include/google/protobuf/metadata_lite.h

// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
// https://developers.google.com/protocol-buffers/
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#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/message_lite.h>
#include <google/protobuf/stubs/port.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_PROTOBUF_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_PROTOBUF_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_PROTOBUF_ATTRIBUTE_ALWAYS_INLINE Arena* arena() const {
    if (GOOGLE_PREDICT_FALSE(have_unknown_fields())) {
      return PtrValue<Container>()->arena;
    } else {
      return PtrValue<Arena>();
    }
  }

  GOOGLE_PROTOBUF_ATTRIBUTE_ALWAYS_INLINE bool have_unknown_fields() const {
    return PtrTag() == kTagContainer;
  }

  GOOGLE_PROTOBUF_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_PROTOBUF_ATTRIBUTE_ALWAYS_INLINE void MergeFrom(const Derived& other) {
    if (other.have_unknown_fields()) {
      static_cast<Derived*>(this)->DoMergeFrom(other.unknown_fields());
    }
  }

  GOOGLE_PROTOBUF_ATTRIBUTE_ALWAYS_INLINE void Clear() {
    if (have_unknown_fields()) {
      static_cast<Derived*>(this)->DoClear();
    }
  }

  GOOGLE_PROTOBUF_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_PROTOBUF_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_PROTOBUF_ATTRIBUTE_NOINLINE T* mutable_unknown_fields_slow() {
    Arena* my_arena = arena();
    Container* container = Arena::Create<Container>(my_arena);
    // Two-step assignment works around a bug in clang's static analyzer:
    // https://bugs.llvm.org/show_bug.cgi?id=34198.
    ptr_ = container;
    ptr_ = reinterpret_cast<void*>(
        reinterpret_cast<intptr_t>(ptr_) | 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<string,
                                      InternalMetadataWithArenaLite>(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();
  }
};

// This helper RAII class is needed to efficiently parse unknown fields. We
// should only call mutable_unknown_fields if there are actual unknown fields.
// The obvious thing to just use a stack string and swap it at the end of the
// parse won't work, because the destructor of StringOutputStream needs to be
// called before we can modify the string (it check-fails). Using
// LiteUnknownFieldSetter setter(&_internal_metadata_);
// StringOutputStream stream(setter.buffer());
// guarantees that the string is only swapped after stream is destroyed.
class LIBPROTOBUF_EXPORT LiteUnknownFieldSetter {
 public:
  explicit LiteUnknownFieldSetter(InternalMetadataWithArenaLite* metadata)
      : metadata_(metadata) {
    if (metadata->have_unknown_fields()) {
      buffer_.swap(*metadata->mutable_unknown_fields());
    }
  }
  ~LiteUnknownFieldSetter() {
    if (!buffer_.empty()) metadata_->mutable_unknown_fields()->swap(buffer_);
  }
  string* buffer() { return &buffer_; }

 private:
  InternalMetadataWithArenaLite* metadata_;
  string buffer_;
};

}  // namespace internal
}  // namespace protobuf

}  // namespace google
#endif  // GOOGLE_PROTOBUF_METADATA_LITE_H__