arrayOop.hpp revision 1472:c18cbe5936b8 
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24
25// arrayOopDesc is the abstract baseclass for all arrays.  It doesn't
26// declare pure virtual to enforce this because that would allocate a vtbl
27// in each instance, which we don't want.
28
29// The layout of array Oops is:
30//
31//  markOop
32//  klassOop  // 32 bits if compressed but declared 64 in LP64.
33//  length    // shares klass memory or allocated after declared fields.
34
35
36class arrayOopDesc : public oopDesc {
37  friend class VMStructs;
38
39  // Interpreter/Compiler offsets
40
41  // Header size computation.
42  // The header is considered the oop part of this type plus the length.
43  // Returns the aligned header_size_in_bytes.  This is not equivalent to
44  // sizeof(arrayOopDesc) which should not appear in the code.
45  static int header_size_in_bytes() {
46    size_t hs = align_size_up(length_offset_in_bytes() + sizeof(int),
47                              HeapWordSize);
48#ifdef ASSERT
49    // make sure it isn't called before UseCompressedOops is initialized.
50    static size_t arrayoopdesc_hs = 0;
51    if (arrayoopdesc_hs == 0) arrayoopdesc_hs = hs;
52    assert(arrayoopdesc_hs == hs, "header size can't change");
53#endif // ASSERT
54    return (int)hs;
55  }
56
57 public:
58  // The _length field is not declared in C++.  It is allocated after the
59  // declared nonstatic fields in arrayOopDesc if not compressed, otherwise
60  // it occupies the second half of the _klass field in oopDesc.
61  static int length_offset_in_bytes() {
62    return UseCompressedOops ? klass_gap_offset_in_bytes() :
63                               sizeof(arrayOopDesc);
64  }
65
66  // Returns the offset of the first element.
67  static int base_offset_in_bytes(BasicType type) {
68    return header_size(type) * HeapWordSize;
69  }
70
71  // Returns the address of the first element.
72  void* base(BasicType type) const {
73    return (void*) (((intptr_t) this) + base_offset_in_bytes(type));
74  }
75
76  // Tells whether index is within bounds.
77  bool is_within_bounds(int index) const        { return 0 <= index && index < length(); }
78
79  // Accessors for instance variable which is not a C++ declared nonstatic
80  // field.
81  int length() const {
82    return *(int*)(((intptr_t)this) + length_offset_in_bytes());
83  }
84  void set_length(int length) {
85    *(int*)(((intptr_t)this) + length_offset_in_bytes()) = length;
86  }
87
88  // Should only be called with constants as argument
89  // (will not constant fold otherwise)
90  // Returns the header size in words aligned to the requirements of the
91  // array object type.
92  static int header_size(BasicType type) {
93    size_t typesize_in_bytes = header_size_in_bytes();
94    return (int)(Universe::element_type_should_be_aligned(type)
95      ? align_object_size(typesize_in_bytes/HeapWordSize)
96      : typesize_in_bytes/HeapWordSize);
97  }
98
99  // Return the maximum length of an array of BasicType.  The length can passed
100  // to typeArrayOop::object_size(scale, length, header_size) without causing an
101  // overflow.
102  static int32_t max_array_length(BasicType type) {
103    assert(type >= 0 && type < T_CONFLICT, "wrong type");
104    assert(type2aelembytes(type) != 0, "wrong type");
105    const int bytes_per_element = type2aelembytes(type);
106    if (bytes_per_element < HeapWordSize) {
107      return max_jint;
108    }
109
110    const int32_t max_words = align_size_down(max_jint, MinObjAlignment);
111    const int32_t max_element_words = max_words - header_size(type);
112    const int32_t words_per_element = bytes_per_element >> LogHeapWordSize;
113    return max_element_words / words_per_element;
114  }
115};
116