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 24 
 25 #ifndef SHARE_OOPS_ACCESSDECORATORS_HPP
 26 #define SHARE_OOPS_ACCESSDECORATORS_HPP
 27 
 28 #include "gc/shared/barrierSetConfig.hpp"
 29 #include "memory/allStatic.hpp"
 30 #include "utilities/globalDefinitions.hpp"
 31 
 32 #include <type_traits>
 33 
 34 // A decorator is an attribute or property that affects the way a memory access is performed in some way.
 35 // There are different groups of decorators. Some have to do with memory ordering, others to do with,
 36 // e.g. strength of references, strength of GC barriers, or whether compression should be applied or not.
 37 // Some decorators are set at buildtime, such as whether primitives require GC barriers or not, others
 38 // at callsites such as whether an access is in the heap or not, and others are resolved at runtime
 39 // such as GC-specific barriers and encoding/decoding compressed oops.
 40 typedef uint64_t DecoratorSet;
 41 
 42 // The HasDecorator trait can help at compile-time determining whether a decorator set
 43 // has an intersection with a certain other decorator set
 44 template <DecoratorSet decorators, DecoratorSet decorator>
 45 struct HasDecorator: public std::integral_constant<bool, (decorators & decorator) != 0> {};
 46 
 47 // == General Decorators ==
 48 // * DECORATORS_NONE: This is the name for the empty decorator set (in absence of other decorators).
 49 const DecoratorSet DECORATORS_NONE                   = UCONST64(0);
 50 
 51 // == Internal Decorators - do not use ==
 52 // * INTERNAL_CONVERT_COMPRESSED_OOPS: This is an oop access that will require converting an oop
 53 //   to a narrowOop or vice versa, if UseCompressedOops is known to be set.
 54 // * INTERNAL_VALUE_IS_OOP: Remember that the involved access is on oop rather than primitive.
 55 const DecoratorSet INTERNAL_CONVERT_COMPRESSED_OOP   = UCONST64(1) << 1;
 56 const DecoratorSet INTERNAL_VALUE_IS_OOP             = UCONST64(1) << 2;
 57 
 58 // == Internal run-time Decorators ==
 59 // * INTERNAL_RT_USE_COMPRESSED_OOPS: This decorator will be set in runtime resolved
 60 //   access backends iff UseCompressedOops is true.
 61 const DecoratorSet INTERNAL_RT_USE_COMPRESSED_OOPS   = UCONST64(1) << 5;
 62 
 63 const DecoratorSet INTERNAL_DECORATOR_MASK           = INTERNAL_CONVERT_COMPRESSED_OOP | INTERNAL_VALUE_IS_OOP |
 64                                                        INTERNAL_RT_USE_COMPRESSED_OOPS;
 65 
 66 // == Memory Ordering Decorators ==
 67 // The memory ordering decorators can be described in the following way:
 68 // === Decorator Rules ===
 69 // The different types of memory ordering guarantees have a strict order of strength.
 70 // Explicitly specifying the stronger ordering implies that the guarantees of the weaker
 71 // property holds too. The names come from the C++11 atomic operations, and typically
 72 // have a JMM equivalent property.
 73 // The equivalence may be viewed like this:
 74 // MO_UNORDERED is equivalent to JMM plain.
 75 // MO_RELAXED is equivalent to JMM opaque.
 76 // MO_ACQUIRE is equivalent to JMM acquire.
 77 // MO_RELEASE is equivalent to JMM release.
 78 // MO_SEQ_CST is equivalent to JMM volatile.
 79 //
 80 // === Stores ===
 81 //  * MO_UNORDERED (Default): No guarantees.
 82 //    - The compiler and hardware are free to reorder aggressively. And they will.
 83 //  * MO_RELAXED: Relaxed atomic stores.
 84 //    - The stores are atomic.
 85 //    - The stores are not reordered by the compiler (but possibly the HW) w.r.t
 86 //      other ordered accesses in program order.
 87 //    - Also used for C++ volatile stores, since actual usage of volatile
 88 //      requires no word tearing.
 89 //  * MO_RELEASE: Releasing stores.
 90 //    - The releasing store will make its preceding memory accesses observable to memory accesses
 91 //      subsequent to an acquiring load observing this releasing store.
 92 //    - Guarantees from relaxed stores hold.
 93 //  * MO_SEQ_CST: Sequentially consistent stores.
 94 //    - The stores are observed in the same order by MO_SEQ_CST loads on other processors
 95 //    - Preceding loads and stores in program order are not reordered with subsequent loads and stores in program order.
 96 //    - Guarantees from releasing stores hold.
 97 // === Loads ===
 98 //  * MO_UNORDERED (Default): No guarantees
 99 //    - The compiler and hardware are free to reorder aggressively. And they will.
100 //  * MO_RELAXED: Relaxed atomic loads.
101 //    - The loads are atomic.
102 //    - The loads are not reordered by the compiler (but possibly the HW) w.r.t.
103 //      other ordered accesses in program order.
104 //    - Also used for C++ volatile loads, since actual usage of volatile
105 //      requires no word tearing.
106 //  * MO_ACQUIRE: Acquiring loads.
107 //    - An acquiring load will make subsequent memory accesses observe the memory accesses
108 //      preceding the releasing store that the acquiring load observed.
109 //    - Guarantees from relaxed loads hold.
110 //  * MO_SEQ_CST: Sequentially consistent loads.
111 //    - These loads observe MO_SEQ_CST stores in the same order on other processors
112 //    - Preceding loads and stores in program order are not reordered with subsequent loads and stores in program order.
113 //    - Guarantees from acquiring loads hold.
114 // === Atomic Cmpxchg ===
115 //  * MO_RELAXED: Atomic but relaxed cmpxchg.
116 //    - Guarantees from MO_RELAXED loads and MO_RELAXED stores hold unconditionally.
117 //  * MO_SEQ_CST: Sequentially consistent cmpxchg.
118 //    - Guarantees from MO_SEQ_CST loads and MO_SEQ_CST stores hold unconditionally.
119 // === Atomic Xchg ===
120 //  * MO_RELAXED: Atomic but relaxed atomic xchg.
121 //    - Guarantees from MO_RELAXED loads and MO_RELAXED stores hold.
122 //  * MO_SEQ_CST: Sequentially consistent xchg.
123 //    - Guarantees from MO_SEQ_CST loads and MO_SEQ_CST stores hold.
124 const DecoratorSet MO_UNORDERED      = UCONST64(1) << 6;
125 const DecoratorSet MO_RELAXED        = UCONST64(1) << 7;
126 const DecoratorSet MO_ACQUIRE        = UCONST64(1) << 8;
127 const DecoratorSet MO_RELEASE        = UCONST64(1) << 9;
128 const DecoratorSet MO_SEQ_CST        = UCONST64(1) << 10;
129 const DecoratorSet MO_DECORATOR_MASK = MO_UNORDERED | MO_RELAXED |
130                                        MO_ACQUIRE | MO_RELEASE | MO_SEQ_CST;
131 
132 // === Barrier Strength Decorators ===
133 // * AS_RAW: The access will translate into a raw memory access, hence ignoring all semantic concerns
134 //   except memory ordering and compressed oops. This will bypass runtime function pointer dispatching
135 //   in the pipeline and hardwire to raw accesses without going through the GC access barriers.
136 //  - Accesses on oop* translate to raw memory accesses without runtime checks
137 //  - Accesses on narrowOop* translate to encoded/decoded memory accesses without runtime checks
138 //  - Accesses on HeapWord* translate to a runtime check choosing one of the above
139 //  - Accesses on other types translate to raw memory accesses without runtime checks
140 // * AS_NO_KEEPALIVE: The barrier is used only on oop references and will not keep any involved objects
141 //   alive, regardless of the type of reference being accessed. It will however perform the memory access
142 //   in a consistent way w.r.t. e.g. concurrent compaction, so that the right field is being accessed,
143 //   or maintain, e.g. intergenerational or interregional pointers if applicable. This should be used with
144 //   extreme caution in isolated scopes.
145 // * AS_NORMAL: The accesses will be resolved to an accessor on the BarrierSet class, giving the
146 //   responsibility of performing the access and what barriers to be performed to the GC. This is the default.
147 //   Note that primitive accesses will only be resolved on the barrier set if the appropriate build-time
148 //   decorator for enabling primitive barriers is enabled for the build.
149 const DecoratorSet AS_RAW                  = UCONST64(1) << 11;
150 const DecoratorSet AS_NO_KEEPALIVE         = UCONST64(1) << 12;
151 const DecoratorSet AS_NORMAL               = UCONST64(1) << 13;
152 const DecoratorSet AS_DECORATOR_MASK       = AS_RAW | AS_NO_KEEPALIVE | AS_NORMAL;
153 
154 // === Reference Strength Decorators ===
155 // These decorators only apply to accesses on oop-like types (oop/narrowOop).
156 // * ON_STRONG_OOP_REF: Memory access is performed on a strongly reachable reference.
157 // * ON_WEAK_OOP_REF: The memory access is performed on a weakly reachable reference.
158 // * ON_PHANTOM_OOP_REF: The memory access is performed on a phantomly reachable reference.
159 //   This is the same ring of strength as jweak and weak oops in the VM.
160 // * ON_UNKNOWN_OOP_REF: The memory access is performed on a reference of unknown strength.
161 //   This could for example come from the unsafe API.
162 // * Default (no explicit reference strength specified): ON_STRONG_OOP_REF
163 const DecoratorSet ON_STRONG_OOP_REF  = UCONST64(1) << 14;
164 const DecoratorSet ON_WEAK_OOP_REF    = UCONST64(1) << 15;
165 const DecoratorSet ON_PHANTOM_OOP_REF = UCONST64(1) << 16;
166 const DecoratorSet ON_UNKNOWN_OOP_REF = UCONST64(1) << 17;
167 const DecoratorSet ON_DECORATOR_MASK  = ON_STRONG_OOP_REF | ON_WEAK_OOP_REF |
168                                         ON_PHANTOM_OOP_REF | ON_UNKNOWN_OOP_REF;
169 
170 // === Access Location ===
171 // Accesses can take place in, e.g. the heap, old or young generation, different native roots, or native memory off the heap.
172 // The location is important to the GC as it may imply different actions. The following decorators are used:
173 // * IN_HEAP: The access is performed in the heap. Many barriers such as card marking will
174 //   be omitted if this decorator is not set.
175 // * IN_NATIVE: The access is performed in an off-heap data structure.
176 // * IN_NMETHOD: The access is performed inside of an nmethod.
177 const DecoratorSet IN_HEAP            = UCONST64(1) << 18;
178 const DecoratorSet IN_NATIVE          = UCONST64(1) << 19;
179 const DecoratorSet IN_NMETHOD         = UCONST64(1) << 20;
180 const DecoratorSet IN_DECORATOR_MASK  = IN_HEAP | IN_NATIVE | IN_NMETHOD;
181 
182 // == Boolean Flag Decorators ==
183 // * IS_ARRAY: The access is performed on a heap allocated array. This is sometimes a special case
184 //   for some GCs.
185 // * IS_DEST_UNINITIALIZED: This property can be important to e.g. SATB barriers by
186 //   marking that the previous value is uninitialized nonsense rather than a real value.
187 // * IS_NOT_NULL: This property can make certain barriers faster such as compressing oops.
188 const DecoratorSet IS_ARRAY              = UCONST64(1) << 21;
189 const DecoratorSet IS_DEST_UNINITIALIZED = UCONST64(1) << 22;
190 const DecoratorSet IS_NOT_NULL           = UCONST64(1) << 23;
191 
192 // == Arraycopy Decorators ==
193 // * ARRAYCOPY_CHECKCAST: This property means that the class of the objects in source
194 //   are not guaranteed to be subclasses of the class of the destination array. This requires
195 //   a check-cast barrier during the copying operation. If this is not set, it is assumed
196 //   that the array is covariant: (the source array type is-a destination array type)
197 // * ARRAYCOPY_DISJOINT: This property means that it is known that the two array ranges
198 //   are disjoint.
199 // * ARRAYCOPY_ARRAYOF: The copy is in the arrayof form.
200 // * ARRAYCOPY_ATOMIC: The accesses have to be atomic over the size of its elements.
201 // * ARRAYCOPY_ALIGNED: The accesses have to be aligned on a HeapWord.
202 const DecoratorSet ARRAYCOPY_CHECKCAST            = UCONST64(1) << 24;
203 const DecoratorSet ARRAYCOPY_DISJOINT             = UCONST64(1) << 25;
204 const DecoratorSet ARRAYCOPY_ARRAYOF              = UCONST64(1) << 26;
205 const DecoratorSet ARRAYCOPY_ATOMIC               = UCONST64(1) << 27;
206 const DecoratorSet ARRAYCOPY_ALIGNED              = UCONST64(1) << 28;
207 const DecoratorSet ARRAYCOPY_DECORATOR_MASK       = ARRAYCOPY_CHECKCAST | ARRAYCOPY_DISJOINT |
208                                                     ARRAYCOPY_DISJOINT | ARRAYCOPY_ARRAYOF |
209                                                     ARRAYCOPY_ATOMIC | ARRAYCOPY_ALIGNED;
210 
211 // == Resolve barrier decorators ==
212 // * ACCESS_READ: Indicate that the resolved object is accessed read-only. This allows the GC
213 //   backend to use weaker and more efficient barriers.
214 // * ACCESS_WRITE: Indicate that the resolved object is used for write access.
215 const DecoratorSet ACCESS_READ                    = UCONST64(1) << 29;
216 const DecoratorSet ACCESS_WRITE                   = UCONST64(1) << 30;
217 
218 // Keep track of the last decorator.
219 const DecoratorSet DECORATOR_LAST = UCONST64(1) << 30;
220 
221 namespace AccessInternal {
222   // This class adds implied decorators that follow according to decorator rules.
223   // For example adding default reference strength and default memory ordering
224   // semantics.
225   template <DecoratorSet input_decorators>
226   struct DecoratorFixup: AllStatic {
227     // If no reference strength has been picked, then strong will be picked
228     static const DecoratorSet ref_strength_default = input_decorators |
229       (((ON_DECORATOR_MASK & input_decorators) == 0 && (INTERNAL_VALUE_IS_OOP & input_decorators) != 0) ?
230        ON_STRONG_OOP_REF : DECORATORS_NONE);
231     // If no memory ordering has been picked, unordered will be picked
232     static const DecoratorSet memory_ordering_default = ref_strength_default |
233       ((MO_DECORATOR_MASK & ref_strength_default) == 0 ? MO_UNORDERED : DECORATORS_NONE);
234     // If no barrier strength has been picked, normal will be used
235     static const DecoratorSet barrier_strength_default = memory_ordering_default |
236       ((AS_DECORATOR_MASK & memory_ordering_default) == 0 ? AS_NORMAL : DECORATORS_NONE);
237     static const DecoratorSet value = barrier_strength_default;
238   };
239 
240   // This function implements the above DecoratorFixup rules, but without meta
241   // programming for code generation that does not use templates.
242   inline DecoratorSet decorator_fixup(DecoratorSet input_decorators, BasicType type) {
243     // Some call-sites don't specify that the access is performed on oops
244     DecoratorSet with_oop_decorators = input_decorators |= (is_reference_type(type) ? INTERNAL_VALUE_IS_OOP : 0);
245     // If no reference strength has been picked, then strong will be picked
246     DecoratorSet ref_strength_default = with_oop_decorators |
247       (((ON_DECORATOR_MASK & with_oop_decorators) == 0 && (INTERNAL_VALUE_IS_OOP & input_decorators) != 0) ?
248        ON_STRONG_OOP_REF : DECORATORS_NONE);
249     // If no memory ordering has been picked, unordered will be picked
250     DecoratorSet memory_ordering_default = ref_strength_default |
251       ((MO_DECORATOR_MASK & ref_strength_default) == 0 ? MO_UNORDERED : DECORATORS_NONE);
252     // If no barrier strength has been picked, normal will be used
253     DecoratorSet barrier_strength_default = memory_ordering_default |
254       ((AS_DECORATOR_MASK & memory_ordering_default) == 0 ? AS_NORMAL : DECORATORS_NONE);
255     return barrier_strength_default;
256   }
257 }
258 
259 #endif // SHARE_OOPS_ACCESSDECORATORS_HPP