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