.NewHandle(linker->FindDexCache(*dex_file)));
+    Handle<mirror::DexCache> dex_cache(hs.NewHandle(linker->RegisterDexFile(*dex_file)));
 
     if (kPreloadDexCachesStrings) {
       for (size_t j = 0; j < dex_cache->NumStrings(); j++) {
diff --git a/runtime/runtime-inl.h b/runtime/runtime-inl.h
index 380e72b..bfa8c54 100644
--- a/runtime/runtime-inl.h
+++ b/runtime/runtime-inl.h
@@ -20,6 +20,7 @@
 #include "runtime.h"
 
 #include "art_method.h"
+#include "class_linker.h"
 #include "read_barrier-inl.h"
 
 namespace art {
diff --git a/runtime/runtime.cc b/runtime/runtime.cc
index 1912314..49451ad 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -791,6 +791,12 @@
   return failure_count;
 }
 
+void Runtime::SetSentinel(mirror::Object* sentinel) {
+  CHECK(sentinel_.Read() == nullptr);
+  CHECK(sentinel != nullptr);
+  sentinel_ = GcRoot<mirror::Object>(sentinel);
+}
+
 bool Runtime::Init(const RuntimeOptions& raw_options, bool ignore_unrecognized) {
   ATRACE_BEGIN("Runtime::Init");
   CHECK_EQ(sysconf(_SC_PAGE_SIZE), kPageSize);
@@ -1054,10 +1060,6 @@
 
   CHECK(class_linker_ != nullptr);
 
-  // Initialize the special sentinel_ value early.
-  sentinel_ = GcRoot<mirror::Object>(class_linker_->AllocObject(self));
-  CHECK(sentinel_.Read() != nullptr);
-
   verifier::MethodVerifier::Init();
 
   if (runtime_options.Exists(Opt::MethodTrace)) {
diff --git a/runtime/runtime.h b/runtime/runtime.h
index 4577b75..bd21db1 100644
--- a/runtime/runtime.h
+++ b/runtime/runtime.h
@@ -568,6 +568,9 @@
     return fingerprint_;
   }
 
+  // Called from class linker.
+  void SetSentinel(mirror::Object* sentinel) SHARED_REQUIRES(Locks::mutator_lock_);
+
  private:
   static void InitPlatformSignalHandlers();
 
diff --git a/runtime/trace.cc b/runtime/trace.cc
index 4393430..7579d8d 100644
--- a/runtime/trace.cc
+++ b/runtime/trace.cc
@@ -640,7 +640,8 @@
   ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
   for (auto& e : seen_methods) {
     DexIndexBitSet* bit_set = e.second;
-    mirror::DexCache* dex_cache = class_linker->FindDexCache(*e.first);
+    // TODO: Visit trace methods as roots.
+    mirror::DexCache* dex_cache = class_linker->FindDexCache(*e.first, false);
     for (uint32_t i = 0; i < bit_set->size(); ++i) {
       if ((*bit_set)[i]) {
         visited_methods->insert(dex_cache->GetResolvedMethod(i, sizeof(void*)));
diff --git a/runtime/verifier/method_verifier.cc b/runtime/verifier/method_verifier.cc
index a506071..1693a85 100644
--- a/runtime/verifier/method_verifier.cc
+++ b/runtime/verifier/method_verifier.cc
@@ -3361,6 +3361,7 @@
 ArtMethod* MethodVerifier::ResolveMethodAndCheckAccess(
     uint32_t dex_method_idx, MethodType method_type) {
   const DexFile::MethodId& method_id = dex_file_->GetMethodId(dex_method_idx);
+  // LOG(INFO) << dex_file_->NumTypeIds() << " " << dex_file_->NumClassDefs();
   const RegType& klass_type = ResolveClassAndCheckAccess(method_id.class_idx_);
   if (klass_type.IsConflict()) {
     std::string append(" in attempt to access method ");