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.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
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).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #ifndef SHARE_OOPS_ARRAYOOP_HPP
26 #define SHARE_OOPS_ARRAYOOP_HPP
27
28 #include "oops/oop.hpp"
29 #include "runtime/globals.hpp"
30 #include "utilities/align.hpp"
31 #include "utilities/globalDefinitions.hpp"
32
33 // arrayOopDesc is the abstract baseclass for all arrays. It doesn't
34 // declare pure virtual to enforce this because that would allocate a vtbl
35 // in each instance, which we don't want.
36
37 // The layout of array Oops is:
38 //
39 // markWord
40 // Klass* // 32 bits if compressed but declared 64 in LP64.
41 // length // shares klass memory or allocated after declared fields.
42
43
44 class arrayOopDesc : public oopDesc {
45 friend class VMStructs;
46 friend class arrayOopDescTest;
47
48 // Interpreter/Compiler offsets
49
50 private:
51 // Returns the address of the length "field". See length_offset_in_bytes().
52 static int* length_addr_impl(void* obj_ptr) {
53 char* ptr = static_cast<char*>(obj_ptr);
54 return reinterpret_cast<int*>(ptr + length_offset_in_bytes());
55 }
56
57 // Given a type, return true if elements of that type must be aligned to 64-bit.
58 static bool element_type_should_be_aligned(BasicType type) {
59 if (EnableValhalla && type == T_PRIMITIVE_OBJECT) {
60 return true; //CMH: tighten the alignment when removing T_PRIMITIVE_OBJECT
61 }
62 #ifdef _LP64
63 if (type == T_OBJECT || type == T_ARRAY) {
64 return !UseCompressedOops;
65 }
66 #endif
67 return type == T_DOUBLE || type == T_LONG;
68 }
69
70 public:
71 // Header size computation.
72 // The header is considered the oop part of this type plus the length.
73 // This is not equivalent to sizeof(arrayOopDesc) which should not appear in the code.
74 static int header_size_in_bytes() {
75 int hs = length_offset_in_bytes() + (int)sizeof(int);
76 #ifdef ASSERT
77 // make sure it isn't called before UseCompressedOops is initialized.
78 static int arrayoopdesc_hs = 0;
79 if (arrayoopdesc_hs == 0) arrayoopdesc_hs = hs;
80 assert(arrayoopdesc_hs == hs, "header size can't change");
81 #endif // ASSERT
82 return (int)hs;
83 }
84
85 // The _length field is not declared in C++. It is allocated after the
86 // declared nonstatic fields in arrayOopDesc if not compressed, otherwise
87 // it occupies the second half of the _klass field in oopDesc.
88 static int length_offset_in_bytes() {
89 return UseCompressedClassPointers ? klass_gap_offset_in_bytes() :
90 (int)sizeof(arrayOopDesc);
91 }
92
93 // Returns the offset of the first element.
94 static int base_offset_in_bytes(BasicType type) {
95 int hs = header_size_in_bytes();
96 return element_type_should_be_aligned(type) ? align_up(hs, BytesPerLong) : hs;
97 }
98
99 // Returns the address of the first element. The elements in the array will not
100 // relocate from this address until a subsequent thread transition.
101 void* base(BasicType type) const {
102 return reinterpret_cast<void*>(cast_from_oop<intptr_t>(as_oop()) + base_offset_in_bytes(type));
103 }
104
105 template <typename T>
106 static T* obj_offset_to_raw(arrayOop obj, size_t offset_in_bytes, T* raw) {
107 if (obj != nullptr) {
108 assert(raw == nullptr, "either raw or in-heap");
109 char* base = reinterpret_cast<char*>((void*) obj);
110 raw = reinterpret_cast<T*>(base + offset_in_bytes);
111 } else {
112 assert(raw != nullptr, "either raw or in-heap");
113 }
114 return raw;
115 }
116
117 // Tells whether index is within bounds.
118 bool is_within_bounds(int index) const { return 0 <= index && index < length(); }
119
120 // Accessors for array length. There's not a member variable for
121 // it; see length_offset_in_bytes().
122 int length() const { return *length_addr_impl(const_cast<arrayOopDesc*>(this)); }
123 void set_length(int length) { *length_addr_impl(this) = length; }
124
125 int* length_addr() {
126 return length_addr_impl(this);
127 }
128
129 static void set_length(HeapWord* mem, int length) {
130 *length_addr_impl(mem) = length;
131 }
132
133 // Return the maximum length of an array of BasicType. The length can be passed
134 // to typeArrayOop::object_size(scale, length, header_size) without causing an
135 // overflow. We also need to make sure that this will not overflow a size_t on
136 // 32 bit platforms when we convert it to a byte size.
137 static int32_t max_array_length(BasicType type) {
138 assert(type < T_CONFLICT, "wrong type");
139 assert(type2aelembytes(type) != 0, "wrong type");
140
141 int hdr_size_in_bytes = base_offset_in_bytes(type);
142 // This is rounded-up and may overlap with the first array elements.
143 int hdr_size_in_words = align_up(hdr_size_in_bytes, HeapWordSize) / HeapWordSize;
144
145 const size_t max_element_words_per_size_t =
146 align_down((SIZE_MAX/HeapWordSize - (size_t)hdr_size_in_words), MinObjAlignment);
147 const size_t max_elements_per_size_t =
148 HeapWordSize * max_element_words_per_size_t / (size_t)type2aelembytes(type);
149 if ((size_t)max_jint < max_elements_per_size_t) {
150 // It should be ok to return max_jint here, but parts of the code
151 // (CollectedHeap, Klass::oop_oop_iterate(), and more) uses an int for
152 // passing around the size (in words) of an object. So, we need to avoid
153 // overflowing an int when we add the header. See CRs 4718400 and 7110613.
154 return align_down(max_jint - hdr_size_in_words, MinObjAlignment);
155 }
156 return (int32_t)max_elements_per_size_t;
157 }
158
159 };
160
161 #endif // SHARE_OOPS_ARRAYOOP_HPP