Tentative implementation of string slices (hidden under the flag --string-slices).

src/objects.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // 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
@@ skipping 68 matching lines @@
 //           - CompilationCacheTable
 //           - CodeCacheHashTable
 //           - MapCache
 //         - Context
 //         - JSFunctionResultCache
 //         - SerializedScopeInfo
 //       - String
 //         - SeqString
 //           - SeqAsciiString
 //           - SeqTwoByteString
+//         - SlicedString
 //         - ConsString
 //         - ExternalString
 //           - ExternalAsciiString
 //           - ExternalTwoByteString
 //       - HeapNumber
 //       - Code
 //       - Map
 //       - Oddball
 //       - Foreign
 //       - SharedFunctionInfo
@@ skipping 174 matching lines @@
   V(ASCII_SYMBOL_TYPE)                                                         \
   V(CONS_SYMBOL_TYPE)                                                          \
   V(CONS_ASCII_SYMBOL_TYPE)                                                    \
   V(EXTERNAL_SYMBOL_TYPE)                                                      \
   V(EXTERNAL_SYMBOL_WITH_ASCII_DATA_TYPE)                                      \
   V(EXTERNAL_ASCII_SYMBOL_TYPE)                                                \
   V(STRING_TYPE)                                                               \
   V(ASCII_STRING_TYPE)                                                         \
   V(CONS_STRING_TYPE)                                                          \
   V(CONS_ASCII_STRING_TYPE)                                                    \
+  V(SLICED_STRING_TYPE)                                                        \
   V(EXTERNAL_STRING_TYPE)                                                      \
   V(EXTERNAL_STRING_WITH_ASCII_DATA_TYPE)                                      \
   V(EXTERNAL_ASCII_STRING_TYPE)                                                \
   V(PRIVATE_EXTERNAL_ASCII_STRING_TYPE)                                        \
                                                                                \
   V(MAP_TYPE)                                                                  \
   V(CODE_TYPE)                                                                 \
   V(ODDBALL_TYPE)                                                              \
   V(JS_GLOBAL_PROPERTY_CELL_TYPE)                                              \
                                                                                \
@@ skipping 68 matching lines @@
     ascii_symbol,                                                              \
     AsciiSymbol)                                                               \
   V(CONS_SYMBOL_TYPE,                                                          \
     ConsString::kSize,                                                         \
     cons_symbol,                                                               \
     ConsSymbol)                                                                \
   V(CONS_ASCII_SYMBOL_TYPE,                                                    \
     ConsString::kSize,                                                         \
     cons_ascii_symbol,                                                         \
     ConsAsciiSymbol)                                                           \
+  V(SLICED_SYMBOL_TYPE,                                                        \
+    SlicedString::kSize,                                                       \
+    sliced_symbol,                                                             \
+    SlicedSymbol)                                                              \
+  V(SLICED_ASCII_SYMBOL_TYPE,                                                  \
+    SlicedString::kSize,                                                       \
+    sliced_ascii_symbol,                                                       \
+    SlicedAsciiSymbol)                                                         \
   V(EXTERNAL_SYMBOL_TYPE,                                                      \
     ExternalTwoByteString::kSize,                                              \
     external_symbol,                                                           \
     ExternalSymbol)                                                            \
   V(EXTERNAL_SYMBOL_WITH_ASCII_DATA_TYPE,                                      \
     ExternalTwoByteString::kSize,                                              \
     external_symbol_with_ascii_data,                                           \
     ExternalSymbolWithAsciiData)                                               \
   V(EXTERNAL_ASCII_SYMBOL_TYPE,                                                \
     ExternalAsciiString::kSize,                                                \
     external_ascii_symbol,                                                     \
     ExternalAsciiSymbol)                                                       \
   V(STRING_TYPE,                                                               \
     kVariableSizeSentinel,                                                     \
     string,                                                                    \
     String)                                                                    \
   V(ASCII_STRING_TYPE,                                                         \
     kVariableSizeSentinel,                                                     \
     ascii_string,                                                              \
     AsciiString)                                                               \
   V(CONS_STRING_TYPE,                                                          \
     ConsString::kSize,                                                         \
     cons_string,                                                               \
     ConsString)                                                                \
   V(CONS_ASCII_STRING_TYPE,                                                    \
     ConsString::kSize,                                                         \
     cons_ascii_string,                                                         \
     ConsAsciiString)                                                           \
+  V(SLICED_STRING_TYPE,                                                        \
+    SlicedString::kSize,                                                       \
+    sliced_string,                                                             \
+    SlicedString)                                                              \
+  V(SLICED_ASCII_STRING_TYPE,                                                  \
+    SlicedString::kSize,                                                       \
+    sliced_ascii_string,                                                       \
+    SlicedAsciiString)                                                         \
   V(EXTERNAL_STRING_TYPE,                                                      \
     ExternalTwoByteString::kSize,                                              \
     external_string,                                                           \
     ExternalString)                                                            \
   V(EXTERNAL_STRING_WITH_ASCII_DATA_TYPE,                                      \
     ExternalTwoByteString::kSize,                                              \
     external_string_with_ascii_data,                                           \
     ExternalStringWithAsciiData)                                               \
   V(EXTERNAL_ASCII_STRING_TYPE,                                                \
     ExternalAsciiString::kSize,                                                \
@@ skipping 53 matching lines @@
 const uint32_t kStringEncodingMask = 0x4;
 const uint32_t kTwoByteStringTag = 0x0;
 const uint32_t kAsciiStringTag = 0x4;
 
 // If bit 7 is clear, the low-order 2 bits indicate the representation
 // of the string.
 const uint32_t kStringRepresentationMask = 0x03;
 enum StringRepresentationTag {
   kSeqStringTag = 0x0,
   kConsStringTag = 0x1,
-  kExternalStringTag = 0x2
+  kExternalStringTag = 0x2,
+  kSlicedStringTag = 0x3
 };
-const uint32_t kIsConsStringMask = 0x1;
+const uint32_t kIsIndirectStringMask = 0x1;
+const uint32_t kIsIndirectStringTag = 0x1;

[antonm, 2011/07/27 12:16:39]

maybe worth adding some documentation why it works.  And would probably added a
helper function right here which checks if type is indirect.

 
 // If bit 7 is clear, then bit 3 indicates whether this two-byte
 // string actually contains ascii data.
 const uint32_t kAsciiDataHintMask = 0x08;
 const uint32_t kAsciiDataHintTag = 0x08;
 
 
 // A ConsString with an empty string as the right side is a candidate
 // for being shortcut by the garbage collector unless it is a
 // symbol. It's not common to have non-flat symbols, so we do not
 // shortcut them thereby avoiding turning symbols into strings. See
 // heap.cc and mark-compact.cc.
 const uint32_t kShortcutTypeMask =
     kIsNotStringMask |
     kIsSymbolMask |
     kStringRepresentationMask;
 const uint32_t kShortcutTypeTag = kConsStringTag;
 
 
 enum InstanceType {
   // String types.
   SYMBOL_TYPE = kTwoByteStringTag | kSymbolTag | kSeqStringTag,
   ASCII_SYMBOL_TYPE = kAsciiStringTag | kSymbolTag | kSeqStringTag,
   CONS_SYMBOL_TYPE = kTwoByteStringTag | kSymbolTag | kConsStringTag,
   CONS_ASCII_SYMBOL_TYPE = kAsciiStringTag | kSymbolTag | kConsStringTag,
+  SLICED_SYMBOL_TYPE = kTwoByteStringTag | kSymbolTag | kSlicedStringTag,
+  SLICED_ASCII_SYMBOL_TYPE = kAsciiStringTag | kSymbolTag | kSlicedStringTag,
   EXTERNAL_SYMBOL_TYPE = kTwoByteStringTag | kSymbolTag | kExternalStringTag,
   EXTERNAL_SYMBOL_WITH_ASCII_DATA_TYPE =
       kTwoByteStringTag | kSymbolTag | kExternalStringTag | kAsciiDataHintTag,
   EXTERNAL_ASCII_SYMBOL_TYPE =
       kAsciiStringTag | kSymbolTag | kExternalStringTag,
   STRING_TYPE = kTwoByteStringTag | kSeqStringTag,
   ASCII_STRING_TYPE = kAsciiStringTag | kSeqStringTag,
   CONS_STRING_TYPE = kTwoByteStringTag | kConsStringTag,
   CONS_ASCII_STRING_TYPE = kAsciiStringTag | kConsStringTag,
+  SLICED_STRING_TYPE = kTwoByteStringTag | kSlicedStringTag,
+  SLICED_ASCII_STRING_TYPE = kAsciiStringTag | kSlicedStringTag,
   EXTERNAL_STRING_TYPE = kTwoByteStringTag | kExternalStringTag,
   EXTERNAL_STRING_WITH_ASCII_DATA_TYPE =
       kTwoByteStringTag | kExternalStringTag | kAsciiDataHintTag,
   // LAST_STRING_TYPE
   EXTERNAL_ASCII_STRING_TYPE = kAsciiStringTag | kExternalStringTag,
   PRIVATE_EXTERNAL_ASCII_STRING_TYPE = EXTERNAL_ASCII_STRING_TYPE,
 
   // Objects allocated in their own spaces (never in new space).
   MAP_TYPE = kNotStringTag,  // FIRST_NONSTRING_TYPE
   CODE_TYPE,
@@ skipping 183 matching lines @@
   V(HeapObject)                                \
   V(Number)                                    \
 
 #define HEAP_OBJECT_TYPE_LIST(V)               \
   V(HeapNumber)                                \
   V(String)                                    \
   V(Symbol)                                    \
   V(SeqString)                                 \
   V(ExternalString)                            \
   V(ConsString)                                \
+  V(SlicedString)                              \
   V(ExternalTwoByteString)                     \
   V(ExternalAsciiString)                       \
   V(SeqTwoByteString)                          \
   V(SeqAsciiString)                            \
                                                \
   V(ExternalArray)                             \
   V(ExternalByteArray)                         \
   V(ExternalUnsignedByteArray)                 \
   V(ExternalShortArray)                        \
   V(ExternalUnsignedShortArray)                \
@@ skipping 4980 matching lines @@
 // prone and so we no longer put StringShapes in variables unless there is a
 // concrete performance benefit at that particular point in the code.
 class StringShape BASE_EMBEDDED {
  public:
   inline explicit StringShape(String* s);
   inline explicit StringShape(Map* s);
   inline explicit StringShape(InstanceType t);
   inline bool IsSequential();
   inline bool IsExternal();
   inline bool IsCons();
+  inline bool IsSliced();
   inline bool IsExternalAscii();
   inline bool IsExternalTwoByte();
   inline bool IsSequentialAscii();
   inline bool IsSequentialTwoByte();
   inline bool IsSymbol();
   inline StringRepresentationTag representation_tag();
   inline uint32_t full_representation_tag();
   inline uint32_t size_tag();
 #ifdef DEBUG
   inline uint32_t type() { return type_; }
@@ skipping 499 matching lines @@
   static const int kMinLength = 13;
 
   typedef FixedBodyDescriptor<kFirstOffset, kSecondOffset + kPointerSize, kSize>
           BodyDescriptor;
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(ConsString);
 };
 
 
+class SlicedString: public String {
+ public:
+
+  inline String* parent();
+  inline void set_parent(String* parent, WriteBarrierMode mode);
+  inline int offset();
+  inline void set_offset(int offset);
+
+  // Dispatched behavior.
+  uint16_t SlicedStringGet(int index);
+
+  // Casting.
+  static inline SlicedString* cast(Object* obj);
+
+  // Layout description.
+  static const int kParentOffset = POINTER_SIZE_ALIGN(String::kSize);
+  static const int kOffsetOffset = kParentOffset + kPointerSize;
+  static const int kSize = kOffsetOffset + kPointerSize;
+
+  // Support for StringInputBuffer
+  inline const unibrow::byte* SlicedStringReadBlock(ReadBlockBuffer* buffer,
+                                                    unsigned* offset_ptr,
+                                                    unsigned chars);
+  inline void SlicedStringReadBlockIntoBuffer(ReadBlockBuffer* buffer,
+                                              unsigned* offset_ptr,
+                                              unsigned chars);
+  // Minimum length for a sliced string.
+  static const int kMinLength = 13;
+
+  typedef FixedBodyDescriptor<kParentOffset,
+                              kOffsetOffset + kPointerSize, kSize>
+          BodyDescriptor;
+
+ private:
+  DISALLOW_IMPLICIT_CONSTRUCTORS(SlicedString);
+};
+
+
 // The ExternalString class describes string values that are backed by
 // a string resource that lies outside the V8 heap.  ExternalStrings
 // consist of the length field common to all strings, a pointer to the
 // external resource.  It is important to ensure (externally) that the
 // resource is not deallocated while the ExternalString is live in the
 // V8 heap.
 //
 // The API expects that all ExternalStrings are created through the
 // API.  Therefore, ExternalStrings should not be used internally.
 class ExternalString: public String {
@@ skipping 962 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_