1 /*
2 * Copyright (c) 1997, 2018, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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5 * This code is free software; you can redistribute it and/or modify it
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7 * published by the Free Software Foundation.
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11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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13 * accompanied this code).
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24
25 #ifndef SHARE_VM_OOPS_ARRAYOOP_HPP
26 #define SHARE_VM_OOPS_ARRAYOOP_HPP
27
28 #include "memory/universe.inline.hpp"
29 #include "oops/oop.hpp"
30 #include "utilities/align.hpp"
31
32 // arrayOopDesc is the abstract baseclass for all arrays. It doesn't
33 // declare pure virtual to enforce this because that would allocate a vtbl
34 // in each instance, which we don't want.
35
36 // The layout of array Oops is:
37 //
38 // markOop
39 // Klass* // 32 bits if compressed but declared 64 in LP64.
40 // length // shares klass memory or allocated after declared fields.
41
42
43 class arrayOopDesc : public oopDesc {
44 friend class VMStructs;
45 friend class arrayOopDescTest;
46
47 // Interpreter/Compiler offsets
48
49 // Header size computation.
50 // The header is considered the oop part of this type plus the length.
51 // Returns the aligned header_size_in_bytes. This is not equivalent to
52 // sizeof(arrayOopDesc) which should not appear in the code.
53 static int header_size_in_bytes() {
54 size_t hs = align_up(length_offset_in_bytes() + sizeof(int),
55 HeapWordSize);
56 #ifdef ASSERT
57 // make sure it isn't called before UseCompressedOops is initialized.
58 static size_t arrayoopdesc_hs = 0;
59 if (arrayoopdesc_hs == 0) arrayoopdesc_hs = hs;
60 assert(arrayoopdesc_hs == hs, "header size can't change");
61 #endif // ASSERT
62 return (int)hs;
63 }
64
65 public:
66 // The _length field is not declared in C++. It is allocated after the
67 // declared nonstatic fields in arrayOopDesc if not compressed, otherwise
68 // it occupies the second half of the _klass field in oopDesc.
69 static int length_offset_in_bytes() {
70 return UseCompressedClassPointers ? klass_gap_offset_in_bytes() :
71 sizeof(arrayOopDesc);
72 }
73
74 // Returns the offset of the first element.
75 static int base_offset_in_bytes(BasicType type) {
76 return header_size(type) * HeapWordSize;
77 }
78
79 // Returns the address of the first element. The elements in the array will not
80 // relocate from this address until a subsequent thread transition.
81 inline void* base(BasicType type) const;
82 inline void* base_raw(BasicType type) const; // GC barrier invariant
83
84 // Tells whether index is within bounds.
85 bool is_within_bounds(int index) const { return 0 <= index && index < length(); }
86
87 // Accessors for instance variable which is not a C++ declared nonstatic
88 // field.
89 int length() const {
90 return *(int*)(((intptr_t)this) + length_offset_in_bytes());
91 }
92 void set_length(int length) {
93 *(int*)(((intptr_t)this) + length_offset_in_bytes()) = length;
94 }
95
96 // Should only be called with constants as argument
97 // (will not constant fold otherwise)
98 // Returns the header size in words aligned to the requirements of the
99 // array object type.
100 static int header_size(BasicType type) {
101 size_t typesize_in_bytes = header_size_in_bytes();
102 return (int)(Universe::element_type_should_be_aligned(type)
103 ? align_object_offset(typesize_in_bytes/HeapWordSize)
104 : typesize_in_bytes/HeapWordSize);
105 }
106
107 // Return the maximum length of an array of BasicType. The length can passed
108 // to typeArrayOop::object_size(scale, length, header_size) without causing an
109 // overflow. We also need to make sure that this will not overflow a size_t on
110 // 32 bit platforms when we convert it to a byte size.
111 static int32_t max_array_length(BasicType type) {
112 assert(type >= 0 && type < T_CONFLICT, "wrong type");
113 assert(type2aelembytes(type) != 0, "wrong type");
114
115 const size_t max_element_words_per_size_t =
116 align_down((SIZE_MAX/HeapWordSize - header_size(type)), MinObjAlignment);
117 const size_t max_elements_per_size_t =
118 HeapWordSize * max_element_words_per_size_t / type2aelembytes(type);
119 if ((size_t)max_jint < max_elements_per_size_t) {
120 // It should be ok to return max_jint here, but parts of the code
121 // (CollectedHeap, Klass::oop_oop_iterate(), and more) uses an int for
122 // passing around the size (in words) of an object. So, we need to avoid
123 // overflowing an int when we add the header. See CRs 4718400 and 7110613.
124 return align_down(max_jint - header_size(type), MinObjAlignment);
125 }
126 return (int32_t)max_elements_per_size_t;
127 }
128
129 };
130
131 #endif // SHARE_VM_OOPS_ARRAYOOP_HPP