1 /*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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24
25 #ifndef SHARE_VM_RUNTIME_ATOMIC_HPP
26 #define SHARE_VM_RUNTIME_ATOMIC_HPP
27
28 #include "memory/allocation.hpp"
29 #include "metaprogramming/conditional.hpp"
30 #include "metaprogramming/enableIf.hpp"
31 #include "metaprogramming/isIntegral.hpp"
32 #include "metaprogramming/isPointer.hpp"
33 #include "metaprogramming/isSame.hpp"
34 #include "metaprogramming/primitiveConversions.hpp"
35 #include "metaprogramming/removeCV.hpp"
36 #include "metaprogramming/removePointer.hpp"
37 #include "utilities/align.hpp"
38 #include "utilities/macros.hpp"
39
40 enum cmpxchg_memory_order {
41 memory_order_relaxed,
42 // Use value which doesn't interfere with C++2011. We need to be more conservative.
43 memory_order_conservative = 8
44 };
45
46 class Atomic : AllStatic {
47 public:
48 // Atomic operations on jlong types are not available on all 32-bit
49 // platforms. If atomic ops on jlongs are defined here they must only
50 // be used from code that verifies they are available at runtime and
51 // can provide an alternative action if not - see supports_cx8() for
52 // a means to test availability.
53
54 // The memory operations that are mentioned with each of the atomic
55 // function families come from src/share/vm/runtime/orderAccess.hpp,
56 // e.g., <fence> is described in that file and is implemented by the
57 // OrderAccess::fence() function. See that file for the gory details
58 // on the Memory Access Ordering Model.
59
60 // All of the atomic operations that imply a read-modify-write action
61 // guarantee a two-way memory barrier across that operation. Historically
62 // these semantics reflect the strength of atomic operations that are
63 // provided on SPARC/X86. We assume that strength is necessary unless
64 // we can prove that a weaker form is sufficiently safe.
65
66 // Atomically store to a location
67 inline static void store (jbyte store_value, jbyte* dest);
68 inline static void store (jshort store_value, jshort* dest);
69 inline static void store (jint store_value, jint* dest);
70 // See comment above about using jlong atomics on 32-bit platforms
71 inline static void store (jlong store_value, jlong* dest);
72 inline static void store_ptr(intptr_t store_value, intptr_t* dest);
73 inline static void store_ptr(void* store_value, void* dest);
74
75 inline static void store (jbyte store_value, volatile jbyte* dest);
76 inline static void store (jshort store_value, volatile jshort* dest);
77 inline static void store (jint store_value, volatile jint* dest);
78 // See comment above about using jlong atomics on 32-bit platforms
79 inline static void store (jlong store_value, volatile jlong* dest);
80 inline static void store_ptr(intptr_t store_value, volatile intptr_t* dest);
81 inline static void store_ptr(void* store_value, volatile void* dest);
82
83 // See comment above about using jlong atomics on 32-bit platforms
84 inline static jlong load(const volatile jlong* src);
85
86 // Atomically add to a location. Returns updated value. add*() provide:
87 // <fence> add-value-to-dest <membar StoreLoad|StoreStore>
88
89 template<typename I, typename D>
90 inline static D add(I add_value, D volatile* dest);
91
92 inline static intptr_t add_ptr(intptr_t add_value, volatile intptr_t* dest) {
93 return add(add_value, dest);
94 }
95
96 inline static void* add_ptr(intptr_t add_value, volatile void* dest) {
97 return add(add_value, reinterpret_cast<char* volatile*>(dest));
98 }
99
100 // Atomically increment location. inc() provide:
101 // <fence> increment-dest <membar StoreLoad|StoreStore>
102 // The type D may be either a pointer type, or an integral
103 // type. If it is a pointer type, then the increment is
104 // scaled to the size of the type pointed to by the pointer.
105 template<typename D>
106 inline static void inc(D volatile* dest);
107
108 // Atomically decrement a location. dec() provide:
109 // <fence> decrement-dest <membar StoreLoad|StoreStore>
110 // The type D may be either a pointer type, or an integral
111 // type. If it is a pointer type, then the decrement is
112 // scaled to the size of the type pointed to by the pointer.
113 template<typename D>
114 inline static void dec(D volatile* dest);
115
116 // Performs atomic exchange of *dest with exchange_value. Returns old
117 // prior value of *dest. xchg*() provide:
118 // <fence> exchange-value-with-dest <membar StoreLoad|StoreStore>
119 // The type T must be either a pointer type convertible to or equal
120 // to D, an integral/enum type equal to D, or a type equal to D that
121 // is primitive convertible using PrimitiveConversions.
122 template<typename T, typename D>
123 inline static D xchg(T exchange_value, volatile D* dest);
124
125 inline static intptr_t xchg_ptr(intptr_t exchange_value, volatile intptr_t* dest) {
126 return xchg(exchange_value, dest);
127 }
128
129 inline static void* xchg_ptr(void* exchange_value, volatile void* dest) {
130 return xchg(exchange_value, reinterpret_cast<void* volatile*>(dest));
131 }
132
133 // Performs atomic compare of *dest and compare_value, and exchanges
134 // *dest with exchange_value if the comparison succeeded. Returns prior
135 // value of *dest. cmpxchg*() provide:
136 // <fence> compare-and-exchange <membar StoreLoad|StoreStore>
137
138 template<typename T, typename D, typename U>
139 inline static D cmpxchg(T exchange_value,
140 D volatile* dest,
141 U compare_value,
142 cmpxchg_memory_order order = memory_order_conservative);
143
144 // Performs atomic compare of *dest and NULL, and replaces *dest
145 // with exchange_value if the comparison succeeded. Returns true if
146 // the comparison succeeded and the exchange occurred. This is
147 // often used as part of lazy initialization, as a lock-free
148 // alternative to the Double-Checked Locking Pattern.
149 template<typename T, typename D>
150 inline static bool replace_if_null(T* value, D* volatile* dest,
151 cmpxchg_memory_order order = memory_order_conservative);
152
153 inline static intptr_t cmpxchg_ptr(intptr_t exchange_value,
154 volatile intptr_t* dest,
155 intptr_t compare_value,
156 cmpxchg_memory_order order = memory_order_conservative) {
157 return cmpxchg(exchange_value, dest, compare_value, order);
158 }
159
160 inline static void* cmpxchg_ptr(void* exchange_value,
161 volatile void* dest,
162 void* compare_value,
163 cmpxchg_memory_order order = memory_order_conservative) {
164 return cmpxchg(exchange_value,
165 reinterpret_cast<void* volatile*>(dest),
166 compare_value,
167 order);
168 }
169
170 private:
171 // Test whether From is implicitly convertible to To.
172 // From and To must be pointer types.
173 // Note: Provides the limited subset of C++11 std::is_convertible
174 // that is needed here.
175 template<typename From, typename To> struct IsPointerConvertible;
176
177 // Dispatch handler for add. Provides type-based validity checking
178 // and limited conversions around calls to the platform-specific
179 // implementation layer provided by PlatformAdd.
180 template<typename I, typename D, typename Enable = void>
181 struct AddImpl;
182
183 // Platform-specific implementation of add. Support for sizes of 4
184 // bytes and (if different) pointer size bytes are required. The
185 // class is a function object that must be default constructable,
186 // with these requirements:
187 //
188 // - dest is of type D*, an integral or pointer type.
189 // - add_value is of type I, an integral type.
190 // - sizeof(I) == sizeof(D).
191 // - if D is an integral type, I == D.
192 // - platform_add is an object of type PlatformAdd<sizeof(D)>.
193 //
194 // Then
195 // platform_add(add_value, dest)
196 // must be a valid expression, returning a result convertible to D.
197 //
198 // No definition is provided; all platforms must explicitly define
199 // this class and any needed specializations.
200 template<size_t byte_size> struct PlatformAdd;
201
202 // Helper base classes for defining PlatformAdd. To use, define
203 // PlatformAdd or a specialization that derives from one of these,
204 // and include in the PlatformAdd definition the support function
205 // (described below) required by the base class.
206 //
207 // These classes implement the required function object protocol for
208 // PlatformAdd, using a support function template provided by the
209 // derived class. Let add_value (of type I) and dest (of type D) be
210 // the arguments the object is called with. If D is a pointer type
211 // P*, then let addend (of type I) be add_value * sizeof(P);
212 // otherwise, addend is add_value.
213 //
214 // FetchAndAdd requires the derived class to provide
215 // fetch_and_add(addend, dest)
216 // atomically adding addend to the value of dest, and returning the
217 // old value.
218 //
219 // AddAndFetch requires the derived class to provide
220 // add_and_fetch(addend, dest)
221 // atomically adding addend to the value of dest, and returning the
222 // new value.
223 //
224 // When D is a pointer type P*, both fetch_and_add and add_and_fetch
225 // treat it as if it were a uintptr_t; they do not perform any
226 // scaling of the addend, as that has already been done by the
227 // caller.
228 public: // Temporary, can't be private: C++03 11.4/2. Fixed by C++11.
229 template<typename Derived> struct FetchAndAdd;
230 template<typename Derived> struct AddAndFetch;
231 private:
232
233 // Support for platforms that implement some variants of add using a
234 // (typically out of line) non-template helper function. The
235 // generic arguments passed to PlatformAdd need to be translated to
236 // the appropriate type for the helper function, the helper function
237 // invoked on the translated arguments, and the result translated
238 // back. Type is the parameter / return type of the helper
239 // function. No scaling of add_value is performed when D is a pointer
240 // type, so this function can be used to implement the support function
241 // required by AddAndFetch.
242 template<typename Type, typename Fn, typename I, typename D>
243 static D add_using_helper(Fn fn, I add_value, D volatile* dest);
244
245 // Dispatch handler for cmpxchg. Provides type-based validity
246 // checking and limited conversions around calls to the
247 // platform-specific implementation layer provided by
248 // PlatformCmpxchg.
249 template<typename T, typename D, typename U, typename Enable = void>
250 struct CmpxchgImpl;
251
252 // Platform-specific implementation of cmpxchg. Support for sizes
253 // of 1, 4, and 8 are required. The class is a function object that
254 // must be default constructable, with these requirements:
255 //
256 // - dest is of type T*.
257 // - exchange_value and compare_value are of type T.
258 // - order is of type cmpxchg_memory_order.
259 // - platform_cmpxchg is an object of type PlatformCmpxchg<sizeof(T)>.
260 //
261 // Then
262 // platform_cmpxchg(exchange_value, dest, compare_value, order)
263 // must be a valid expression, returning a result convertible to T.
264 //
265 // A default definition is provided, which declares a function template
266 // T operator()(T, T volatile*, T, cmpxchg_memory_order) const
267 //
268 // For each required size, a platform must either provide an
269 // appropriate definition of that function, or must entirely
270 // specialize the class template for that size.
271 template<size_t byte_size> struct PlatformCmpxchg;
272
273 // Support for platforms that implement some variants of cmpxchg
274 // using a (typically out of line) non-template helper function.
275 // The generic arguments passed to PlatformCmpxchg need to be
276 // translated to the appropriate type for the helper function, the
277 // helper invoked on the translated arguments, and the result
278 // translated back. Type is the parameter / return type of the
279 // helper function.
280 template<typename Type, typename Fn, typename T>
281 static T cmpxchg_using_helper(Fn fn,
282 T exchange_value,
283 T volatile* dest,
284 T compare_value);
285
286 // Support platforms that do not provide Read-Modify-Write
287 // byte-level atomic access. To use, derive PlatformCmpxchg<1> from
288 // this class.
289 public: // Temporary, can't be private: C++03 11.4/2. Fixed by C++11.
290 struct CmpxchgByteUsingInt;
291 private:
292
293 // Dispatch handler for xchg. Provides type-based validity
294 // checking and limited conversions around calls to the
295 // platform-specific implementation layer provided by
296 // PlatformXchg.
297 template<typename T, typename D, typename Enable = void>
298 struct XchgImpl;
299
300 // Platform-specific implementation of xchg. Support for sizes
301 // of 4, and sizeof(intptr_t) are required. The class is a function
302 // object that must be default constructable, with these requirements:
303 //
304 // - dest is of type T*.
305 // - exchange_value is of type T.
306 // - platform_xchg is an object of type PlatformXchg<sizeof(T)>.
307 //
308 // Then
309 // platform_xchg(exchange_value, dest)
310 // must be a valid expression, returning a result convertible to T.
311 //
312 // A default definition is provided, which declares a function template
313 // T operator()(T, T volatile*, T, cmpxchg_memory_order) const
314 //
315 // For each required size, a platform must either provide an
316 // appropriate definition of that function, or must entirely
317 // specialize the class template for that size.
318 template<size_t byte_size> struct PlatformXchg;
319
320 // Support for platforms that implement some variants of xchg
321 // using a (typically out of line) non-template helper function.
322 // The generic arguments passed to PlatformXchg need to be
323 // translated to the appropriate type for the helper function, the
324 // helper invoked on the translated arguments, and the result
325 // translated back. Type is the parameter / return type of the
326 // helper function.
327 template<typename Type, typename Fn, typename T>
328 static T xchg_using_helper(Fn fn,
329 T exchange_value,
330 T volatile* dest);
331 };
332
333 template<typename From, typename To>
334 struct Atomic::IsPointerConvertible<From*, To*> : AllStatic {
335 // Determine whether From* is implicitly convertible to To*, using
336 // the "sizeof trick".
337 typedef char yes;
338 typedef char (&no)[2];
339
340 static yes test(To*);
341 static no test(...);
342 static From* test_value;
343
344 static const bool value = (sizeof(yes) == sizeof(test(test_value)));
345 };
346
347 // Define FetchAndAdd and AddAndFetch helper classes before including
348 // platform file, which may use these as base classes, requiring they
349 // be complete.
350
351 template<typename Derived>
352 struct Atomic::FetchAndAdd VALUE_OBJ_CLASS_SPEC {
353 template<typename I, typename D>
354 D operator()(I add_value, D volatile* dest) const;
355 };
356
357 template<typename Derived>
358 struct Atomic::AddAndFetch VALUE_OBJ_CLASS_SPEC {
359 template<typename I, typename D>
360 D operator()(I add_value, D volatile* dest) const;
361 };
362
363 template<typename D>
364 inline void Atomic::inc(D volatile* dest) {
365 STATIC_ASSERT(IsPointer<D>::value || IsIntegral<D>::value);
366 typedef typename Conditional<IsPointer<D>::value, ptrdiff_t, D>::type I;
367 Atomic::add(I(1), dest);
368 }
369
370 template<typename D>
371 inline void Atomic::dec(D volatile* dest) {
372 STATIC_ASSERT(IsPointer<D>::value || IsIntegral<D>::value);
373 typedef typename Conditional<IsPointer<D>::value, ptrdiff_t, D>::type I;
374 // Assumes two's complement integer representation.
375 #pragma warning(suppress: 4146)
376 Atomic::add(I(-1), dest);
377 }
378
379 // Define the class before including platform file, which may specialize
380 // the operator definition. No generic definition of specializations
381 // of the operator template are provided, nor are there any generic
382 // specializations of the class. The platform file is responsible for
383 // providing those.
384 template<size_t byte_size>
385 struct Atomic::PlatformCmpxchg VALUE_OBJ_CLASS_SPEC {
386 template<typename T>
387 T operator()(T exchange_value,
388 T volatile* dest,
389 T compare_value,
390 cmpxchg_memory_order order) const;
391 };
392
393 // Define the class before including platform file, which may use this
394 // as a base class, requiring it be complete. The definition is later
395 // in this file, near the other definitions related to cmpxchg.
396 struct Atomic::CmpxchgByteUsingInt VALUE_OBJ_CLASS_SPEC {
397 template<typename T>
398 T operator()(T exchange_value,
399 T volatile* dest,
400 T compare_value,
401 cmpxchg_memory_order order) const;
402 };
403
404 // Define the class before including platform file, which may specialize
405 // the operator definition. No generic definition of specializations
406 // of the operator template are provided, nor are there any generic
407 // specializations of the class. The platform file is responsible for
408 // providing those.
409 template<size_t byte_size>
410 struct Atomic::PlatformXchg VALUE_OBJ_CLASS_SPEC {
411 template<typename T>
412 T operator()(T exchange_value,
413 T volatile* dest) const;
414 };
415
416 // platform specific in-line definitions - must come before shared definitions
417
418 #include OS_CPU_HEADER(atomic)
419
420 // shared in-line definitions
421
422 // size_t casts...
423 #if (SIZE_MAX != UINTPTR_MAX)
424 #error size_t is not WORD_SIZE, interesting platform, but missing implementation here
425 #endif
426
427 template<typename I, typename D>
428 inline D Atomic::add(I add_value, D volatile* dest) {
429 return AddImpl<I, D>()(add_value, dest);
430 }
431
432 template<typename I, typename D>
433 struct Atomic::AddImpl<
434 I, D,
435 typename EnableIf<IsIntegral<I>::value &&
436 IsIntegral<D>::value &&
437 (sizeof(I) <= sizeof(D)) &&
438 (IsSigned<I>::value == IsSigned<D>::value)>::type>
439 VALUE_OBJ_CLASS_SPEC
440 {
441 D operator()(I add_value, D volatile* dest) const {
442 D addend = add_value;
443 return PlatformAdd<sizeof(D)>()(addend, dest);
444 }
445 };
446
447 template<typename I, typename P>
448 struct Atomic::AddImpl<
449 I, P*,
450 typename EnableIf<IsIntegral<I>::value && (sizeof(I) <= sizeof(P*))>::type>
451 VALUE_OBJ_CLASS_SPEC
452 {
453 P* operator()(I add_value, P* volatile* dest) const {
454 STATIC_ASSERT(sizeof(intptr_t) == sizeof(P*));
455 STATIC_ASSERT(sizeof(uintptr_t) == sizeof(P*));
456 typedef typename Conditional<IsSigned<I>::value,
457 intptr_t,
458 uintptr_t>::type CI;
459 CI addend = add_value;
460 return PlatformAdd<sizeof(P*)>()(addend, dest);
461 }
462 };
463
464 // Most platforms do not support atomic add on a 2-byte value. However,
465 // if the value occupies the most significant 16 bits of an aligned 32-bit
466 // word, then we can do this with an atomic add of (add_value << 16)
467 // to the 32-bit word.
468 //
469 // The least significant parts of this 32-bit word will never be affected, even
470 // in case of overflow/underflow.
471 //
472 // Use the ATOMIC_SHORT_PAIR macro (see macros.hpp) to get the desired alignment.
473 template<>
474 struct Atomic::AddImpl<jshort, jshort> VALUE_OBJ_CLASS_SPEC {
475 jshort operator()(jshort add_value, jshort volatile* dest) const {
476 #ifdef VM_LITTLE_ENDIAN
477 assert((intx(dest) & 0x03) == 0x02, "wrong alignment");
478 jint new_value = Atomic::add(add_value << 16, (volatile jint*)(dest-1));
479 #else
480 assert((intx(dest) & 0x03) == 0x00, "wrong alignment");
481 jint new_value = Atomic::add(add_value << 16, (volatile jint*)(dest));
482 #endif
483 return (jshort)(new_value >> 16); // preserves sign
484 }
485 };
486
487 template<typename Derived>
488 template<typename I, typename D>
489 inline D Atomic::FetchAndAdd<Derived>::operator()(I add_value, D volatile* dest) const {
490 I addend = add_value;
491 // If D is a pointer type P*, scale by sizeof(P).
492 if (IsPointer<D>::value) {
493 addend *= sizeof(typename RemovePointer<D>::type);
494 }
495 D old = static_cast<const Derived*>(this)->fetch_and_add(addend, dest);
496 return old + add_value;
497 }
498
499 template<typename Derived>
500 template<typename I, typename D>
501 inline D Atomic::AddAndFetch<Derived>::operator()(I add_value, D volatile* dest) const {
502 // If D is a pointer type P*, scale by sizeof(P).
503 if (IsPointer<D>::value) {
504 add_value *= sizeof(typename RemovePointer<D>::type);
505 }
506 return static_cast<const Derived*>(this)->add_and_fetch(add_value, dest);
507 }
508
509 template<typename Type, typename Fn, typename I, typename D>
510 inline D Atomic::add_using_helper(Fn fn, I add_value, D volatile* dest) {
511 return PrimitiveConversions::cast<D>(
512 fn(PrimitiveConversions::cast<Type>(add_value),
513 reinterpret_cast<Type volatile*>(dest)));
514 }
515
516 template<typename T, typename D, typename U>
517 inline D Atomic::cmpxchg(T exchange_value,
518 D volatile* dest,
519 U compare_value,
520 cmpxchg_memory_order order) {
521 return CmpxchgImpl<T, D, U>()(exchange_value, dest, compare_value, order);
522 }
523
524 template<typename T, typename D>
525 inline bool Atomic::replace_if_null(T* value, D* volatile* dest,
526 cmpxchg_memory_order order) {
527 // Presently using a trivial implementation in terms of cmpxchg.
528 // Consider adding platform support, to permit the use of compiler
529 // intrinsics like gcc's __sync_bool_compare_and_swap.
530 D* expected_null = NULL;
531 return expected_null == cmpxchg(value, dest, expected_null, order);
532 }
533
534 // Handle cmpxchg for integral and enum types.
535 //
536 // All the involved types must be identical.
537 template<typename T>
538 struct Atomic::CmpxchgImpl<
539 T, T, T,
540 typename EnableIf<IsIntegral<T>::value || IsRegisteredEnum<T>::value>::type>
541 VALUE_OBJ_CLASS_SPEC
542 {
543 T operator()(T exchange_value, T volatile* dest, T compare_value,
544 cmpxchg_memory_order order) const {
545 // Forward to the platform handler for the size of T.
546 return PlatformCmpxchg<sizeof(T)>()(exchange_value,
547 dest,
548 compare_value,
549 order);
550 }
551 };
552
553 // Handle cmpxchg for pointer types.
554 //
555 // The destination's type and the compare_value type must be the same,
556 // ignoring cv-qualifiers; we don't care about the cv-qualifiers of
557 // the compare_value.
558 //
559 // The exchange_value must be implicitly convertible to the
560 // destination's type; it must be type-correct to store the
561 // exchange_value in the destination.
562 template<typename T, typename D, typename U>
563 struct Atomic::CmpxchgImpl<
564 T*, D*, U*,
565 typename EnableIf<Atomic::IsPointerConvertible<T*, D*>::value &&
566 IsSame<typename RemoveCV<D>::type,
567 typename RemoveCV<U>::type>::value>::type>
568 VALUE_OBJ_CLASS_SPEC
569 {
570 D* operator()(T* exchange_value, D* volatile* dest, U* compare_value,
571 cmpxchg_memory_order order) const {
572 // Allow derived to base conversion, and adding cv-qualifiers.
573 D* new_value = exchange_value;
574 // Don't care what the CV qualifiers for compare_value are,
575 // but we need to match D* when calling platform support.
576 D* old_value = const_cast<D*>(compare_value);
577 return PlatformCmpxchg<sizeof(D*)>()(new_value, dest, old_value, order);
578 }
579 };
580
581 // Handle cmpxchg for types that have a translator.
582 //
583 // All the involved types must be identical.
584 //
585 // This translates the original call into a call on the decayed
586 // arguments, and returns the recovered result of that translated
587 // call.
588 template<typename T>
589 struct Atomic::CmpxchgImpl<
590 T, T, T,
591 typename EnableIf<PrimitiveConversions::Translate<T>::value>::type>
592 VALUE_OBJ_CLASS_SPEC
593 {
594 T operator()(T exchange_value, T volatile* dest, T compare_value,
595 cmpxchg_memory_order order) const {
596 typedef PrimitiveConversions::Translate<T> Translator;
597 typedef typename Translator::Decayed Decayed;
598 STATIC_ASSERT(sizeof(T) == sizeof(Decayed));
599 return Translator::recover(
600 cmpxchg(Translator::decay(exchange_value),
601 reinterpret_cast<Decayed volatile*>(dest),
602 Translator::decay(compare_value),
603 order));
604 }
605 };
606
607 template<typename Type, typename Fn, typename T>
608 inline T Atomic::cmpxchg_using_helper(Fn fn,
609 T exchange_value,
610 T volatile* dest,
611 T compare_value) {
612 STATIC_ASSERT(sizeof(Type) == sizeof(T));
613 return PrimitiveConversions::cast<T>(
614 fn(PrimitiveConversions::cast<Type>(exchange_value),
615 reinterpret_cast<Type volatile*>(dest),
616 PrimitiveConversions::cast<Type>(compare_value)));
617 }
618
619 template<typename T>
620 inline T Atomic::CmpxchgByteUsingInt::operator()(T exchange_value,
621 T volatile* dest,
622 T compare_value,
623 cmpxchg_memory_order order) const {
624 STATIC_ASSERT(sizeof(T) == sizeof(uint8_t));
625 uint8_t canon_exchange_value = exchange_value;
626 uint8_t canon_compare_value = compare_value;
627 volatile uint32_t* aligned_dest
628 = reinterpret_cast<volatile uint32_t*>(align_down(dest, sizeof(uint32_t)));
629 size_t offset = pointer_delta(dest, aligned_dest, 1);
630 uint32_t cur = *aligned_dest;
631 uint8_t* cur_as_bytes = reinterpret_cast<uint8_t*>(&cur);
632
633 // current value may not be what we are looking for, so force it
634 // to that value so the initial cmpxchg will fail if it is different
635 cur_as_bytes[offset] = canon_compare_value;
636
637 // always execute a real cmpxchg so that we get the required memory
638 // barriers even on initial failure
639 do {
640 // value to swap in matches current value ...
641 uint32_t new_value = cur;
642 // ... except for the one jbyte we want to update
643 reinterpret_cast<uint8_t*>(&new_value)[offset] = canon_exchange_value;
644
645 uint32_t res = cmpxchg(new_value, aligned_dest, cur, order);
646 if (res == cur) break; // success
647
648 // at least one byte in the int changed value, so update
649 // our view of the current int
650 cur = res;
651 // if our byte is still as cur we loop and try again
652 } while (cur_as_bytes[offset] == canon_compare_value);
653
654 return PrimitiveConversions::cast<T>(cur_as_bytes[offset]);
655 }
656
657 // Handle xchg for integral and enum types.
658 //
659 // All the involved types must be identical.
660 template<typename T>
661 struct Atomic::XchgImpl<
662 T, T,
663 typename EnableIf<IsIntegral<T>::value || IsRegisteredEnum<T>::value>::type>
664 VALUE_OBJ_CLASS_SPEC
665 {
666 T operator()(T exchange_value, T volatile* dest) const {
667 // Forward to the platform handler for the size of T.
668 return PlatformXchg<sizeof(T)>()(exchange_value, dest);
669 }
670 };
671
672 // Handle xchg for pointer types.
673 //
674 // The exchange_value must be implicitly convertible to the
675 // destination's type; it must be type-correct to store the
676 // exchange_value in the destination.
677 template<typename T, typename D>
678 struct Atomic::XchgImpl<
679 T*, D*,
680 typename EnableIf<Atomic::IsPointerConvertible<T*, D*>::value>::type>
681 VALUE_OBJ_CLASS_SPEC
682 {
683 D* operator()(T* exchange_value, D* volatile* dest) const {
684 // Allow derived to base conversion, and adding cv-qualifiers.
685 D* new_value = exchange_value;
686 return PlatformXchg<sizeof(D*)>()(new_value, dest);
687 }
688 };
689
690 // Handle xchg for types that have a translator.
691 //
692 // All the involved types must be identical.
693 //
694 // This translates the original call into a call on the decayed
695 // arguments, and returns the recovered result of that translated
696 // call.
697 template<typename T>
698 struct Atomic::XchgImpl<
699 T, T,
700 typename EnableIf<PrimitiveConversions::Translate<T>::value>::type>
701 VALUE_OBJ_CLASS_SPEC
702 {
703 T operator()(T exchange_value, T volatile* dest) const {
704 typedef PrimitiveConversions::Translate<T> Translator;
705 typedef typename Translator::Decayed Decayed;
706 STATIC_ASSERT(sizeof(T) == sizeof(Decayed));
707 return Translator::recover(
708 xchg(Translator::decay(exchange_value),
709 reinterpret_cast<Decayed volatile*>(dest)));
710 }
711 };
712
713 template<typename Type, typename Fn, typename T>
714 inline T Atomic::xchg_using_helper(Fn fn,
715 T exchange_value,
716 T volatile* dest) {
717 STATIC_ASSERT(sizeof(Type) == sizeof(T));
718 return PrimitiveConversions::cast<T>(
719 fn(PrimitiveConversions::cast<Type>(exchange_value),
720 reinterpret_cast<Type volatile*>(dest)));
721 }
722
723 template<typename T, typename D>
724 inline D Atomic::xchg(T exchange_value, volatile D* dest) {
725 return XchgImpl<T, D>()(exchange_value, dest);
726 }
727
728 #endif // SHARE_VM_RUNTIME_ATOMIC_HPP