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24
25 #ifndef SHARE_OOPS_LAYOUTKIND_HPP
26 #define SHARE_OOPS_LAYOUTKIND_HPP
27
28 // LayoutKind is an enum used to indicate which layout has been used for a given value field.
29 // Each layout has its own properties and its own access protocol that is detailed below.
30 //
31 // REFERENCE : this layout uses a pointer to a heap allocated instance (no flattening).
32 // When used, field_flags().is_flat() is false . The field can be nullable or
33 // null-restricted, in the later case, field_flags().is_null_free_inline_type() is true.
34 // In case of a null-restricted field, putfield and putstatic must perform a null-check
35 // before writing a new value. Still for null-restricted fields, if getfield reads a null pointer
36 // from the receiver, it means that the field was not initialized yet, and getfield must substitute
37 // the null reference with the default value of the field's class.
38 // NON_ATOMIC_FLAT : this layout is the simplest form of flattening. Any field embedded inside the flat field
39 // can be accessed independently. The field is null-restricted, meaning putfield must perform a
40 // null-check before performing a field update.
41 // ATOMIC_FLAT : this flat layout is designed for atomic updates, with size and alignment that make use of
42 // atomic instructions possible. All accesses, reads and writes, must be performed atomically.
43 // The field is null-restricted, meaning putfield must perform a null-check before performing a
44 // field update.
45 // NULLABLE_ATOMIC_FLAT : this is the flat layout designed for JEP 401. It is designed for atomic updates,
46 // with size and alignment that make use of atomic instructions possible. All accesses, reads and
47 // writes, must be performed atomically. The layout includes a null marker which indicates if the
48 // field's value must be considered as null or not. The null marker is a byte, with the value zero
49 // meaning the field's value is null, and a non-zero value meaning the field's value is not null.
50 // A getfield must check the value of the null marker before returning a value. If the null marker
51 // is zero, getfield must return the null reference, otherwise it returns the field's value read
52 // from the receiver. When a putfield writes a non-null value to such field, the update, including
53 // the field's value and the null marker, must be performed in a single atomic operation. If the
54 // source of the value is a heap allocated instance of the field's class, it is allowed to set the
55 // null marker to non-zero in the heap allocated instance before copying the value to the receiver
56 // (the BUFFERED layout used in heap allocated values guarantees that the space for the null marker
57 // is included, but has no meaning for the heap allocated instance which is always non-null, and that
58 // the whole payload is correctly aligned for atomic operations). When a putfield writes null to such
59 // field, the null marker must be set to zero. However, if the field contains oops, those oops must be
60 // cleared too in order to prevent memory leaks. In order to simplify such operation, value classes
61 // supporting a NULLABLE_ATOMIC_FLAT layout have a pre-allocated reset value instance, filled with
62 // zeros, which can be used to simply overwrite the whole flat field and reset everything (oops and
63 // null marker). The reset value instance is needed because the VM needs an instance guaranteed to
64 // always be filled with zeros, and the default value could have its null marker set to non-zero if
65 // it is used as a source to update a NULLABLE_ATOMIC_FLAT field.
66 // BUFFERED: this layout is only used in heap buffered instances of a value class. It is computed to be compatible
67 // to be compatible in size and alignment with all other flat layouts supported by the value class.
68 //
69 //
70 // IMPORTANT: The REFERENCE layout must always be associated with the numerical value zero, because the implementation
71 // of the lava.lang.invoke.MemberName class relies on this property.
72
73 enum class LayoutKind : uint32_t {
74 REFERENCE = 0, // indirection to a heap allocated instance
75 BUFFERED = 1, // layout used in heap allocated standalone instances
76 NON_ATOMIC_FLAT = 2, // flat, no guarantee of atomic updates, no null marker
77 ATOMIC_FLAT = 3, // flat, size compatible with atomic updates, alignment requirement is equal to the size
78 NULLABLE_ATOMIC_FLAT = 4, // flat, include a null marker, plus same properties as ATOMIC layout
79 UNKNOWN = 5 // used for uninitialized fields of type LayoutKind
80 };
81
82 #endif // SHARE_OOPS_LAYOUTKIND_HPP