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// The CollectedHeap type requires subtypes to implement a method

// "block_start".  For some subtypes, notably generational

// systems using card-table-based write barriers, the efficiency of this

// operation may be important.  Implementations of the "BlockOffsetArray"

// class may be useful in providing such efficient implementations.

//

// While generally mirroring the structure of the BOT for GenCollectedHeap,

// the following types are tailored more towards G1's uses; these should,

// however, be merged back into a common BOT to avoid code duplication

// and reduce maintenance overhead.

//

//    G1BlockOffsetTable (abstract)

//    -- G1BlockOffsetArray                (uses G1BlockOffsetSharedArray)

//       -- G1BlockOffsetArrayContigSpace

//

// A main impediment to the consolidation of this code might be the

// effect of making some of the block_start*() calls non-const as

// below. Whether that might adversely affect performance optimizations

// that compilers might normally perform in the case of non-G1

// collectors needs to be carefully investigated prior to any such

// consolidation.

// Forward declarations

class ContiguousSpace;

class G1BlockOffsetSharedArray;

class G1BlockOffsetTable VALUE_OBJ_CLASS_SPEC {

  friend class VMStructs;

protected:

  // These members describe the region covered by the table.

  // The space this table is covering.

  HeapWord* _bottom;    // == reserved.start

  HeapWord* _end;       // End of currently allocated region.

public:

  // Initialize the table to cover the given space.

  // The contents of the initial table are undefined.

  G1BlockOffsetTable(HeapWord* bottom, HeapWord* end) :

    _bottom(bottom), _end(end)

    {

      assert(_bottom <= _end, "arguments out of order");

    }

  // Note that the committed size of the covered space may have changed,

  // so the table size might also wish to change.

  virtual void resize(size_t new_word_size) = 0;

  virtual void set_bottom(HeapWord* new_bottom) {

    assert(new_bottom <= _end, "new_bottom > _end");

    _bottom = new_bottom;

    resize(pointer_delta(_end, _bottom));

  }

  // Requires "addr" to be contained by a block, and returns the address of

  // the start of that block.  (May have side effects, namely updating of

  // shared array entries that "point" too far backwards.  This can occur,

  // for example, when LAB allocation is used in a space covered by the

  // table.)

  virtual HeapWord* block_start_unsafe(const void* addr) = 0;

  // Same as above, but does not have any of the possible side effects

  // discussed above.

  virtual HeapWord* block_start_unsafe_const(const void* addr) const = 0;

  // Returns the address of the start of the block containing "addr", or

  // else "null" if it is covered by no block.  (May have side effects,

  // namely updating of shared array entries that "point" too far

  // backwards.  This can occur, for example, when lab allocation is used

  // in a space covered by the table.)

  inline HeapWord* block_start(const void* addr);

  // Same as above, but does not have any of the possible side effects

  // discussed above.

  inline HeapWord* block_start_const(const void* addr) const;

};

// This implementation of "G1BlockOffsetTable" divides the covered region

// into "N"-word subregions (where "N" = 2^"LogN".  An array with an entry

// for each such subregion indicates how far back one must go to find the

// start of the chunk that includes the first word of the subregion.

//

// Each BlockOffsetArray is owned by a Space.  However, the actual array

// may be shared by several BlockOffsetArrays; this is useful

// when a single resizable area (such as a generation) is divided up into

// several spaces in which contiguous allocation takes place,

// such as, for example, in G1 or in the train generation.)

// Here is the shared array type.

class G1BlockOffsetSharedArray: public CHeapObj {

  friend class G1BlockOffsetArray;

  friend class G1BlockOffsetArrayContigSpace;

  friend class VMStructs;

private:

  // The reserved region covered by the shared array.

  MemRegion _reserved;

  // End of the current committed region.

  HeapWord* _end;

  // Array for keeping offsets for retrieving object start fast given an

  // address.

  VirtualSpace _vs;

  u_char* _offset_array;          // byte array keeping backwards offsets

  // Bounds checking accessors:

  // For performance these have to devolve to array accesses in product builds.

  u_char offset_array(size_t index) const {

    assert(index < _vs.committed_size(), "index out of range");

    return _offset_array[index];

  }

  void set_offset_array(size_t index, u_char offset) {

    assert(index < _vs.committed_size(), "index out of range");

    assert(offset <= N_words, "offset too large");

    _offset_array[index] = offset;

  }

  void set_offset_array(size_t index, HeapWord* high, HeapWord* low) {

    assert(index < _vs.committed_size(), "index out of range");

    assert(high >= low, "addresses out of order");

    assert(pointer_delta(high, low) <= N_words, "offset too large");

    _offset_array[index] = (u_char) pointer_delta(high, low);

  }

  void set_offset_array(HeapWord* left, HeapWord* right, u_char offset) {

    assert(index_for(right - 1) < _vs.committed_size(),

           "right address out of range");

    assert(left  < right, "Heap addresses out of order");

    size_t num_cards = pointer_delta(right, left) >> LogN_words;

    memset(&_offset_array[index_for(left)], offset, num_cards);

  }

  void set_offset_array(size_t left, size_t right, u_char offset) {

    assert(right < _vs.committed_size(), "right address out of range");

    assert(left  <= right, "indexes out of order");

    size_t num_cards = right - left + 1;

    memset(&_offset_array[left], offset, num_cards);

  }

  void check_offset_array(size_t index, HeapWord* high, HeapWord* low) const {

    assert(index < _vs.committed_size(), "index out of range");

    assert(high >= low, "addresses out of order");

    assert(pointer_delta(high, low) <= N_words, "offset too large");

    assert(_offset_array[index] == pointer_delta(high, low),

           "Wrong offset");

  }

  bool is_card_boundary(HeapWord* p) const;

  // Return the number of slots needed for an offset array

  // that covers mem_region_words words.

  // We always add an extra slot because if an object

  // ends on a card boundary we put a 0 in the next

  // offset array slot, so we want that slot always

  // to be reserved.

  size_t compute_size(size_t mem_region_words) {

    size_t number_of_slots = (mem_region_words / N_words) + 1;

    return ReservedSpace::page_align_size_up(number_of_slots);

  }

public:

  enum SomePublicConstants {

    LogN = 9,

    LogN_words = LogN - LogHeapWordSize,

    N_bytes = 1 << LogN,

    N_words = 1 << LogN_words

  };

  // Initialize the table to cover from "base" to (at least)

  // "base + init_word_size".  In the future, the table may be expanded

  // (see "resize" below) up to the size of "_reserved" (which must be at

  // least "init_word_size".) The contents of the initial table are

  // undefined; it is the responsibility of the constituent

  // G1BlockOffsetTable(s) to initialize cards.

  G1BlockOffsetSharedArray(MemRegion reserved, size_t init_word_size);

  // Notes a change in the committed size of the region covered by the

  // table.  The "new_word_size" may not be larger than the size of the

  // reserved region this table covers.

  void resize(size_t new_word_size);

  void set_bottom(HeapWord* new_bottom);

  // Updates all the BlockOffsetArray's sharing this shared array to

  // reflect the current "top"'s of their spaces.

  void update_offset_arrays();

  // Return the appropriate index into "_offset_array" for "p".

  inline size_t index_for(const void* p) const;

  // Return the address indicating the start of the region corresponding to

  // "index" in "_offset_array".

  inline HeapWord* address_for_index(size_t index) const;

};

// And here is the G1BlockOffsetTable subtype that uses the array.

class G1BlockOffsetArray: public G1BlockOffsetTable {

  friend class G1BlockOffsetSharedArray;

  friend class G1BlockOffsetArrayContigSpace;

  friend class VMStructs;

private:

  enum SomePrivateConstants {

    N_words = G1BlockOffsetSharedArray::N_words,

    LogN    = G1BlockOffsetSharedArray::LogN

  };

  // The following enums are used by do_block_helper

  enum Action {

    Action_single,      // BOT records a single block (see single_block())

    Action_mark,        // BOT marks the start of a block (see mark_block())

    Action_check        // Check that BOT records block correctly

                        // (see verify_single_block()).

  };

  // This is the array, which can be shared by several BlockOffsetArray's

  // servicing different

  G1BlockOffsetSharedArray* _array;

  // The space that owns this subregion.

  Space* _sp;

  // If "_sp" is a contiguous space, the field below is the view of "_sp"

  // as a contiguous space, else NULL.

  ContiguousSpace* _csp;

  // If true, array entries are initialized to 0; otherwise, they are

  // initialized to point backwards to the beginning of the covered region.

  bool _init_to_zero;

  // The portion [_unallocated_block, _sp.end()) of the space that

  // is a single block known not to contain any objects.

  // NOTE: See BlockOffsetArrayUseUnallocatedBlock flag.

  HeapWord* _unallocated_block;

  // Sets the entries

  // corresponding to the cards starting at "start" and ending at "end"

  // to point back to the card before "start": the interval [start, end)

  // is right-open.

  void set_remainder_to_point_to_start(HeapWord* start, HeapWord* end);

  // Same as above, except that the args here are a card _index_ interval

  // that is closed: [start_index, end_index]

  void set_remainder_to_point_to_start_incl(size_t start, size_t end);

  // A helper function for BOT adjustment/verification work

  void do_block_internal(HeapWord* blk_start, HeapWord* blk_end, Action action);

protected:

  ContiguousSpace* csp() const { return _csp; }

  // Returns the address of a block whose start is at most "addr".

  // If "has_max_index" is true, "assumes "max_index" is the last valid one

  // in the array.

  inline HeapWord* block_at_or_preceding(const void* addr,

                                         bool has_max_index,

                                         size_t max_index) const;

  // "q" is a block boundary that is <= "addr"; "n" is the address of the

  // next block (or the end of the space.)  Return the address of the

  // beginning of the block that contains "addr".  Does so without side

  // effects (see, e.g., spec of  block_start.)

  inline HeapWord*

  forward_to_block_containing_addr_const(HeapWord* q, HeapWord* n,

                                         const void* addr) const;

  // "q" is a block boundary that is <= "addr"; return the address of the

  // beginning of the block that contains "addr".  May have side effects

  // on "this", by updating imprecise entries.

  inline HeapWord* forward_to_block_containing_addr(HeapWord* q,

                                                    const void* addr);

  // "q" is a block boundary that is <= "addr"; "n" is the address of the

  // next block (or the end of the space.)  Return the address of the

  // beginning of the block that contains "addr".  May have side effects

  // on "this", by updating imprecise entries.

  HeapWord* forward_to_block_containing_addr_slow(HeapWord* q,

                                                  HeapWord* n,

                                                  const void* addr);

  // Requires that "*threshold_" be the first array entry boundary at or

  // above "blk_start", and that "*index_" be the corresponding array

  // index.  If the block starts at or crosses "*threshold_", records

  // "blk_start" as the appropriate block start for the array index

  // starting at "*threshold_", and for any other indices crossed by the

  // block.  Updates "*threshold_" and "*index_" to correspond to the first

  // index after the block end.

  void alloc_block_work2(HeapWord** threshold_, size_t* index_,

                         HeapWord* blk_start, HeapWord* blk_end);

public:

  // The space may not have it's bottom and top set yet, which is why the

  // region is passed as a parameter.  If "init_to_zero" is true, the

  // elements of the array are initialized to zero.  Otherwise, they are

  // initialized to point backwards to the beginning.

  G1BlockOffsetArray(G1BlockOffsetSharedArray* array, MemRegion mr,

                     bool init_to_zero);

  // Note: this ought to be part of the constructor, but that would require

  // "this" to be passed as a parameter to a member constructor for

  // the containing concrete subtype of Space.

  // This would be legal C++, but MS VC++ doesn't allow it.

  void set_space(Space* sp);

  // Resets the covered region to the given "mr".

  void set_region(MemRegion mr);

  // Resets the covered region to one with the same _bottom as before but

  // the "new_word_size".

  void resize(size_t new_word_size);

  // These must be guaranteed to work properly (i.e., do nothing)

  // when "blk_start" ("blk" for second version) is "NULL".

  virtual void alloc_block(HeapWord* blk_start, HeapWord* blk_end);

  virtual void alloc_block(HeapWord* blk, size_t size) {

    alloc_block(blk, blk + size);

  }

  // The following methods are useful and optimized for a

  // general, non-contiguous space.

  // The given arguments are required to be the starts of adjacent ("blk1"

  // before "blk2") well-formed blocks covered by "this".  After this call,

  // they should be considered to form one block.

  virtual void join_blocks(HeapWord* blk1, HeapWord* blk2);

  // Given a block [blk_start, blk_start + full_blk_size), and

  // a left_blk_size < full_blk_size, adjust the BOT to show two

  // blocks [blk_start, blk_start + left_blk_size) and

  // [blk_start + left_blk_size, blk_start + full_blk_size).

  // It is assumed (and verified in the non-product VM) that the

  // BOT was correct for the original block.

  void split_block(HeapWord* blk_start, size_t full_blk_size,

                           size_t left_blk_size);

  // Adjust the BOT to show that it has a single block in the

  // range [blk_start, blk_start + size). All necessary BOT

  // cards are adjusted, but _unallocated_block isn't.

  void single_block(HeapWord* blk_start, HeapWord* blk_end);

  void single_block(HeapWord* blk, size_t size) {

    single_block(blk, blk + size);

  }

  // Adjust BOT to show that it has a block in the range

  // [blk_start, blk_start + size). Only the first card

  // of BOT is touched. It is assumed (and verified in the

  // non-product VM) that the remaining cards of the block

  // are correct.

  void mark_block(HeapWord* blk_start, HeapWord* blk_end);

  void mark_block(HeapWord* blk, size_t size) {

    mark_block(blk, blk + size);

  }

  // Adjust _unallocated_block to indicate that a particular

  // block has been newly allocated or freed. It is assumed (and

  // verified in the non-product VM) that the BOT is correct for

  // the given block.

  inline void allocated(HeapWord* blk_start, HeapWord* blk_end) {

    // Verify that the BOT shows [blk, blk + blk_size) to be one block.

    verify_single_block(blk_start, blk_end);

    if (BlockOffsetArrayUseUnallocatedBlock) {

      _unallocated_block = MAX2(_unallocated_block, blk_end);

    }

  }

  inline void allocated(HeapWord* blk, size_t size) {

    allocated(blk, blk + size);

  }

  inline void freed(HeapWord* blk_start, HeapWord* blk_end);

  inline void freed(HeapWord* blk, size_t size);

  virtual HeapWord* block_start_unsafe(const void* addr);

  virtual HeapWord* block_start_unsafe_const(const void* addr) const;

  // Requires "addr" to be the start of a card and returns the

  // start of the block that contains the given address.

  HeapWord* block_start_careful(const void* addr) const;

  // If true, initialize array slots with no allocated blocks to zero.

  // Otherwise, make them point back to the front.

  bool init_to_zero() { return _init_to_zero; }

  // Verification & debugging - ensure that the offset table reflects the fact

  // that the block [blk_start, blk_end) or [blk, blk + size) is a

  // single block of storage. NOTE: can;t const this because of

  // call to non-const do_block_internal() below.

  inline void verify_single_block(HeapWord* blk_start, HeapWord* blk_end) {

    if (VerifyBlockOffsetArray) {

      do_block_internal(blk_start, blk_end, Action_check);

    }

  }

  inline void verify_single_block(HeapWord* blk, size_t size) {

    verify_single_block(blk, blk + size);

  }

  // Verify that the given block is before _unallocated_block

  inline void verify_not_unallocated(HeapWord* blk_start,

                                     HeapWord* blk_end) const {

    if (BlockOffsetArrayUseUnallocatedBlock) {

      assert(blk_start < blk_end, "Block inconsistency?");

      assert(blk_end <= _unallocated_block, "_unallocated_block problem");

    }

  }

  inline void verify_not_unallocated(HeapWord* blk, size_t size) const {

    verify_not_unallocated(blk, blk + size);

  }

  void check_all_cards(size_t left_card, size_t right_card) const;

  virtual void set_for_starts_humongous(HeapWord* new_end);

};

// A subtype of BlockOffsetArray that takes advantage of the fact

// that its underlying space is a ContiguousSpace, so that its "active"

// region can be more efficiently tracked (than for a non-contiguous space).

class G1BlockOffsetArrayContigSpace: public G1BlockOffsetArray {

  friend class VMStructs;

  // allocation boundary at which offset array must be updated

  HeapWord* _next_offset_threshold;

  size_t    _next_offset_index;      // index corresponding to that boundary

  // Work function to be called when allocation start crosses the next

  // threshold in the contig space.

  void alloc_block_work1(HeapWord* blk_start, HeapWord* blk_end) {

    alloc_block_work2(&_next_offset_threshold, &_next_offset_index,

                      blk_start, blk_end);

  }

 public:

  G1BlockOffsetArrayContigSpace(G1BlockOffsetSharedArray* array, MemRegion mr);

  // Initialize the threshold to reflect the first boundary after the

  // bottom of the covered region.

  HeapWord* initialize_threshold();

  // Zero out the entry for _bottom (offset will be zero).

  void      zero_bottom_entry();

  // Return the next threshold, the point at which the table should be

  // updated.

  HeapWord* threshold() const { return _next_offset_threshold; }

  // These must be guaranteed to work properly (i.e., do nothing)

  // when "blk_start" ("blk" for second version) is "NULL".  In this

  // implementation, that's true because NULL is represented as 0, and thus

  // never exceeds the "_next_offset_threshold".

  void alloc_block(HeapWord* blk_start, HeapWord* blk_end) {

    if (blk_end > _next_offset_threshold)

      alloc_block_work1(blk_start, blk_end);

  }

  void alloc_block(HeapWord* blk, size_t size) {

     alloc_block(blk, blk+size);

  }

  HeapWord* block_start_unsafe(const void* addr);

  HeapWord* block_start_unsafe_const(const void* addr) const;

  virtual void set_for_starts_humongous(HeapWord* new_end);

};