1 /*
2 * Copyright (c) 1997, 2024, 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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10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
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24
25 #ifndef SHARE_OOPS_ARRAYOOP_HPP
26 #define SHARE_OOPS_ARRAYOOP_HPP
27
28 #include "oops/oop.hpp"
29 #include "utilities/align.hpp"
30 #include "utilities/globalDefinitions.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 // markWord
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 private:
50 // Returns the address of the length "field". See length_offset_in_bytes().
51 static int* length_addr_impl(void* obj_ptr) {
52 char* ptr = static_cast<char*>(obj_ptr);
53 return reinterpret_cast<int*>(ptr + length_offset_in_bytes());
54 }
55
56 // Given a type, return true if elements of that type must be aligned to 64-bit.
57 static bool element_type_should_be_aligned(BasicType type) {
58 #ifdef _LP64
59 if (type == T_OBJECT || type == T_ARRAY) {
60 return !UseCompressedOops;
61 }
62 #endif
63 return type == T_DOUBLE || type == T_LONG;
64 }
65
66 public:
67 // Header size computation.
68 // The header is considered the oop part of this type plus the length.
69 // This is not equivalent to sizeof(arrayOopDesc) which should not appear in the code.
70 static int header_size_in_bytes() {
71 int hs = length_offset_in_bytes() + (int)sizeof(int);
72 #ifdef ASSERT
73 // make sure it isn't called before UseCompressedOops is initialized.
74 static int arrayoopdesc_hs = 0;
75 if (arrayoopdesc_hs == 0) arrayoopdesc_hs = hs;
76 assert(arrayoopdesc_hs == hs, "header size can't change");
77 #endif // ASSERT
78 assert(!UseCompactObjectHeaders || hs == 8, "array header must be 8 bytes");
79 return (int)hs;
80 }
81
82 // The _length field is not declared in C++. It is allocated after the
83 // mark-word when using compact headers (+UseCompactObjectHeaders), otherwise
84 // after the compressed Klass* when running with compressed class-pointers
85 // (+UseCompressedClassPointers), or else after the full Klass*.
86 static int length_offset_in_bytes() {
87 assert(!UseCompactObjectHeaders || oopDesc::base_offset_in_bytes() == 4, "array length must be at 4 bytes");
88 return oopDesc::base_offset_in_bytes();
89 }
90
91 // Returns the offset of the first element.
92 static int base_offset_in_bytes(BasicType type) {
93 int hs = header_size_in_bytes();
94 return element_type_should_be_aligned(type) ? align_up(hs, BytesPerLong) : hs;
95 }
96
97 // Returns the address of the first element. The elements in the array will not
98 // relocate from this address until a subsequent thread transition.
99 void* base(BasicType type) const {
100 return reinterpret_cast<void*>(cast_from_oop<intptr_t>(as_oop()) + base_offset_in_bytes(type));
101 }
102
103 template <typename T>
104 static T* obj_offset_to_raw(arrayOop obj, size_t offset_in_bytes, T* raw) {
105 if (obj != nullptr) {
106 assert(raw == nullptr, "either raw or in-heap");
107 char* base = reinterpret_cast<char*>((void*) obj);
108 raw = reinterpret_cast<T*>(base + offset_in_bytes);
109 } else {
110 assert(raw != nullptr, "either raw or in-heap");
111 }
112 return raw;
113 }
114
115 // Tells whether index is within bounds.
116 bool is_within_bounds(int index) const { return 0 <= index && index < length(); }
117
118 // Accessors for array length. There's not a member variable for
119 // it; see length_offset_in_bytes().
120 int length() const { return *length_addr_impl(const_cast<arrayOopDesc*>(this)); }
121 void set_length(int length) { *length_addr_impl(this) = length; }
122
123 int* length_addr() {
124 return length_addr_impl(this);
125 }
126
127 static void set_length(HeapWord* mem, int length) {
128 *length_addr_impl(mem) = length;
129 }
130
131 // Return the maximum length of an array of BasicType. The length can be passed
132 // to typeArrayOop::object_size(scale, length, header_size) without causing an
133 // overflow. We also need to make sure that this will not overflow a size_t on
134 // 32 bit platforms when we convert it to a byte size.
135 static int32_t max_array_length(BasicType type) {
136 assert(type < T_CONFLICT, "wrong type");
137 assert(type2aelembytes(type) != 0, "wrong type");
138
139 int hdr_size_in_bytes = base_offset_in_bytes(type);
140 // This is rounded-up and may overlap with the first array elements.
141 int hdr_size_in_words = align_up(hdr_size_in_bytes, HeapWordSize) / HeapWordSize;
142
143 const size_t max_element_words_per_size_t =
144 align_down((SIZE_MAX/HeapWordSize - (size_t)hdr_size_in_words), MinObjAlignment);
145 const size_t max_elements_per_size_t =
146 HeapWordSize * max_element_words_per_size_t / (size_t)type2aelembytes(type);
147 if ((size_t)max_jint < max_elements_per_size_t) {
148 // It should be ok to return max_jint here, but parts of the code
149 // (CollectedHeap, Klass::oop_oop_iterate(), and more) uses an int for
150 // passing around the size (in words) of an object. So, we need to avoid
151 // overflowing an int when we add the header. See CRs 4718400 and 7110613.
152 return align_down(max_jint - hdr_size_in_words, MinObjAlignment);
153 }
154 return (int32_t)max_elements_per_size_t;
155 }
156
157 };
158
159 #endif // SHARE_OOPS_ARRAYOOP_HPP