markOop.hpp

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/* Copyright 1994 - 1996 LongView Technologies L.L.C. $Revision: 1.24 $ */
/* Copyright (c) 2006, Sun Microsystems, Inc.
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// Bit-format of markOop:
//  sentinel:1 near_death:1 tagged_contents:1 age:7 hash:20 tag:2 = 32 bits
//  - sentinel is needed during pointer reversal in garbage collection 
//    to distinguish markOops from roots of oops. markOops are used termination
//    elements in the pointer list.
//    During normal execution the sentinel is 1 but the but may be used for
//    marking during special vm operations like dependency checking.
//  - near_death is set by the memory system iff weak pointers keep the object alive.
//  - tagged_contents indicates the is no untagged data in the object.
//  - age contains the age of the object when residing in the new generation.
//    (during garbage collection, age is used to store the size of the object).
//    PLEASE DO NOT CHANGE THE AGE FIELD since the garbage collector relies on the size.
//  - hash contains the identity hash value.
//  - tag contains the special mark tag

class markOopDesc: public oopDesc {
 private:
  unsigned int value()  const { return (unsigned int) this; }
  friend   int assign_hash(markOop& m);

  enum { no_hash    = 0, 
         first_hash = 1};

  enum { sentinel_bits         = 1,
         near_death_bits       = 1,
         tagged_contents_bits  = 1,
         age_bits              = 7,
         hash_bits             = BitsPerWord - sentinel_bits - near_death_bits - tagged_contents_bits - age_bits - Tag_Size };

  enum { hash_shift            = Tag_Size,
         age_shift             = hash_bits             + hash_shift,
         tagged_contents_shift = age_bits              + age_shift,
         near_death_shift      = tagged_contents_bits  + tagged_contents_shift,
         sentinel_shift        = near_death_bits       + near_death_shift };

  enum { hash_mask                     = nthMask(hash_bits),
         hash_mask_in_place            = hash_mask << hash_shift,
         age_mask                      = nthMask(age_bits),
         age_mask_in_place             = age_mask << age_shift,
         tagged_contents_mask          = nthMask(tagged_contents_bits),
         tagged_contents_mask_in_place = tagged_contents_mask << tagged_contents_shift,
         near_death_mask               = nthMask(near_death_bits),
         near_death_mask_in_place      = near_death_mask << near_death_shift,
         sentinel_mask                 = nthMask(sentinel_bits),
         sentinel_mask_in_place        = sentinel_mask << sentinel_shift };


  enum { no_hash_in_place           = no_hash     << hash_shift,
         first_hash_in_place        = first_hash  << hash_shift,
         untagged_contents_in_place = 1           << tagged_contents_shift };

  enum { sentinel_is_place = 1 << sentinel_shift } ;
 public:
  enum { max_age = age_mask };
  // accessors
  bool    has_sentinel()      const { return maskBits(value(), sentinel_mask_in_place) != 0; }
  markOop set_sentinel()      const { return markOop( sentinel_is_place | value()); } 
  markOop clear_sentinel()    const { return markOop(~sentinel_is_place & value()); }

  bool has_tagged_contents()  const { return maskBits(value(), tagged_contents_mask_in_place) != 0; }

  bool    is_near_death()     const { return maskBits(value(), near_death_mask_in_place) != 0; }
  markOop set_near_death()    const { return markOop( near_death_mask_in_place | value()); } 
  markOop clear_near_death()  const { return markOop(~near_death_mask_in_place & value()); }

  // klass invalidation (via sentinel bit)
  bool    is_klass_invalid()  const { return !has_sentinel();  }
  markOop set_klass_invalid() const { return clear_sentinel(); }
  markOop clear_klass_invalid() const { return set_sentinel(); }

  // tells if context has forward reference to an unoptimized context(via sentinel bit)
  static int context_forward_bit_mask() { return near_death_mask_in_place; }
  bool    has_context_forward() const { return is_near_death();  }
  markOop set_context_forward() const { return set_near_death(); }

  // notification queue check
  bool    is_queued()    const { return !has_sentinel();  }
  markOop set_queued()   const { return clear_sentinel(); }
  markOop clear_queued() const { return set_sentinel();   }

  // age operations
  int        age()            const { return maskBits(value(), age_mask_in_place)  >> age_shift; }
  markOop    set_age(int v)   const {
    assert((v & ~age_mask) == 0, "shouldn't overflow field");
    return markOop((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
  }
  markOop    incr_age()       const { return age() == max_age ? markOop(this) : set_age(age() + 1); }

  // hash operations
  int        hash()           const { return maskBits(value(), hash_mask_in_place) >> hash_shift; }
  markOop    set_hash(int v)  const {
    if ((v & hash_mask) == 0) v = first_hash; // avoid no_hash
    markOop val = markOop((value() & ~hash_mask_in_place) | ((v & hash_mask) << hash_shift));
    assert(val->hash() != no_hash, "should have hash now");
    return val;
  }
  bool       has_valid_hash() const { return hash() != no_hash; }

  // markOop prototypes
  static markOop tagged_prototype() {
    return markOop(sentinel_is_place | no_hash_in_place | Mark_Tag);
  }
  static markOop untagged_prototype() {
    return markOop(sentinel_is_place | untagged_contents_in_place | no_hash_in_place | Mark_Tag);
  }

  // badOop
  static markOop bad() {
    return markOop(sentinel_is_place | first_hash_in_place | Mark_Tag);
  }

  friend int hash_markOop(markOop& m) {
    int v = m->hash();
    return v == no_hash ? assign_hash(m) : v;
  }

  // printing
  void print_on(outputStream* st);

  static int masked_hash(int v) { return v & hash_mask; }
};

# define badOop markOopDesc::bad()

// tells whether p is a root to an oop or a markOop
// used during pointer reversal during GC.
inline bool is_oop_root(oop* p) { return !markOop(p)->has_sentinel(); }

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