1 /*
2 * Copyright (c) 2025, 2026, 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_LAYOUTKIND_HPP
26 #define SHARE_OOPS_LAYOUTKIND_HPP
27
28 #include "memory/allStatic.hpp"
29 #include "utilities/globalDefinitions.hpp"
30 #include "utilities/ostream.hpp"
31
32 // LayoutKind is an enum used to indicate which layout has been used for a given value field.
33 // Each layout has its own properties and its own access protocol that is detailed below.
34 //
35 // REFERENCE : This layout uses a pointer to a heap allocated instance (no flattening).
36 // When used, field_flags().is_flat() is false . The field can be nullable or
37 // null-restricted, in the later case, field_flags().is_null_free_inline_type() is true.
38 // In case of a null-restricted field, putfield and putstatic must perform a null-check
39 // before writing a new value. Still for null-restricted fields, if getfield reads a null pointer
40 // from the receiver, it means that the field was not initialized yet, and getfield must substitute
41 // the null reference with the default value of the field's class.
42 // NULL_FREE_NON_ATOMIC_FLAT : This layout is the simplest form of flattening. Any field embedded inside the flat field
43 // can be accessed independently. The field is null-restricted, meaning putfield must perform a
44 // null-check before performing a field update.
45 // NULL_FREE_ATOMIC_FLAT : This flat layout is designed for atomic updates, with size and alignment that make use of
46 // atomic instructions possible. All accesses, reads and writes, must be performed atomically.
47 // The field is null-restricted, meaning putfield must perform a null-check before performing a
48 // field update.
49 // NULLABLE_ATOMIC_FLAT : This is the flat layout designed for JEP 401. It is designed for atomic updates,
50 // with size and alignment that make use of atomic instructions possible. All accesses, reads and
51 // writes, must be performed atomically. The layout includes a null marker which indicates if the
52 // field's value must be considered as null or not. The null marker is a byte, with the value zero
53 // meaning the field's value is null, and a non-zero value meaning the field's value is not null.
54 // A getfield must check the value of the null marker before returning a value. If the null marker
55 // is zero, getfield must return the null reference, otherwise it returns the field's value read
56 // from the receiver. When a putfield writes a non-null value to such field, the update, including
57 // the field's value and the null marker, must be performed in a single atomic operation. If the
58 // source of the value is a heap allocated instance of the field's class, it is allowed to set the
59 // null marker to non-zero in the heap allocated instance before copying the value to the receiver
60 // (the BUFFERED layout used in heap allocated values guarantees that the space for the null marker
61 // is included, but has no meaning for the heap allocated instance which is always non-null, and that
62 // the whole payload is correctly aligned for atomic operations). When a putfield writes null to such
63 // field, the null marker must be set to zero. However, if the field contains oops, those oops must be
64 // cleared too in order to prevent memory leaks. In order to simplify such operation, value classes
65 // supporting a NULLABLE_ATOMIC_FLAT layout have a pre-allocated reset value instance, filled with
66 // zeros, which can be used to simply overwrite the whole flat field and reset everything (oops and
67 // null marker). The reset value instance is needed because the VM needs an instance guaranteed to
68 // always be filled with zeros, and the default value could have its null marker set to non-zero if
69 // it is used as a source to update a NULLABLE_ATOMIC_FLAT field.
70 // NULLABLE_NON_ATOMIC_FLAT: This is a special layout, only used for strict final non-static fields. Because strict
71 // final non-static fields cannot be updated after the call to the super constructor, there's no
72 // concurrency issue on those fields, so they can be flattened even if they are nullable. During the
73 // construction of the instance, the uninitializedThis reference cannot escape before the call to
74 // the super's constructor, so no concurrent reads are possible when the field is initialized. After
75 // the call to the super's constructor, no update is possible because the field is strict and final,
76 // so no write possible during a read. This field has a null marker similar to the one of the
77 // NULLABLE_ATOMIC_FLAT layout. However, there's no requirement to read the null marker and the
78 // rest of the value atomically. If the null marker indicates a non-null value, the fields of the
79 // field's value can be read independently. Same rules for a putfield, no atomicity requirement,
80 // as long as all fields and the null marker are up to date at the end of the putfield.
81 // BUFFERED: This layout is only used in heap buffered instances of a value class. It is computed to be compatible
82 // in size and alignment with all other flat layouts supported by the value class.
83 //
84 //
85 // IMPORTANT: The REFERENCE layout must always be associated with the numerical value zero, because the implementation
86 // of the lava.lang.invoke.MemberName class relies on this property.
87
88 enum class LayoutKind : uint32_t {
89 REFERENCE = 0, // indirection to a heap allocated instance
90 BUFFERED = 1, // layout used in heap allocated standalone instances
91 NULL_FREE_NON_ATOMIC_FLAT = 2, // flat, null-free (no null marker), no guarantee of atomic updates
92 NULL_FREE_ATOMIC_FLAT = 3, // flat, null-free, size compatible with atomic updates, alignment requirement is equal to the size
93 NULLABLE_ATOMIC_FLAT = 4, // flat, include a null marker, plus same size/alignment properties as ATOMIC layout
94 NULLABLE_NON_ATOMIC_FLAT = 5, // flat, include a null marker, non-atomic, only used for strict final non-static fields
95 UNKNOWN = 6 // used for uninitialized fields of type LayoutKind
96 };
97
98 class outputStream;
99
100 class LayoutKindHelper : AllStatic {
101 public:
102 static LayoutKind get_copy_layout(LayoutKind src, LayoutKind dst) {
103 assert(src == dst || src == LayoutKind::BUFFERED || dst == LayoutKind::BUFFERED,
104 "Only same or from/to BUFFERED is supported. src: %s, dst: %s",
105 layout_kind_as_string(src), layout_kind_as_string(dst));
106 return src == LayoutKind::BUFFERED ? dst : src;
107 }
108
109 static bool is_flat(LayoutKind lk) {
110 assert(lk != LayoutKind::UNKNOWN, "Sanity check");
111 return lk == LayoutKind::NULL_FREE_NON_ATOMIC_FLAT ||
112 lk == LayoutKind::NULL_FREE_ATOMIC_FLAT ||
113 lk == LayoutKind::NULLABLE_ATOMIC_FLAT ||
114 lk == LayoutKind::NULLABLE_NON_ATOMIC_FLAT;
115 }
116 static bool is_atomic_flat(LayoutKind lk) {
117 return lk == LayoutKind::NULL_FREE_ATOMIC_FLAT ||
118 lk == LayoutKind::NULLABLE_ATOMIC_FLAT;
119 }
120 static bool is_nullable_flat(LayoutKind lk) {
121 return lk == LayoutKind::NULLABLE_ATOMIC_FLAT ||
122 lk == LayoutKind::NULLABLE_NON_ATOMIC_FLAT;
123 }
124 static bool is_null_free_flat(LayoutKind lk) {
125 return lk == LayoutKind::NULL_FREE_ATOMIC_FLAT ||
126 lk == LayoutKind::NULL_FREE_NON_ATOMIC_FLAT;
127 }
128 static const char* layout_kind_as_string(LayoutKind lk);
129
130 static void print_on(LayoutKind lk, outputStream* st) NOT_DEBUG_RETURN;
131 };
132
133 #endif // SHARE_OOPS_LAYOUTKIND_HPP