1 // Protocol Buffers - Google's data interchange format
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30 
31 // This header file defines an internal class that encapsulates internal message
32 // metadata (Unknown-field set, Arena pointer, ...) and allows its
33 // representation to be made more space-efficient via various optimizations.
34 //
35 // Note that this is distinct from google::protobuf::Metadata, which encapsulates
36 // Descriptor and Reflection pointers.
37 
38 #ifndef GOOGLE_PROTOBUF_METADATA_H__
39 #define GOOGLE_PROTOBUF_METADATA_H__
40 
41 #include <google/protobuf/stubs/common.h>
42 #include <google/protobuf/arena.h>
43 #include <google/protobuf/unknown_field_set.h>
44 
45 namespace google {
46 namespace protobuf {
47 namespace internal {
48 
49 // This is the representation for messages that support arena allocation. It
50 // uses a tagged pointer to either store the Arena pointer, if there are no
51 // unknown fields, or a pointer to a block of memory with both the Arena pointer
52 // and the UnknownFieldSet, if there are unknown fields. This optimization
53 // allows for "zero-overhead" storage of the Arena pointer, relative to the
54 // above baseline implementation.
55 //
56 // The tagged pointer uses the LSB to disambiguate cases, and uses bit 0 == 0 to
57 // indicate an arena pointer and bit 0 == 1 to indicate a UFS+Arena-container
58 // pointer.
59 class LIBPROTOBUF_EXPORT InternalMetadataWithArena {
60  public:
InternalMetadataWithArena()61   InternalMetadataWithArena() : ptr_(NULL) {}
InternalMetadataWithArena(Arena * arena)62   explicit InternalMetadataWithArena(Arena* arena)
63       : ptr_ (arena) {}
64 
~InternalMetadataWithArena()65   ~InternalMetadataWithArena() {
66     if (have_unknown_fields() && arena() == NULL) {
67       delete PtrValue<Container>();
68     }
69     ptr_ = NULL;
70   }
71 
unknown_fields()72   GOOGLE_ATTRIBUTE_ALWAYS_INLINE const UnknownFieldSet& unknown_fields() const {
73     if (GOOGLE_PREDICT_FALSE(have_unknown_fields())) {
74       return PtrValue<Container>()->unknown_fields_;
75     } else {
76       return *UnknownFieldSet::default_instance();
77     }
78   }
79 
mutable_unknown_fields()80   GOOGLE_ATTRIBUTE_ALWAYS_INLINE UnknownFieldSet* mutable_unknown_fields() {
81     if (GOOGLE_PREDICT_TRUE(have_unknown_fields())) {
82       return &PtrValue<Container>()->unknown_fields_;
83     } else {
84       return mutable_unknown_fields_slow();
85     }
86   }
87 
arena()88   GOOGLE_ATTRIBUTE_ALWAYS_INLINE Arena* arena() const {
89     if (GOOGLE_PREDICT_FALSE(have_unknown_fields())) {
90       return PtrValue<Container>()->arena_;
91     } else {
92       return PtrValue<Arena>();
93     }
94   }
95 
have_unknown_fields()96   GOOGLE_ATTRIBUTE_ALWAYS_INLINE bool have_unknown_fields() const {
97     return PtrTag() == kTagContainer;
98   }
99 
Swap(InternalMetadataWithArena * other)100   GOOGLE_ATTRIBUTE_ALWAYS_INLINE void Swap(InternalMetadataWithArena* other) {
101     // Semantics here are that we swap only the unknown fields, not the arena
102     // pointer. We cannot simply swap ptr_ with other->ptr_ because we need to
103     // maintain our own arena ptr. Also, our ptr_ and other's ptr_ may be in
104     // different states (direct arena pointer vs. container with UFS) so we
105     // cannot simply swap ptr_ and then restore the arena pointers. We reuse
106     // UFS's swap implementation instead.
107     if (have_unknown_fields() || other->have_unknown_fields()) {
108       mutable_unknown_fields()->Swap(other->mutable_unknown_fields());
109     }
110   }
111 
raw_arena_ptr()112   GOOGLE_ATTRIBUTE_ALWAYS_INLINE void* raw_arena_ptr() const {
113     return ptr_;
114   }
115 
116  private:
117   void* ptr_;
118 
119   // Tagged pointer implementation.
120   enum {
121     // ptr_ is an Arena*.
122     kTagArena = 0,
123     // ptr_ is a Container*.
124     kTagContainer = 1,
125   };
126   static const intptr_t kPtrTagMask = 1;
127   static const intptr_t kPtrValueMask = ~kPtrTagMask;
128 
129   // Accessors for pointer tag and pointer value.
PtrTag()130   GOOGLE_ATTRIBUTE_ALWAYS_INLINE int PtrTag() const {
131     return reinterpret_cast<intptr_t>(ptr_) & kPtrTagMask;
132   }
133 
PtrValue()134   template<typename T> T* PtrValue() const {
135     return reinterpret_cast<T*>(
136         reinterpret_cast<intptr_t>(ptr_) & kPtrValueMask);
137   }
138 
139   // If ptr_'s tag is kTagContainer, it points to an instance of this struct.
140   struct Container {
141     UnknownFieldSet unknown_fields_;
142     Arena* arena_;
143   };
144 
mutable_unknown_fields_slow()145   GOOGLE_ATTRIBUTE_NOINLINE UnknownFieldSet* mutable_unknown_fields_slow() {
146     Arena* my_arena = arena();
147     Container* container = Arena::Create<Container>(my_arena);
148     // Two-step assignment works around a bug in clang's static analyzer:
149     // https://bugs.llvm.org/show_bug.cgi?id=34198.
150     ptr_ = container;
151     ptr_ = reinterpret_cast<void*>(
152         reinterpret_cast<intptr_t>(ptr_) | kTagContainer);
153     container->arena_ = my_arena;
154     return &(container->unknown_fields_);
155   }
156 };
157 
158 // Temporary compatibility typedef. Remove once this is released in components
159 // and upb CL is submitted.
160 typedef InternalMetadataWithArena InternalMetadata;
161 
162 }  // namespace internal
163 }  // namespace protobuf
164 
165 }  // namespace google
166 #endif  // GOOGLE_PROTOBUF_METADATA_H__
167