Introduce FieldIndex to unify and abstract property/field offset

src/field-index.h

+// Copyright 2014 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef V8_FIELD_INDEX_H_
+#define V8_FIELD_INDEX_H_
+
+#include "src/utils.h"
+#include "src/property-details.h"
+
+namespace v8 {
+namespace internal {
+
+class Map;
+
+// Wrapper class to hold a field index, usually but not necessarily generated
+// from a property index. When available, the wrapper class captures additional
+// information to allow the field index to be translated back into the property
+// index it was originally generated from.
+class FieldIndex V8_FINAL {
+ public:
+  static FieldIndex ForPropertyIndex(Map* map,
+                                     int index,
+                                     bool is_double = false);
+  static FieldIndex ForInObjectOffset(int offset, Map* map = NULL);
+  static FieldIndex ForLookupResult(const LookupResult* result);
+  static FieldIndex ForDescriptor(Map* map, int descriptor_index);
+  static FieldIndex ForKeyedLookupCacheIndex(Map* map, int index) {
+    return ForPropertyIndex(map, index);
+  }
+  static FieldIndex ForLoadByFieldIndex(Map* map, int index);
+
+  bool is_inobject() const {
+    return IsInObjectBits::decode(bit_field_);
+  }
+
+  bool is_double() const {
+    return IsDoubleBits::decode(bit_field_);
+  }
+
+  int offset() const {
+    return index() * kPointerSize;
+  }
+
+  int index() const {
+    return IndexBits::decode(bit_field_);
+  }
+
+  int property_index() const {
+    return index() - header_size() / kPointerSize;
+  }
+
+  int GetLoadByFieldIndex() const {
+    // For efficiency, the LoadByFieldIndex instruction takes an index that is
+    // optimized for quick access. If the property is inline, the index is
+    // positive. If it's out-of-line, the encoded index is -raw_index - 1 to
+    // disambiguate the zero out-of-line index from the zero inobject case.
+    // The index itself is shifted up by one bit, the lower-most bit
+    // signifying if the field is a mutable double box (1) or not (0).
+    int result = property_index();
+    if (!is_inobject()) {
+      result = -result - 1;
+    }
+    result <<= 1;
+    return is_double() ? (result | 1) : result;
+  }
+
+  int GetKeyedLookupCacheIndex() const {
+    int result = property_index();
+    if (!is_inobject()) {
+      result += InObjectPropertyBits::decode(bit_field_);
+    }
+    return result;
+  }
+
+  int GetLoadFieldStubKey() const {
+    return bit_field_ &
+        (IsInObjectBits::kMask | IsDoubleBits::kMask | IndexBits::kMask);
+  }
+
+ private:
+  FieldIndex(bool is_inobject, int local_index, bool is_double,
+             int inobject_properties, int header_size,
+             bool is_hidden = false) {
+    ASSERT((header_size & (kPointerSize - 1)) == 0);
+    bit_field_ = IsInObjectBits::encode(is_inobject) |
+      IsDoubleBits::encode(is_double) |
+      HeaderSizeBits::encode(header_size) |
+      IsHiddenField::encode(is_hidden) |
+      IndexBits::encode(local_index) |
+      InObjectPropertyBits::encode(inobject_properties);
+  }
+
+  int header_size() const {
+    ASSERT(!IsHiddenField::decode(bit_field_));
+    return HeaderSizeBits::decode(bit_field_);
+  }
+
+  static const int kIndexBitsSize = kDescriptorIndexBitCount + 1;
+
+  class IndexBits : public BitField<int, 0, kIndexBitsSize> {};
+  class IsInObjectBits : public BitField<bool, IndexBits::kNext, 1> {};
+  class IsDoubleBits : public BitField<bool, IsInObjectBits::kNext, 1> {};
+  class InObjectPropertyBits : public BitField<int, IsDoubleBits::kNext,
+                                               kDescriptorIndexBitCount> {};
+  class HeaderSizeBits : public BitField<int, InObjectPropertyBits::kNext,
+                                         7> {};
+  class IsHiddenField : public BitField<bool, HeaderSizeBits::kNext, 1> {};
+  STATIC_ASSERT(IsHiddenField::kNext <= 32);
+
+  int bit_field_;
+};
+
+} }  // namespace v8::internal
+
+#endif