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diff --git a/src/google/protobuf/metadata_lite.h b/src/google/protobuf/metadata_lite.h
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+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef GOOGLE_PROTOBUF_METADATA_LITE_H__
+#define GOOGLE_PROTOBUF_METADATA_LITE_H__
+
+#include <google/protobuf/stubs/common.h>
+#include <google/protobuf/arena.h>
+#include <google/protobuf/generated_message_util.h>
+
+namespace google {
+namespace protobuf {
+namespace internal {
+
+// This is the representation for messages that support arena allocation. It
+// uses a tagged pointer to either store the Arena pointer, if there are no
+// unknown fields, or a pointer to a block of memory with both the Arena pointer
+// and the UnknownFieldSet, if there are unknown fields. This optimization
+// allows for "zero-overhead" storage of the Arena pointer, relative to the
+// above baseline implementation.
+//
+// The tagged pointer uses the LSB to disambiguate cases, and uses bit 0 == 0 to
+// indicate an arena pointer and bit 0 == 1 to indicate a UFS+Arena-container
+// pointer.
+template <class T, class Derived>
+class InternalMetadataWithArenaBase {
+ public:
+  InternalMetadataWithArenaBase() : ptr_(NULL) {}
+  explicit InternalMetadataWithArenaBase(Arena* arena) : ptr_(arena) {}
+
+  ~InternalMetadataWithArenaBase() {
+    if (have_unknown_fields() && arena() == NULL) {
+      delete PtrValue<Container>();
+    }
+    ptr_ = NULL;
+  }
+
+  GOOGLE_ATTRIBUTE_ALWAYS_INLINE const T& unknown_fields() const {
+    if (GOOGLE_PREDICT_FALSE(have_unknown_fields())) {
+      return PtrValue<Container>()->unknown_fields;
+    } else {
+      return Derived::default_instance();
+    }
+  }
+
+  GOOGLE_ATTRIBUTE_ALWAYS_INLINE T* mutable_unknown_fields() {
+    if (GOOGLE_PREDICT_TRUE(have_unknown_fields())) {
+      return &PtrValue<Container>()->unknown_fields;
+    } else {
+      return mutable_unknown_fields_slow();
+    }
+  }
+
+  GOOGLE_ATTRIBUTE_ALWAYS_INLINE Arena* arena() const {
+    if (GOOGLE_PREDICT_FALSE(have_unknown_fields())) {
+      return PtrValue<Container>()->arena;
+    } else {
+      return PtrValue<Arena>();
+    }
+  }
+
+  GOOGLE_ATTRIBUTE_ALWAYS_INLINE bool have_unknown_fields() const {
+    return PtrTag() == kTagContainer;
+  }
+
+  GOOGLE_ATTRIBUTE_ALWAYS_INLINE void Swap(Derived* other) {
+    // Semantics here are that we swap only the unknown fields, not the arena
+    // pointer. We cannot simply swap ptr_ with other->ptr_ because we need to
+    // maintain our own arena ptr. Also, our ptr_ and other's ptr_ may be in
+    // different states (direct arena pointer vs. container with UFS) so we
+    // cannot simply swap ptr_ and then restore the arena pointers. We reuse
+    // UFS's swap implementation instead.
+    if (have_unknown_fields() || other->have_unknown_fields()) {
+      static_cast<Derived*>(this)->DoSwap(other->mutable_unknown_fields());
+    }
+  }
+
+  GOOGLE_ATTRIBUTE_ALWAYS_INLINE void MergeFrom(const Derived& other) {
+    if (other.have_unknown_fields()) {
+      static_cast<Derived*>(this)->DoMergeFrom(other.unknown_fields());
+    }
+  }
+
+  GOOGLE_ATTRIBUTE_ALWAYS_INLINE void Clear() {
+    if (have_unknown_fields()) {
+      static_cast<Derived*>(this)->DoClear();
+    }
+  }
+
+  GOOGLE_ATTRIBUTE_ALWAYS_INLINE void* raw_arena_ptr() const {
+    return ptr_;
+  }
+
+ private:
+  void* ptr_;
+
+  // Tagged pointer implementation.
+  enum {
+    // ptr_ is an Arena*.
+    kTagArena = 0,
+    // ptr_ is a Container*.
+    kTagContainer = 1,
+  };
+  static const intptr_t kPtrTagMask = 1;
+  static const intptr_t kPtrValueMask = ~kPtrTagMask;
+
+  // Accessors for pointer tag and pointer value.
+  GOOGLE_ATTRIBUTE_ALWAYS_INLINE int PtrTag() const {
+    return reinterpret_cast<intptr_t>(ptr_) & kPtrTagMask;
+  }
+
+  template<typename U> U* PtrValue() const {
+    return reinterpret_cast<U*>(
+        reinterpret_cast<intptr_t>(ptr_) & kPtrValueMask);
+  }
+
+  // If ptr_'s tag is kTagContainer, it points to an instance of this struct.
+  struct Container {
+    T unknown_fields;
+    Arena* arena;
+  };
+
+  GOOGLE_ATTRIBUTE_NOINLINE T* mutable_unknown_fields_slow() {
+    Arena* my_arena = arena();
+    Container* container = Arena::Create<Container>(my_arena);
+    ptr_ = reinterpret_cast<void*>(
+        reinterpret_cast<intptr_t>(container) | kTagContainer);
+    container->arena = my_arena;
+    return &(container->unknown_fields);
+  }
+};
+
+// We store unknown fields as a string right now, because there is currently no
+// good interface for reading unknown fields into an ArenaString.  We may want
+// to revisit this to allow unknown fields to be parsed onto the Arena.
+class InternalMetadataWithArenaLite
+    : public InternalMetadataWithArenaBase<string,
+                                           InternalMetadataWithArenaLite> {
+ public:
+  InternalMetadataWithArenaLite() {}
+
+  explicit InternalMetadataWithArenaLite(Arena* arena)
+      : InternalMetadataWithArenaBase(arena) {}
+
+  void DoSwap(string* other) {
+    mutable_unknown_fields()->swap(*other);
+  }
+
+  void DoMergeFrom(const string& other) {
+    mutable_unknown_fields()->append(other);
+  }
+
+  void DoClear() {
+    mutable_unknown_fields()->clear();
+  }
+
+  static const string& default_instance() {
+    return GetEmptyStringAlreadyInited();
+  }
+};
+
+}  // namespace internal
+}  // namespace protobuf
+
+}  // namespace google
+#endif  // GOOGLE_PROTOBUF_METADATA_LITE_H__