atomic.hpp revision 13249:a2753984d2c1 
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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/align.hpp"
30#include "utilities/macros.hpp"
31
32enum cmpxchg_memory_order {
33  memory_order_relaxed,
34  // Use value which doesn't interfere with C++2011. We need to be more conservative.
35  memory_order_conservative = 8
36};
37
38class Atomic : AllStatic {
39 public:
40  // Atomic operations on jlong types are not available on all 32-bit
41  // platforms. If atomic ops on jlongs are defined here they must only
42  // be used from code that verifies they are available at runtime and
43  // can provide an alternative action if not - see supports_cx8() for
44  // a means to test availability.
45
46  // The memory operations that are mentioned with each of the atomic
47  // function families come from src/share/vm/runtime/orderAccess.hpp,
48  // e.g., <fence> is described in that file and is implemented by the
49  // OrderAccess::fence() function. See that file for the gory details
50  // on the Memory Access Ordering Model.
51
52  // All of the atomic operations that imply a read-modify-write action
53  // guarantee a two-way memory barrier across that operation. Historically
54  // these semantics reflect the strength of atomic operations that are
55  // provided on SPARC/X86. We assume that strength is necessary unless
56  // we can prove that a weaker form is sufficiently safe.
57
58  // Atomically store to a location
59  inline static void store    (jbyte    store_value, jbyte*    dest);
60  inline static void store    (jshort   store_value, jshort*   dest);
61  inline static void store    (jint     store_value, jint*     dest);
62  // See comment above about using jlong atomics on 32-bit platforms
63  inline static void store    (jlong    store_value, jlong*    dest);
64  inline static void store_ptr(intptr_t store_value, intptr_t* dest);
65  inline static void store_ptr(void*    store_value, void*     dest);
66
67  inline static void store    (jbyte    store_value, volatile jbyte*    dest);
68  inline static void store    (jshort   store_value, volatile jshort*   dest);
69  inline static void store    (jint     store_value, volatile jint*     dest);
70  // See comment above about using jlong atomics on 32-bit platforms
71  inline static void store    (jlong    store_value, volatile jlong*    dest);
72  inline static void store_ptr(intptr_t store_value, volatile intptr_t* dest);
73  inline static void store_ptr(void*    store_value, volatile void*     dest);
74
75  // See comment above about using jlong atomics on 32-bit platforms
76  inline static jlong load(const volatile jlong* src);
77
78  // Atomically add to a location. Returns updated value. add*() provide:
79  // <fence> add-value-to-dest <membar StoreLoad|StoreStore>
80  inline static jshort   add    (jshort   add_value, volatile jshort*   dest);
81  inline static jint     add    (jint     add_value, volatile jint*     dest);
82  inline static size_t   add    (size_t   add_value, volatile size_t*   dest);
83  inline static intptr_t add_ptr(intptr_t add_value, volatile intptr_t* dest);
84  inline static void*    add_ptr(intptr_t add_value, volatile void*     dest);
85
86  // Atomically increment location. inc*() provide:
87  // <fence> increment-dest <membar StoreLoad|StoreStore>
88  inline static void inc    (volatile jint*     dest);
89  inline static void inc    (volatile jshort*   dest);
90  inline static void inc    (volatile size_t*   dest);
91  inline static void inc_ptr(volatile intptr_t* dest);
92  inline static void inc_ptr(volatile void*     dest);
93
94  // Atomically decrement a location. dec*() provide:
95  // <fence> decrement-dest <membar StoreLoad|StoreStore>
96  inline static void dec    (volatile jint*     dest);
97  inline static void dec    (volatile jshort*   dest);
98  inline static void dec    (volatile size_t*   dest);
99  inline static void dec_ptr(volatile intptr_t* dest);
100  inline static void dec_ptr(volatile void*     dest);
101
102  // Performs atomic exchange of *dest with exchange_value. Returns old
103  // prior value of *dest. xchg*() provide:
104  // <fence> exchange-value-with-dest <membar StoreLoad|StoreStore>
105  inline static jint         xchg    (jint         exchange_value, volatile jint*         dest);
106  inline static unsigned int xchg    (unsigned int exchange_value, volatile unsigned int* dest);
107  inline static intptr_t     xchg_ptr(intptr_t     exchange_value, volatile intptr_t*     dest);
108  inline static void*        xchg_ptr(void*        exchange_value, volatile void*         dest);
109
110  // Performs atomic compare of *dest and compare_value, and exchanges
111  // *dest with exchange_value if the comparison succeeded. Returns prior
112  // value of *dest. cmpxchg*() provide:
113  // <fence> compare-and-exchange <membar StoreLoad|StoreStore>
114  inline static jbyte        cmpxchg    (jbyte        exchange_value, volatile jbyte*        dest, jbyte        compare_value, cmpxchg_memory_order order = memory_order_conservative);
115  inline static jint         cmpxchg    (jint         exchange_value, volatile jint*         dest, jint         compare_value, cmpxchg_memory_order order = memory_order_conservative);
116  // See comment above about using jlong atomics on 32-bit platforms
117  inline static jlong        cmpxchg    (jlong        exchange_value, volatile jlong*        dest, jlong        compare_value, cmpxchg_memory_order order = memory_order_conservative);
118  inline static unsigned int cmpxchg    (unsigned int exchange_value, volatile unsigned int* dest, unsigned int compare_value, cmpxchg_memory_order order = memory_order_conservative);
119  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);
120  inline static void*        cmpxchg_ptr(void*        exchange_value, volatile void*         dest, void*        compare_value, cmpxchg_memory_order order = memory_order_conservative);
121};
122
123// platform specific in-line definitions - must come before shared definitions
124
125#include OS_CPU_HEADER(atomic)
126
127// shared in-line definitions
128
129// size_t casts...
130#if (SIZE_MAX != UINTPTR_MAX)
131#error size_t is not WORD_SIZE, interesting platform, but missing implementation here
132#endif
133
134inline size_t Atomic::add(size_t add_value, volatile size_t* dest) {
135  return (size_t) add_ptr((intptr_t) add_value, (volatile intptr_t*) dest);
136}
137
138inline void Atomic::inc(volatile size_t* dest) {
139  inc_ptr((volatile intptr_t*) dest);
140}
141
142inline void Atomic::dec(volatile size_t* dest) {
143  dec_ptr((volatile intptr_t*) dest);
144}
145
146#ifndef VM_HAS_SPECIALIZED_CMPXCHG_BYTE
147/*
148 * This is the default implementation of byte-sized cmpxchg. It emulates jbyte-sized cmpxchg
149 * in terms of jint-sized cmpxchg. Platforms may override this by defining their own inline definition
150 * as well as defining VM_HAS_SPECIALIZED_CMPXCHG_BYTE. This will cause the platform specific
151 * implementation to be used instead.
152 */
153inline jbyte Atomic::cmpxchg(jbyte exchange_value, volatile jbyte* dest,
154                             jbyte compare_value, cmpxchg_memory_order order) {
155  STATIC_ASSERT(sizeof(jbyte) == 1);
156  volatile jint* dest_int =
157      reinterpret_cast<volatile jint*>(align_down(dest, sizeof(jint)));
158  size_t offset = pointer_delta(dest, dest_int, 1);
159  jint cur = *dest_int;
160  jbyte* cur_as_bytes = reinterpret_cast<jbyte*>(&cur);
161
162  // current value may not be what we are looking for, so force it
163  // to that value so the initial cmpxchg will fail if it is different
164  cur_as_bytes[offset] = compare_value;
165
166  // always execute a real cmpxchg so that we get the required memory
167  // barriers even on initial failure
168  do {
169    // value to swap in matches current value ...
170    jint new_value = cur;
171    // ... except for the one jbyte we want to update
172    reinterpret_cast<jbyte*>(&new_value)[offset] = exchange_value;
173
174    jint res = cmpxchg(new_value, dest_int, cur, order);
175    if (res == cur) break; // success
176
177    // at least one jbyte in the jint changed value, so update
178    // our view of the current jint
179    cur = res;
180    // if our jbyte is still as cur we loop and try again
181  } while (cur_as_bytes[offset] == compare_value);
182
183  return cur_as_bytes[offset];
184}
185
186#endif // VM_HAS_SPECIALIZED_CMPXCHG_BYTE
187
188inline unsigned Atomic::xchg(unsigned int exchange_value, volatile unsigned int* dest) {
189  assert(sizeof(unsigned int) == sizeof(jint), "more work to do");
190  return (unsigned int)Atomic::xchg((jint)exchange_value, (volatile jint*)dest);
191}
192
193inline unsigned Atomic::cmpxchg(unsigned int exchange_value,
194                         volatile unsigned int* dest, unsigned int compare_value,
195                         cmpxchg_memory_order order) {
196  assert(sizeof(unsigned int) == sizeof(jint), "more work to do");
197  return (unsigned int)Atomic::cmpxchg((jint)exchange_value, (volatile jint*)dest,
198                                       (jint)compare_value, order);
199}
200
201inline jshort Atomic::add(jshort add_value, volatile jshort* dest) {
202  // Most platforms do not support atomic add on a 2-byte value. However,
203  // if the value occupies the most significant 16 bits of an aligned 32-bit
204  // word, then we can do this with an atomic add of (add_value << 16)
205  // to the 32-bit word.
206  //
207  // The least significant parts of this 32-bit word will never be affected, even
208  // in case of overflow/underflow.
209  //
210  // Use the ATOMIC_SHORT_PAIR macro (see macros.hpp) to get the desired alignment.
211#ifdef VM_LITTLE_ENDIAN
212  assert((intx(dest) & 0x03) == 0x02, "wrong alignment");
213  jint new_value = Atomic::add(add_value << 16, (volatile jint*)(dest-1));
214#else
215  assert((intx(dest) & 0x03) == 0x00, "wrong alignment");
216  jint new_value = Atomic::add(add_value << 16, (volatile jint*)(dest));
217#endif
218  return (jshort)(new_value >> 16); // preserves sign
219}
220
221inline void Atomic::inc(volatile jshort* dest) {
222  (void)add(1, dest);
223}
224
225inline void Atomic::dec(volatile jshort* dest) {
226  (void)add(-1, dest);
227}
228
229#endif // SHARE_VM_RUNTIME_ATOMIC_HPP
230