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24
25 // Memory Access Ordering Model
26 //
27 // This interface is based on the JSR-133 Cookbook for Compiler Writers
28 // and on the IA64 memory model. It is the dynamic equivalent of the
29 // C/C++ volatile specifier. I.e., volatility restricts compile-time
30 // memory access reordering in a way similar to what we want to occur
31 // at runtime.
32 //
33 // In the following, the terms 'previous', 'subsequent', 'before',
34 // 'after', 'preceding' and 'succeeding' refer to program order. The
35 // terms 'down' and 'below' refer to forward load or store motion
36 // relative to program order, while 'up' and 'above' refer to backward
37 // motion.
38 //
39 //
40 // We define four primitive memory barrier operations.
41 //
42 // LoadLoad: Load1(s); LoadLoad; Load2
43 //
44 // Ensures that Load1 completes (obtains the value it loads from memory)
45 // before Load2 and any subsequent load operations. Loads before Load1
46 // may *not* float below Load2 and any subsequent load operations.
47 //
48 // StoreStore: Store1(s); StoreStore; Store2
49 //
50 // Ensures that Store1 completes (the effect on memory of Store1 is made
51 // visible to other processors) before Store2 and any subsequent store
52 // operations. Stores before Store1 may *not* float below Store2 and any
53 // subsequent store operations.
54 //
55 // LoadStore: Load1(s); LoadStore; Store2
56 //
57 // Ensures that Load1 completes before Store2 and any subsequent store
58 // operations. Loads before Load1 may *not* float below Store2 and any
59 // subseqeuent store operations.
60 //
61 // StoreLoad: Store1(s); StoreLoad; Load2
62 //
63 // Ensures that Store1 completes before Load2 and any subsequent load
64 // operations. Stores before Store1 may *not* float below Load2 and any
65 // subseqeuent load operations.
66 //
67 //
68 // We define two further operations, 'release' and 'acquire'. They are
69 // mirror images of each other.
70 //
71 // Execution by a processor of release makes the effect of all memory
72 // accesses issued by it previous to the release visible to all
73 // processors *before* the release completes. The effect of subsequent
74 // memory accesses issued by it *may* be made visible *before* the
75 // release. I.e., subsequent memory accesses may float above the
76 // release, but prior ones may not float below it.
77 //
78 // Execution by a processor of acquire makes the effect of all memory
79 // accesses issued by it subsequent to the acquire visible to all
80 // processors *after* the acquire completes. The effect of prior memory
81 // accesses issued by it *may* be made visible *after* the acquire.
82 // I.e., prior memory accesses may float below the acquire, but
83 // subsequent ones may not float above it.
84 //
85 // Finally, we define a 'fence' operation, which conceptually is a
86 // release combined with an acquire. In the real world these operations
87 // require one or more machine instructions which can float above and
88 // below the release or acquire, so we usually can't just issue the
89 // release-acquire back-to-back. All machines we know of implement some
90 // sort of memory fence instruction.
91 //
92 //
93 // The standalone implementations of release and acquire need an associated
94 // dummy volatile store or load respectively. To avoid redundant operations,
95 // we can define the composite operators: 'release_store', 'store_fence' and
96 // 'load_acquire'. Here's a summary of the machine instructions corresponding
97 // to each operation.
98 //
99 // sparc RMO ia64 x86
100 // ---------------------------------------------------------------------
101 // fence membar #LoadStore | mf lock addl 0,(sp)
102 // #StoreStore |
103 // #LoadLoad |
104 // #StoreLoad
105 //
106 // release membar #LoadStore | st.rel [sp]=r0 movl $0,<dummy>
107 // #StoreStore
108 // st %g0,[]
109 //
110 // acquire ld [%sp],%g0 ld.acq <r>=[sp] movl (sp),<r>
111 // membar #LoadLoad |
112 // #LoadStore
113 //
114 // release_store membar #LoadStore | st.rel <store>
115 // #StoreStore
116 // st
117 //
118 // store_fence st st lock xchg
119 // fence mf
120 //
121 // load_acquire ld ld.acq <load>
122 // membar #LoadLoad |
123 // #LoadStore
124 //
125 // Using only release_store and load_acquire, we can implement the
126 // following ordered sequences.
127 //
128 // 1. load, load == load_acquire, load
129 // or load_acquire, load_acquire
130 // 2. load, store == load, release_store
131 // or load_acquire, store
132 // or load_acquire, release_store
133 // 3. store, store == store, release_store
134 // or release_store, release_store
135 //
136 // These require no membar instructions for sparc-TSO and no extra
137 // instructions for ia64.
138 //
139 // Ordering a load relative to preceding stores requires a store_fence,
140 // which implies a membar #StoreLoad between the store and load under
141 // sparc-TSO. A fence is required by ia64. On x86, we use locked xchg.
142 //
143 // 4. store, load == store_fence, load
144 //
145 // Use store_fence to make sure all stores done in an 'interesting'
146 // region are made visible prior to both subsequent loads and stores.
147 //
148 // Conventional usage is to issue a load_acquire for ordered loads. Use
149 // release_store for ordered stores when you care only that prior stores
150 // are visible before the release_store, but don't care exactly when the
151 // store associated with the release_store becomes visible. Use
152 // release_store_fence to update values like the thread state, where we
153 // don't want the current thread to continue until all our prior memory
154 // accesses (including the new thread state) are visible to other threads.
155 //
156 //
157 // C++ Volatility
158 //
159 // C++ guarantees ordering at operations termed 'sequence points' (defined
160 // to be volatile accesses and calls to library I/O functions). 'Side
161 // effects' (defined as volatile accesses, calls to library I/O functions
162 // and object modification) previous to a sequence point must be visible
163 // at that sequence point. See the C++ standard, section 1.9, titled
164 // "Program Execution". This means that all barrier implementations,
165 // including standalone loadload, storestore, loadstore, storeload, acquire
166 // and release must include a sequence point, usually via a volatile memory
167 // access. Other ways to guarantee a sequence point are, e.g., use of
168 // indirect calls and linux's __asm__ volatile.
169 // Note: as of 6973570, we have replaced the originally static "dummy" field
170 // (see above) by a volatile store to the stack. All of the versions of the
171 // compilers that we currently use (SunStudio, gcc and VC++) respect the
172 // semantics of volatile here. If you build HotSpot using other
173 // compilers, you may need to verify that no compiler reordering occurs
174 // across the sequence point respresented by the volatile access.
175 //
176 //
177 // os::is_MP Considered Redundant
178 //
179 // Callers of this interface do not need to test os::is_MP() before
180 // issuing an operation. The test is taken care of by the implementation
181 // of the interface (depending on the vm version and platform, the test
182 // may or may not be actually done by the implementation).
183 //
184 //
185 // A Note on Memory Ordering and Cache Coherency
186 //
187 // Cache coherency and memory ordering are orthogonal concepts, though they
188 // interact. E.g., all existing itanium machines are cache-coherent, but
189 // the hardware can freely reorder loads wrt other loads unless it sees a
190 // load-acquire instruction. All existing sparc machines are cache-coherent
191 // and, unlike itanium, TSO guarantees that the hardware orders loads wrt
192 // loads and stores, and stores wrt to each other.
193 //
194 // Consider the implementation of loadload. *If* your platform *isn't*
195 // cache-coherent, then loadload must not only prevent hardware load
196 // instruction reordering, but it must *also* ensure that subsequent
197 // loads from addresses that could be written by other processors (i.e.,
198 // that are broadcast by other processors) go all the way to the first
199 // level of memory shared by those processors and the one issuing
200 // the loadload.
201 //
202 // So if we have a MP that has, say, a per-processor D$ that doesn't see
203 // writes by other processors, and has a shared E$ that does, the loadload
204 // barrier would have to make sure that either
205 //
206 // 1. cache lines in the issuing processor's D$ that contained data from
207 // addresses that could be written by other processors are invalidated, so
208 // subsequent loads from those addresses go to the E$, (it could do this
209 // by tagging such cache lines as 'shared', though how to tell the hardware
210 // to do the tagging is an interesting problem), or
211 //
212 // 2. there never are such cache lines in the issuing processor's D$, which
213 // means all references to shared data (however identified: see above)
214 // bypass the D$ (i.e., are satisfied from the E$).
215 //
216 // If your machine doesn't have an E$, substitute 'main memory' for 'E$'.
217 //
218 // Either of these alternatives is a pain, so no current machine we know of
219 // has incoherent caches.
220 //
221 // If loadload didn't have these properties, the store-release sequence for
222 // publishing a shared data structure wouldn't work, because a processor
223 // trying to read data newly published by another processor might go to
224 // its own incoherent caches to satisfy the read instead of to the newly
225 // written shared memory.
226 //
227 //
228 // NOTE WELL!!
229 //
230 // A Note on MutexLocker and Friends
231 //
232 // See mutexLocker.hpp. We assume throughout the VM that MutexLocker's
233 // and friends' constructors do a fence, a lock and an acquire *in that
234 // order*. And that their destructors do a release and unlock, in *that*
235 // order. If their implementations change such that these assumptions
236 // are violated, a whole lot of code will break.
237
238 class OrderAccess : AllStatic {
239 public:
240 static void loadload();
241 static void storestore();
242 static void loadstore();
243 static void storeload();
244
245 static void acquire();
246 static void release();
247 static void fence();
248
249 static jbyte load_acquire(volatile jbyte* p);
250 static jshort load_acquire(volatile jshort* p);
251 static jint load_acquire(volatile jint* p);
252 static jlong load_acquire(volatile jlong* p);
253 static jubyte load_acquire(volatile jubyte* p);
254 static jushort load_acquire(volatile jushort* p);
255 static juint load_acquire(volatile juint* p);
256 static julong load_acquire(volatile julong* p);
257 static jfloat load_acquire(volatile jfloat* p);
258 static jdouble load_acquire(volatile jdouble* p);
259
260 static intptr_t load_ptr_acquire(volatile intptr_t* p);
261 static void* load_ptr_acquire(volatile void* p);
262 static void* load_ptr_acquire(const volatile void* p);
263
264 static void release_store(volatile jbyte* p, jbyte v);
265 static void release_store(volatile jshort* p, jshort v);
266 static void release_store(volatile jint* p, jint v);
267 static void release_store(volatile jlong* p, jlong v);
268 static void release_store(volatile jubyte* p, jubyte v);
269 static void release_store(volatile jushort* p, jushort v);
270 static void release_store(volatile juint* p, juint v);
271 static void release_store(volatile julong* p, julong v);
272 static void release_store(volatile jfloat* p, jfloat v);
273 static void release_store(volatile jdouble* p, jdouble v);
274
275 static void release_store_ptr(volatile intptr_t* p, intptr_t v);
276 static void release_store_ptr(volatile void* p, void* v);
277
278 static void store_fence(jbyte* p, jbyte v);
279 static void store_fence(jshort* p, jshort v);
280 static void store_fence(jint* p, jint v);
281 static void store_fence(jlong* p, jlong v);
282 static void store_fence(jubyte* p, jubyte v);
283 static void store_fence(jushort* p, jushort v);
284 static void store_fence(juint* p, juint v);
285 static void store_fence(julong* p, julong v);
286 static void store_fence(jfloat* p, jfloat v);
287 static void store_fence(jdouble* p, jdouble v);
288
289 static void store_ptr_fence(intptr_t* p, intptr_t v);
290 static void store_ptr_fence(void** p, void* v);
291
292 static void release_store_fence(volatile jbyte* p, jbyte v);
293 static void release_store_fence(volatile jshort* p, jshort v);
294 static void release_store_fence(volatile jint* p, jint v);
295 static void release_store_fence(volatile jlong* p, jlong v);
296 static void release_store_fence(volatile jubyte* p, jubyte v);
297 static void release_store_fence(volatile jushort* p, jushort v);
298 static void release_store_fence(volatile juint* p, juint v);
299 static void release_store_fence(volatile julong* p, julong v);
300 static void release_store_fence(volatile jfloat* p, jfloat v);
301 static void release_store_fence(volatile jdouble* p, jdouble v);
302
303 static void release_store_ptr_fence(volatile intptr_t* p, intptr_t v);
304 static void release_store_ptr_fence(volatile void* p, void* v);
305
306 private:
307 // This is a helper that invokes the StubRoutines::fence_entry()
308 // routine if it exists, It should only be used by platforms that
309 // don't another way to do the inline eassembly.
310 static void StubRoutines_fence();
311 };