src/google/protobuf/metadata.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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//
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// This header file defines an internal class that encapsulates internal message
// metadata (Unknown-field set, Arena pointer, ...) and allows its
// representation to be made more space-efficient via various optimizations.
//
// Note that this is distinct from google::protobuf::Metadata, which encapsulates
// Descriptor and Reflection pointers.

#ifndef GOOGLE_PROTOBUF_METADATA_H__
#define GOOGLE_PROTOBUF_METADATA_H__

#include <google/protobuf/stubs/common.h>
#include <google/protobuf/arena.h>
#include <google/protobuf/unknown_field_set.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.
class LIBPROTOBUF_EXPORT InternalMetadataWithArena {
 public:
  InternalMetadataWithArena() : ptr_(NULL) {}
  explicit InternalMetadataWithArena(Arena* arena)
      : ptr_ (arena) {}

  ~InternalMetadataWithArena() {
    if (have_unknown_fields() && arena() == NULL) {
      delete PtrValue<Container>();
    }
    ptr_ = NULL;
  }

  GOOGLE_ATTRIBUTE_ALWAYS_INLINE const UnknownFieldSet& unknown_fields() const {
    if (GOOGLE_PREDICT_FALSE(have_unknown_fields())) {
      return PtrValue<Container>()->unknown_fields_;
    } else {
      return *UnknownFieldSet::default_instance();
    }
  }

  GOOGLE_ATTRIBUTE_ALWAYS_INLINE UnknownFieldSet* 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(InternalMetadataWithArena* 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()) {
      mutable_unknown_fields()->Swap(other->mutable_unknown_fields());
    }
  }

  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 T> T* PtrValue() const {
    return reinterpret_cast<T*>(
        reinterpret_cast<intptr_t>(ptr_) & kPtrValueMask);
  }

  // If ptr_'s tag is kTagContainer, it points to an instance of this struct.
  struct Container {
    UnknownFieldSet unknown_fields_;
    Arena* arena_;
  };

  GOOGLE_ATTRIBUTE_NOINLINE UnknownFieldSet* 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_);
  }
};

// Temporary compatibility typedef. Remove once this is released in components
// and upb CL is submitted.
typedef InternalMetadataWithArena InternalMetadata;

}  // namespace internal
}  // namespace protobuf

}  // namespace google
#endif  // GOOGLE_PROTOBUF_METADATA_H__