atomic.hpp revision 13242:fcb4803050e8 
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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 "utilities/macros.hpp"
30
31enum cmpxchg_memory_order {
32  memory_order_relaxed,
33  // Use value which doesn't interfere with C++2011. We need to be more conservative.
34  memory_order_conservative = 8
35};
36
37class Atomic : AllStatic {
38 public:
39  // Atomic operations on jlong types are not available on all 32-bit
40  // platforms. If atomic ops on jlongs are defined here they must only
41  // be used from code that verifies they are available at runtime and
42  // can provide an alternative action if not - see supports_cx8() for
43  // a means to test availability.
44
45  // The memory operations that are mentioned with each of the atomic
46  // function families come from src/share/vm/runtime/orderAccess.hpp,
47  // e.g., <fence> is described in that file and is implemented by the
48  // OrderAccess::fence() function. See that file for the gory details
49  // on the Memory Access Ordering Model.
50
51  // All of the atomic operations that imply a read-modify-write action
52  // guarantee a two-way memory barrier across that operation. Historically
53  // these semantics reflect the strength of atomic operations that are
54  // provided on SPARC/X86. We assume that strength is necessary unless
55  // we can prove that a weaker form is sufficiently safe.
56
57  // Atomically store to a location
58  inline static void store    (jbyte    store_value, jbyte*    dest);
59  inline static void store    (jshort   store_value, jshort*   dest);
60  inline static void store    (jint     store_value, jint*     dest);
61  // See comment above about using jlong atomics on 32-bit platforms
62  inline static void store    (jlong    store_value, jlong*    dest);
63  inline static void store_ptr(intptr_t store_value, intptr_t* dest);
64  inline static void store_ptr(void*    store_value, void*     dest);
65
66  inline static void store    (jbyte    store_value, volatile jbyte*    dest);
67  inline static void store    (jshort   store_value, volatile jshort*   dest);
68  inline static void store    (jint     store_value, volatile jint*     dest);
69  // See comment above about using jlong atomics on 32-bit platforms
70  inline static void store    (jlong    store_value, volatile jlong*    dest);
71  inline static void store_ptr(intptr_t store_value, volatile intptr_t* dest);
72  inline static void store_ptr(void*    store_value, volatile void*     dest);
73
74  // See comment above about using jlong atomics on 32-bit platforms
75  inline static jlong load(const volatile jlong* src);
76
77  // Atomically add to a location. Returns updated value. add*() provide:
78  // <fence> add-value-to-dest <membar StoreLoad|StoreStore>
79  inline static jshort   add    (jshort   add_value, volatile jshort*   dest);
80  inline static jint     add    (jint     add_value, volatile jint*     dest);
81  inline static size_t   add    (size_t   add_value, volatile size_t*   dest);
82  inline static intptr_t add_ptr(intptr_t add_value, volatile intptr_t* dest);
83  inline static void*    add_ptr(intptr_t add_value, volatile void*     dest);
84
85  // Atomically increment location. inc*() provide:
86  // <fence> increment-dest <membar StoreLoad|StoreStore>
87  inline static void inc    (volatile jint*     dest);
88  inline static void inc    (volatile jshort*   dest);
89  inline static void inc    (volatile size_t*   dest);
90  inline static void inc_ptr(volatile intptr_t* dest);
91  inline static void inc_ptr(volatile void*     dest);
92
93  // Atomically decrement a location. dec*() provide:
94  // <fence> decrement-dest <membar StoreLoad|StoreStore>
95  inline static void dec    (volatile jint*     dest);
96  inline static void dec    (volatile jshort*   dest);
97  inline static void dec    (volatile size_t*   dest);
98  inline static void dec_ptr(volatile intptr_t* dest);
99  inline static void dec_ptr(volatile void*     dest);
100
101  // Performs atomic exchange of *dest with exchange_value. Returns old
102  // prior value of *dest. xchg*() provide:
103  // <fence> exchange-value-with-dest <membar StoreLoad|StoreStore>
104  inline static jint         xchg    (jint         exchange_value, volatile jint*         dest);
105  inline static unsigned int xchg    (unsigned int exchange_value, volatile unsigned int* dest);
106  inline static intptr_t     xchg_ptr(intptr_t     exchange_value, volatile intptr_t*     dest);
107  inline static void*        xchg_ptr(void*        exchange_value, volatile void*         dest);
108
109  // Performs atomic compare of *dest and compare_value, and exchanges
110  // *dest with exchange_value if the comparison succeeded. Returns prior
111  // value of *dest. cmpxchg*() provide:
112  // <fence> compare-and-exchange <membar StoreLoad|StoreStore>
113  inline static jbyte        cmpxchg    (jbyte        exchange_value, volatile jbyte*        dest, jbyte        compare_value, cmpxchg_memory_order order = memory_order_conservative);
114  inline static jint         cmpxchg    (jint         exchange_value, volatile jint*         dest, jint         compare_value, cmpxchg_memory_order order = memory_order_conservative);
115  // See comment above about using jlong atomics on 32-bit platforms
116  inline static jlong        cmpxchg    (jlong        exchange_value, volatile jlong*        dest, jlong        compare_value, cmpxchg_memory_order order = memory_order_conservative);
117  inline static unsigned int cmpxchg    (unsigned int exchange_value, volatile unsigned int* dest, unsigned int compare_value, cmpxchg_memory_order order = memory_order_conservative);
118  inline static intptr_t     cmpxchg_ptr(intptr_t     exchange_value, volatile intptr_t*     dest, intptr_t     compare_value, cmpxchg_memory_order order = memory_order_conservative);
119  inline static void*        cmpxchg_ptr(void*        exchange_value, volatile void*         dest, void*        compare_value, cmpxchg_memory_order order = memory_order_conservative);
120};
121
122// platform specific in-line definitions - must come before shared definitions
123
124#include OS_CPU_HEADER(atomic)
125
126// shared in-line definitions
127
128// size_t casts...
129#if (SIZE_MAX != UINTPTR_MAX)
130#error size_t is not WORD_SIZE, interesting platform, but missing implementation here
131#endif
132
133inline size_t Atomic::add(size_t add_value, volatile size_t* dest) {
134  return (size_t) add_ptr((intptr_t) add_value, (volatile intptr_t*) dest);
135}
136
137inline void Atomic::inc(volatile size_t* dest) {
138  inc_ptr((volatile intptr_t*) dest);
139}
140
141inline void Atomic::dec(volatile size_t* dest) {
142  dec_ptr((volatile intptr_t*) dest);
143}
144
145#ifndef VM_HAS_SPECIALIZED_CMPXCHG_BYTE
146/*
147 * This is the default implementation of byte-sized cmpxchg. It emulates jbyte-sized cmpxchg
148 * in terms of jint-sized cmpxchg. Platforms may override this by defining their own inline definition
149 * as well as defining VM_HAS_SPECIALIZED_CMPXCHG_BYTE. This will cause the platform specific
150 * implementation to be used instead.
151 */
152inline jbyte Atomic::cmpxchg(jbyte exchange_value, volatile jbyte* dest,
153                             jbyte compare_value, cmpxchg_memory_order order) {
154  STATIC_ASSERT(sizeof(jbyte) == 1);
155  volatile jint* dest_int =
156      reinterpret_cast<volatile jint*>(align_ptr_down(dest, sizeof(jint)));
157  size_t offset = pointer_delta(dest, dest_int, 1);
158  jint cur = *dest_int;
159  jbyte* cur_as_bytes = reinterpret_cast<jbyte*>(&cur);
160
161  // current value may not be what we are looking for, so force it
162  // to that value so the initial cmpxchg will fail if it is different
163  cur_as_bytes[offset] = compare_value;
164
165  // always execute a real cmpxchg so that we get the required memory
166  // barriers even on initial failure
167  do {
168    // value to swap in matches current value ...
169    jint new_value = cur;
170    // ... except for the one jbyte we want to update
171    reinterpret_cast<jbyte*>(&new_value)[offset] = exchange_value;
172
173    jint res = cmpxchg(new_value, dest_int, cur, order);
174    if (res == cur) break; // success
175
176    // at least one jbyte in the jint changed value, so update
177    // our view of the current jint
178    cur = res;
179    // if our jbyte is still as cur we loop and try again
180  } while (cur_as_bytes[offset] == compare_value);
181
182  return cur_as_bytes[offset];
183}
184
185#endif // VM_HAS_SPECIALIZED_CMPXCHG_BYTE
186
187inline unsigned Atomic::xchg(unsigned int exchange_value, volatile unsigned int* dest) {
188  assert(sizeof(unsigned int) == sizeof(jint), "more work to do");
189  return (unsigned int)Atomic::xchg((jint)exchange_value, (volatile jint*)dest);
190}
191
192inline unsigned Atomic::cmpxchg(unsigned int exchange_value,
193                         volatile unsigned int* dest, unsigned int compare_value,
194                         cmpxchg_memory_order order) {
195  assert(sizeof(unsigned int) == sizeof(jint), "more work to do");
196  return (unsigned int)Atomic::cmpxchg((jint)exchange_value, (volatile jint*)dest,
197                                       (jint)compare_value, order);
198}
199
200inline jshort Atomic::add(jshort add_value, volatile jshort* dest) {
201  // Most platforms do not support atomic add on a 2-byte value. However,
202  // if the value occupies the most significant 16 bits of an aligned 32-bit
203  // word, then we can do this with an atomic add of (add_value << 16)
204  // to the 32-bit word.
205  //
206  // The least significant parts of this 32-bit word will never be affected, even
207  // in case of overflow/underflow.
208  //
209  // Use the ATOMIC_SHORT_PAIR macro (see macros.hpp) to get the desired alignment.
210#ifdef VM_LITTLE_ENDIAN
211  assert((intx(dest) & 0x03) == 0x02, "wrong alignment");
212  jint new_value = Atomic::add(add_value << 16, (volatile jint*)(dest-1));
213#else
214  assert((intx(dest) & 0x03) == 0x00, "wrong alignment");
215  jint new_value = Atomic::add(add_value << 16, (volatile jint*)(dest));
216#endif
217  return (jshort)(new_value >> 16); // preserves sign
218}
219
220inline void Atomic::inc(volatile jshort* dest) {
221  (void)add(1, dest);
222}
223
224inline void Atomic::dec(volatile jshort* dest) {
225  (void)add(-1, dest);
226}
227
228#endif // SHARE_VM_RUNTIME_ATOMIC_HPP
229