src/layout-descriptor.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_LAYOUT_DESCRIPTOR_H_
#define V8_LAYOUT_DESCRIPTOR_H_

#include <iosfwd>

#include "src/objects/fixed-array.h"

// Has to be the last include (doesn't have include guards):
#include "src/objects/object-macros.h"

namespace v8 {
namespace internal {

// LayoutDescriptor is a bit vector defining which fields contain non-tagged
// values. It could either be a fixed typed array (slow form) or a Smi
// if the length fits (fast form).
// Each bit in the layout represents a FIELD. The bits are referenced by
// field_index which is a field number. If the bit is set then the corresponding
// field contains a non-tagged value and therefore must be skipped by GC.
// Otherwise the field is considered tagged. If the queried bit lays "outside"
// of the descriptor then the field is also considered tagged.
// Once a layout descriptor is created it is allowed only to append properties
// to it. GC uses layout descriptors to iterate objects. Avoid heap pointers
// in a layout descriptor because they can lead to data races in GC when
// GC moves objects in parallel.
class LayoutDescriptor : public ByteArray {
 public:
  V8_INLINE bool IsTagged(int field_index);

  // Queries the contiguous region of fields that are either tagged or not.
  // Returns true if the given field is tagged or false otherwise and writes
  // the length of the contiguous region to |out_sequence_length|.
  // If the sequence is longer than |max_sequence_length| then
  // |out_sequence_length| is set to |max_sequence_length|.
  bool IsTagged(int field_index, int max_sequence_length,
                int* out_sequence_length);

  // Returns true if this is a layout of the object having only tagged fields.
  V8_INLINE bool IsFastPointerLayout();
  V8_INLINE static bool IsFastPointerLayout(Object layout_descriptor);

  // Returns true if the layout descriptor is in non-Smi form.
  V8_INLINE bool IsSlowLayout();

  DECL_CAST(LayoutDescriptor)

  V8_INLINE static LayoutDescriptor cast_gc_safe(Object object);

  // Builds layout descriptor optimized for given |map| by |num_descriptors|
  // elements of given descriptors array. The |map|'s descriptors could be
  // different.
  static Handle<LayoutDescriptor> New(Isolate* isolate, Handle<Map> map,
                                      Handle<DescriptorArray> descriptors,
                                      int num_descriptors);

  // Modifies |map|'s layout descriptor or creates a new one if necessary by
  // appending property with |details| to it.
  static Handle<LayoutDescriptor> ShareAppend(Isolate* isolate, Handle<Map> map,
                                              PropertyDetails details);

  // Creates new layout descriptor by appending property with |details| to
  // |map|'s layout descriptor and if it is still fast then returns it.
  // Otherwise the |full_layout_descriptor| is returned.
  static Handle<LayoutDescriptor> AppendIfFastOrUseFull(
      Isolate* isolate, Handle<Map> map, PropertyDetails details,
      Handle<LayoutDescriptor> full_layout_descriptor);

  // Layout descriptor that corresponds to an object all fields of which are
  // tagged (FastPointerLayout).
  V8_INLINE static LayoutDescriptor FastPointerLayout();

  // Check that this layout descriptor corresponds to given map.
  bool IsConsistentWithMap(Map map, bool check_tail = false);

  // Trims this layout descriptor to given number of descriptors. This happens
  // only when corresponding descriptors array is trimmed.
  // The layout descriptor could be trimmed if it was slow or it could
  // become fast.
  LayoutDescriptor Trim(Heap* heap, Map map, DescriptorArray descriptors,
                        int num_descriptors);

#ifdef OBJECT_PRINT
  // For our gdb macros, we should perhaps change these in the future.
  void Print();

  void ShortPrint(std::ostream& os);
  void Print(std::ostream& os);  // NOLINT
#endif

  // Capacity of layout descriptors in bits.
  V8_INLINE int capacity();

  static Handle<LayoutDescriptor> NewForTesting(Isolate* isolate, int length);
  LayoutDescriptor SetTaggedForTesting(int field_index, bool tagged);

 private:
  // Exclude sign-bit to simplify encoding.
  static constexpr int kBitsInSmiLayout =
      SmiValuesAre32Bits() ? 32 : kSmiValueSize - 1;

  static const int kBitsPerLayoutWord = 32;

  V8_INLINE int number_of_layout_words();
  V8_INLINE uint32_t get_layout_word(int index) const;
  V8_INLINE void set_layout_word(int index, uint32_t value);

  V8_INLINE static Handle<LayoutDescriptor> New(Isolate* isolate, int length);
  V8_INLINE static LayoutDescriptor FromSmi(Smi smi);

  V8_INLINE static bool InobjectUnboxedField(int inobject_properties,
                                             PropertyDetails details);

  // Calculates minimal layout descriptor capacity required for given
  // |map|, |descriptors| and |num_descriptors|.
  V8_INLINE static int CalculateCapacity(Map map, DescriptorArray descriptors,
                                         int num_descriptors);

  // Calculates the length of the slow-mode backing store array by given layout
  // descriptor length.
  V8_INLINE static int GetSlowModeBackingStoreLength(int length);

  // Fills in clean |layout_descriptor| according to given |map|, |descriptors|
  // and |num_descriptors|.
  V8_INLINE static LayoutDescriptor Initialize(
      LayoutDescriptor layout_descriptor, Map map, DescriptorArray descriptors,
      int num_descriptors);

  static Handle<LayoutDescriptor> EnsureCapacity(
      Isolate* isolate, Handle<LayoutDescriptor> layout_descriptor,
      int new_capacity);

  // Returns false if requested field_index is out of bounds.
  V8_INLINE bool GetIndexes(int field_index, int* layout_word_index,
                            int* layout_bit_index);

  V8_INLINE V8_WARN_UNUSED_RESULT LayoutDescriptor SetRawData(int field_index);

  V8_INLINE V8_WARN_UNUSED_RESULT LayoutDescriptor SetTagged(int field_index,
                                                             bool tagged);

  OBJECT_CONSTRUCTORS(LayoutDescriptor, ByteArray);
};


// LayoutDescriptorHelper is a helper class for querying layout descriptor
// about whether the field at given offset is tagged or not.
class LayoutDescriptorHelper {
 public:
  inline explicit LayoutDescriptorHelper(Map map);

  bool all_fields_tagged() { return all_fields_tagged_; }
  inline bool IsTagged(int offset_in_bytes);

  // Queries the contiguous region of fields that are either tagged or not.
  // Returns true if fields starting at |offset_in_bytes| are tagged or false
  // otherwise and writes the offset of the end of the contiguous region to
  // |out_end_of_contiguous_region_offset|. The |end_offset| value is the
  // upper bound for |out_end_of_contiguous_region_offset|.
  bool IsTagged(int offset_in_bytes, int end_offset,
                int* out_end_of_contiguous_region_offset);

 private:
  bool all_fields_tagged_;
  int header_size_;
  LayoutDescriptor layout_descriptor_;
};
}  // namespace internal
}  // namespace v8

#include "src/objects/object-macros-undef.h"

#endif  // V8_LAYOUT_DESCRIPTOR_H_