Refactoring of snapshots. This simplifies and improves

src/serialize.h

Index: src/serialize.h
===================================================================
--- src/serialize.h	(revision 12497)
+++ src/serialize.h	(working copy)
@@ -170,13 +170,27 @@
     return data_[position_++];
   }
 
+  int32_t GetUnalignedInt() {
+#if defined(V8_HOST_CAN_READ_UNALIGNED) &&  __BYTE_ORDER == __LITTLE_ENDIAN
+    int32_t answer;
+    ASSERT(position_ + sizeof(answer) <= length_ + 0u);
+    answer = *reinterpret_cast<const int32_t*>(data_ + position_);
+#else
+    int32_t answer = data_[position_];
+    answer |= data_[position_ + 1] << 8;
+    answer |= data_[position_ + 2] << 16;
+    answer |= data_[position_ + 3] << 24;
+#endif
+    return answer;
+  }
+
+  void Advance(int by) { position_ += by; }
+
   inline void CopyRaw(byte* to, int number_of_bytes);
 
   inline int GetInt();
 
-  bool AtEOF() {
-    return position_ == length_;
-  }
+  bool AtEOF();
 
   int position() { return position_; }
 
@@ -187,23 +201,6 @@
 };
 
 
-#define COMMON_RAW_LENGTHS(f)        \
-  f(1, 1)  \
-  f(2, 2)  \
-  f(3, 3)  \
-  f(4, 4)  \
-  f(5, 5)  \
-  f(6, 6)  \
-  f(7, 7)  \
-  f(8, 8)  \
-  f(9, 12)  \
-  f(10, 16) \
-  f(11, 20) \
-  f(12, 24) \
-  f(13, 28) \
-  f(14, 32) \
-  f(15, 36)
-
 // The Serializer/Deserializer class is a common superclass for Serializer and
 // Deserializer which is used to store common constants and methods used by
 // both.
@@ -211,23 +208,24 @@
  public:
   static void Iterate(ObjectVisitor* visitor);
 
+  static int nop() { return kNop; }
+
  protected:
   // Where the pointed-to object can be found:
   enum Where {
     kNewObject = 0,                 // Object is next in snapshot.
-    // 1-8                             One per space.
+    // 1-6                             One per space.
     kRootArray = 0x9,               // Object is found in root array.
     kPartialSnapshotCache = 0xa,    // Object is in the cache.
     kExternalReference = 0xb,       // Pointer to an external reference.
-    kSkip = 0xc,                    // Skip a pointer sized cell.
-    // 0xd-0xf                         Free.
-    kBackref = 0x10,                 // Object is described relative to end.
-    // 0x11-0x18                       One per space.
-    // 0x19-0x1f                       Free.
-    kFromStart = 0x20,              // Object is described relative to start.
-    // 0x21-0x28                       One per space.
-    // 0x29-0x2f                       Free.
-    // 0x30-0x3f                       Used by misc. tags below.
+    kSkip = 0xc,                    // Skip n bytes.
+    kNop = 0xd,                     // Does nothing, used to pad.
+    // 0xe-0xf                         Free.
+    kBackref = 0x10,                // Object is described relative to end.
+    // 0x11-0x16                       One per space.
+    kBackrefWithSkip = 0x18,        // Object is described relative to end.
+    // 0x19-0x1e                       One per space.
+    // 0x20-0x3f                       Used by misc. tags below.
     kPointedToMask = 0x3f
   };
 
@@ -239,6 +237,13 @@
     kHowToCodeMask = 0x40
   };
 
+  // For kRootArrayConstants
+  enum WithSkip {
+    kNoSkipDistance = 0,
+    kHasSkipDistance = 0x40,
+    kWithSkipMask = 0x40
+  };
+
   // Where to point within the object.
   enum WhereToPoint {
     kStartOfObject = 0,
@@ -247,9 +252,12 @@
   };
 
   // Misc.
-  // Raw data to be copied from the snapshot.
-  static const int kRawData = 0x30;
-  // Some common raw lengths: 0x31-0x3f
+  // Raw data to be copied from the snapshot.  This byte code does not advance
+  // the current pointer, which is used for code objects, where we write the
+  // entire code in one memcpy, then fix up stuff with kSkip and other byte
+  // codes that overwrite data.
+  static const int kRawData = 0x20;
+  // Some common raw lengths: 0x21-0x3f.  These autoadvance the current pointer.
   // A tag emitted at strategic points in the snapshot to delineate sections.
   // If the deserializer does not find these at the expected moments then it
   // is an indication that the snapshot and the VM do not fit together.
@@ -259,64 +267,44 @@
   // Used for the source code of the natives, which is in the executable, but
   // is referred to from external strings in the snapshot.
   static const int kNativesStringResource = 0x71;
-  static const int kNewPage = 0x72;
-  static const int kRepeat = 0x73;
-  static const int kConstantRepeat = 0x74;
-  // 0x74-0x7f            Repeat last word (subtract 0x73 to get the count).
-  static const int kMaxRepeats = 0x7f - 0x73;
+  static const int kRepeat = 0x72;
+  static const int kConstantRepeat = 0x73;
+  // 0x73-0x7f            Repeat last word (subtract 0x72 to get the count).
+  static const int kMaxRepeats = 0x7f - 0x72;
   static int CodeForRepeats(int repeats) {
     ASSERT(repeats >= 1 && repeats <= kMaxRepeats);
-    return 0x73 + repeats;
+    return 0x72 + repeats;
   }
   static int RepeatsForCode(int byte_code) {
     ASSERT(byte_code >= kConstantRepeat && byte_code <= 0x7f);
-    return byte_code - 0x73;
+    return byte_code - 0x72;
   }
-  static const int kRootArrayLowConstants = 0xb0;
-  // 0xb0-0xbf            Things from the first 16 elements of the root array.
-  static const int kRootArrayHighConstants = 0xf0;
-  // 0xf0-0xff            Things from the next 16 elements of the root array.
+  static const int kRootArrayConstants = 0xa0;
+  // 0xa0-0xbf            Things from the first 32 elements of the root array.
   static const int kRootArrayNumberOfConstantEncodings = 0x20;
-  static const int kRootArrayNumberOfLowConstantEncodings = 0x10;
   static int RootArrayConstantFromByteCode(int byte_code) {
-    int constant = (byte_code & 0xf) | ((byte_code & 0x40) >> 2);
-    ASSERT(constant >= 0 && constant < kRootArrayNumberOfConstantEncodings);
-    return constant;
+    return byte_code & 0x1f;
   }
 
-
-  static const int kLargeData = LAST_SPACE;
-  static const int kLargeCode = kLargeData + 1;
-  static const int kLargeFixedArray = kLargeCode + 1;
-  static const int kNumberOfSpaces = kLargeFixedArray + 1;
+  static const int kNumberOfSpaces = LO_SPACE;
   static const int kAnyOldSpace = -1;
 
   // A bitmask for getting the space out of an instruction.
-  static const int kSpaceMask = 15;
-
-  static inline bool SpaceIsLarge(int space) { return space >= kLargeData; }
-  static inline bool SpaceIsPaged(int space) {
-    return space >= FIRST_PAGED_SPACE && space <= LAST_PAGED_SPACE;
-  }
+  static const int kSpaceMask = 7;
 };
 
 
 int SnapshotByteSource::GetInt() {
-  // A little unwind to catch the really small ints.
-  int snapshot_byte = Get();
-  if ((snapshot_byte & 0x80) == 0) {
-    return snapshot_byte;
-  }
-  int accumulator = (snapshot_byte & 0x7f) << 7;
-  while (true) {
-    snapshot_byte = Get();
-    if ((snapshot_byte & 0x80) == 0) {
-      return accumulator | snapshot_byte;
-    }
-    accumulator = (accumulator | (snapshot_byte & 0x7f)) << 7;
-  }
-  UNREACHABLE();
-  return accumulator;
+  // This way of variable-length encoding integers does not suffer from branch
+  // mispredictions.
+  uint32_t answer = GetUnalignedInt();
+  int bytes = answer & 3;
+  Advance(bytes);
+  uint32_t mask = 0xffffffffu;
+  mask >>= 32 - (bytes << 3);
+  answer &= mask;
+  answer >>= 2;
+  return answer;
 }
 
 
@@ -340,6 +328,12 @@
   // Deserialize a single object and the objects reachable from it.
   void DeserializePartial(Object** root);
 
+  void set_reservation(int space_number, uintptr_t reservation) {
+    ASSERT(space_number >= 0);
+    ASSERT(space_number <= LAST_SPACE);
+    reservations_[space_number] = reservation;
+  }
+
  private:
   virtual void VisitPointers(Object** start, Object** end);
 
@@ -358,29 +352,37 @@
   // the heap.
   void ReadChunk(
       Object** start, Object** end, int space, Address object_address);
-  HeapObject* GetAddressFromStart(int space);
-  inline HeapObject* GetAddressFromEnd(int space);
-  Address Allocate(int space_number, Space* space, int size);
-  void ReadObject(int space_number, Space* space, Object** write_back);
+  void ReadObject(int space_number, Object** write_back);
 
+  // This routine both allocates a new object, and also keeps
+  // track of where objects have been allocated so that we can
+  // fix back references when deserializing.
+  Address Allocate(int space_index, int size) {
+    Address address = high_water_[space_index];
+    high_water_[space_index] = address + size;
+    return address;
+  }
+
+  // This returns the address of an object that has been described in the
+  // snapshot as being offset bytes back in a particular space.
+  HeapObject* GetAddressFromEnd(int space) {
+    int offset = source_->GetInt();
+    offset <<= kObjectAlignmentBits;
+    return HeapObject::FromAddress(high_water_[space] - offset);
+  }
+
+
   // Cached current isolate.
   Isolate* isolate_;
 
-  // Keep track of the pages in the paged spaces.
-  // (In large object space we are keeping track of individual objects
-  // rather than pages.)  In new space we just need the address of the
-  // first object and the others will flow from that.
-  List<Address> pages_[SerializerDeserializer::kNumberOfSpaces];
-
   SnapshotByteSource* source_;
   // This is the address of the next object that will be allocated in each
   // space.  It is used to calculate the addresses of back-references.
   Address high_water_[LAST_SPACE + 1];
-  // This is the address of the most recent object that was allocated.  It
-  // is used to set the location of the new page when we encounter a
-  // START_NEW_PAGE_SERIALIZATION tag.
-  Address last_object_address_;
 
+  intptr_t reservations_[LAST_SPACE + 1];
+  static const intptr_t kUninitializedReservation = -1;
+
   ExternalReferenceDecoder* external_reference_decoder_;
 
   DISALLOW_COPY_AND_ASSIGN(Deserializer);
@@ -461,7 +463,7 @@
   // You can call this after serialization to find out how much space was used
   // in each space.
   int CurrentAllocationAddress(int space) {
-    if (SpaceIsLarge(space)) return large_object_total_;
+    ASSERT(space < kNumberOfSpaces);
     return fullness_[space];
   }
 
@@ -478,8 +480,11 @@
   static void TooLateToEnableNow() { too_late_to_enable_now_ = true; }
   static bool enabled() { return serialization_enabled_; }
   SerializationAddressMapper* address_mapper() { return &address_mapper_; }
-  void PutRoot(
-      int index, HeapObject* object, HowToCode how, WhereToPoint where);
+  void PutRoot(int index,
+               HeapObject* object,
+               HowToCode how,
+               WhereToPoint where,
+               int skip);
 
  protected:
   static const int kInvalidRootIndex = -1;
@@ -503,7 +508,9 @@
         object_(HeapObject::cast(o)),
         sink_(sink),
         reference_representation_(how_to_code + where_to_point),
-        bytes_processed_so_far_(0) { }
+        bytes_processed_so_far_(0),
+        code_object_(o->IsCode()),
+        code_has_been_output_(false) { }
     void Serialize();
     void VisitPointers(Object** start, Object** end);
     void VisitEmbeddedPointer(RelocInfo* target);
@@ -523,34 +530,36 @@
     }
 
    private:
-    void OutputRawData(Address up_to);
+    enum ReturnSkip { kCanReturnSkipInsteadOfSkipping, kIgnoringReturn };
+    // This function outputs or skips the raw data between the last pointer and
+    // up to the current position.  It optionally can just return the number of
+    // bytes to skip instead of performing a skip instruction, in case the skip
+    // can be merged into the next instruction.
+    int OutputRawData(Address up_to, ReturnSkip return_skip = kIgnoringReturn);
 
     Serializer* serializer_;
     HeapObject* object_;
     SnapshotByteSink* sink_;
     int reference_representation_;
     int bytes_processed_so_far_;
+    bool code_object_;
+    bool code_has_been_output_;
   };
 
   virtual void SerializeObject(Object* o,
                                HowToCode how_to_code,
-                               WhereToPoint where_to_point) = 0;
+                               WhereToPoint where_to_point,
+                               int skip) = 0;
   void SerializeReferenceToPreviousObject(
       int space,
       int address,
       HowToCode how_to_code,
-      WhereToPoint where_to_point);
+      WhereToPoint where_to_point,
+      int skip);
   void InitializeAllocators();
-  // This will return the space for an object.  If the object is in large
-  // object space it may return kLargeCode or kLargeFixedArray in order
-  // to indicate to the deserializer what kind of large object allocation
-  // to make.
+  // This will return the space for an object.
   static int SpaceOfObject(HeapObject* object);
-  // This just returns the space of the object.  It will return LO_SPACE
-  // for all large objects since you can't check the type of the object
-  // once the map has been used for the serialization address.
-  static int SpaceOfAlreadySerializedObject(HeapObject* object);
-  int Allocate(int space, int size, bool* new_page_started);
+  int Allocate(int space, int size);
   int EncodeExternalReference(Address addr) {
     return external_reference_encoder_->Encode(addr);
   }
@@ -559,9 +568,7 @@
 
   Isolate* isolate_;
   // Keep track of the fullness of each space in order to generate
-  // relative addresses for back references.  Large objects are
-  // just numbered sequentially since relative addresses make no
-  // sense in large object space.
+  // relative addresses for back references.
   int fullness_[LAST_SPACE + 1];
   SnapshotByteSink* sink_;
   int current_root_index_;
@@ -569,9 +576,9 @@
   static bool serialization_enabled_;
   // Did we already make use of the fact that serialization was not enabled?
   static bool too_late_to_enable_now_;
-  int large_object_total_;
   SerializationAddressMapper address_mapper_;
   intptr_t root_index_wave_front_;
+  void Pad();
 
   friend class ObjectSerializer;
   friend class Deserializer;
@@ -594,7 +601,8 @@
   virtual void Serialize(Object** o);
   virtual void SerializeObject(Object* o,
                                HowToCode how_to_code,
-                               WhereToPoint where_to_point);
+                               WhereToPoint where_to_point,
+                               int skip);
 
  protected:
   virtual int PartialSnapshotCacheIndex(HeapObject* o);
@@ -632,11 +640,13 @@
   virtual void SerializeStrongReferences();
   virtual void SerializeObject(Object* o,
                                HowToCode how_to_code,
-                               WhereToPoint where_to_point);
+                               WhereToPoint where_to_point,
+                               int skip);
   void SerializeWeakReferences();
   void Serialize() {
     SerializeStrongReferences();
     SerializeWeakReferences();
+    Pad();
   }
 
  private: