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//                Memory Access Ordering Model

//

// This interface is based on the JSR-133 Cookbook for Compiler Writers

// and on the IA64 memory model.  It is the dynamic equivalent of the

// C/C++ volatile specifier.  I.e., volatility restricts compile-time

// memory access reordering in a way similar to what we want to occur

// at runtime.

//

// In the following, the terms 'previous', 'subsequent', 'before',

// 'after', 'preceding' and 'succeeding' refer to program order.  The

// terms 'down' and 'below' refer to forward load or store motion

// relative to program order, while 'up' and 'above' refer to backward

// motion.

//

//

// We define four primitive memory barrier operations.

//

// LoadLoad:   Load1(s); LoadLoad; Load2

//

// Ensures that Load1 completes (obtains the value it loads from memory)

// before Load2 and any subsequent load operations.  Loads before Load1

// may *not* float below Load2 and any subsequent load operations.

//

// StoreStore: Store1(s); StoreStore; Store2

//

// Ensures that Store1 completes (the effect on memory of Store1 is made

// visible to other processors) before Store2 and any subsequent store

// operations.  Stores before Store1 may *not* float below Store2 and any

// subsequent store operations.

//

// LoadStore:  Load1(s); LoadStore; Store2

//

// Ensures that Load1 completes before Store2 and any subsequent store

// operations.  Loads before Load1 may *not* float below Store2 and any

// subseqeuent store operations.

//

// StoreLoad:  Store1(s); StoreLoad; Load2

//

// Ensures that Store1 completes before Load2 and any subsequent load

// operations.  Stores before Store1 may *not* float below Load2 and any

// subseqeuent load operations.

//

//

// We define two further operations, 'release' and 'acquire'.  They are

// mirror images of each other.

//

// Execution by a processor of release makes the effect of all memory

// accesses issued by it previous to the release visible to all

// processors *before* the release completes.  The effect of subsequent

// memory accesses issued by it *may* be made visible *before* the

// release.  I.e., subsequent memory accesses may float above the

// release, but prior ones may not float below it.

//

// Execution by a processor of acquire makes the effect of all memory

// accesses issued by it subsequent to the acquire visible to all

// processors *after* the acquire completes.  The effect of prior memory

// accesses issued by it *may* be made visible *after* the acquire.

// I.e., prior memory accesses may float below the acquire, but

// subsequent ones may not float above it.

//

// Finally, we define a 'fence' operation, which conceptually is a

// release combined with an acquire.  In the real world these operations

// require one or more machine instructions which can float above and

// below the release or acquire, so we usually can't just issue the

// release-acquire back-to-back.  All machines we know of implement some

// sort of memory fence instruction.

//

//

// The standalone implementations of release and acquire need an associated

// dummy volatile store or load respectively.  To avoid redundant operations,

// we can define the composite operators: 'release_store', 'store_fence' and

// 'load_acquire'.  Here's a summary of the machine instructions corresponding

// to each operation.

//

//               sparc RMO             ia64             x86

// ---------------------------------------------------------------------

// fence         membar #LoadStore |   mf               lock addl 0,(sp)

//                      #StoreStore |

//                      #LoadLoad |

//                      #StoreLoad

//

// release       membar #LoadStore |   st.rel [sp]=r0   movl $0,<dummy>

//                      #StoreStore

//               st %g0,[]

//

// acquire       ld [%sp],%g0          ld.acq <r>=[sp]  movl (sp),<r>

//               membar #LoadLoad |

//                      #LoadStore

//

// release_store membar #LoadStore |   st.rel           <store>

//                      #StoreStore

//               st

//

// store_fence   st                    st               lock xchg

//               fence                 mf

//

// load_acquire  ld                    ld.acq           <load>

//               membar #LoadLoad |

//                      #LoadStore

//

// Using only release_store and load_acquire, we can implement the

// following ordered sequences.

//

// 1. load, load   == load_acquire,  load

//                 or load_acquire,  load_acquire

// 2. load, store  == load,          release_store

//                 or load_acquire,  store

//                 or load_acquire,  release_store

// 3. store, store == store,         release_store

//                 or release_store, release_store

//

// These require no membar instructions for sparc-TSO and no extra

// instructions for ia64.

//

// Ordering a load relative to preceding stores requires a store_fence,

// which implies a membar #StoreLoad between the store and load under

// sparc-TSO.  A fence is required by ia64.  On x86, we use locked xchg.

//

// 4. store, load  == store_fence, load

//

// Use store_fence to make sure all stores done in an 'interesting'

// region are made visible prior to both subsequent loads and stores.

//

// Conventional usage is to issue a load_acquire for ordered loads.  Use

// release_store for ordered stores when you care only that prior stores

// are visible before the release_store, but don't care exactly when the

// store associated with the release_store becomes visible.  Use

// release_store_fence to update values like the thread state, where we

// don't want the current thread to continue until all our prior memory

// accesses (including the new thread state) are visible to other threads.

//

//

//                C++ Volatility

//

// C++ guarantees ordering at operations termed 'sequence points' (defined

// to be volatile accesses and calls to library I/O functions).  'Side

// effects' (defined as volatile accesses, calls to library I/O functions

// and object modification) previous to a sequence point must be visible

// at that sequence point.  See the C++ standard, section 1.9, titled

// "Program Execution".  This means that all barrier implementations,

// including standalone loadload, storestore, loadstore, storeload, acquire

// and release must include a sequence point, usually via a volatile memory

// access.  Other ways to guarantee a sequence point are, e.g., use of

// indirect calls and linux's __asm__ volatile.

// Note: as of 6973570, we have replaced the originally static "dummy" field

// (see above) by a volatile store to the stack. All of the versions of the

// compilers that we currently use (SunStudio, gcc and VC++) respect the

// semantics of volatile here. If you build HotSpot using other

// compilers, you may need to verify that no compiler reordering occurs

// across the sequence point respresented by the volatile access.

//

//

//                os::is_MP Considered Redundant

//

// Callers of this interface do not need to test os::is_MP() before

// issuing an operation. The test is taken care of by the implementation

// of the interface (depending on the vm version and platform, the test

// may or may not be actually done by the implementation).

//

//

//                A Note on Memory Ordering and Cache Coherency

//

// Cache coherency and memory ordering are orthogonal concepts, though they

// interact.  E.g., all existing itanium machines are cache-coherent, but

// the hardware can freely reorder loads wrt other loads unless it sees a

// load-acquire instruction.  All existing sparc machines are cache-coherent

// and, unlike itanium, TSO guarantees that the hardware orders loads wrt

// loads and stores, and stores wrt to each other.

//

// Consider the implementation of loadload.  *If* your platform *isn't*

// cache-coherent, then loadload must not only prevent hardware load

// instruction reordering, but it must *also* ensure that subsequent

// loads from addresses that could be written by other processors (i.e.,

// that are broadcast by other processors) go all the way to the first

// level of memory shared by those processors and the one issuing

// the loadload.

//

// So if we have a MP that has, say, a per-processor D$ that doesn't see

// writes by other processors, and has a shared E$ that does, the loadload

// barrier would have to make sure that either

//

// 1. cache lines in the issuing processor's D$ that contained data from

// addresses that could be written by other processors are invalidated, so

// subsequent loads from those addresses go to the E$, (it could do this

// by tagging such cache lines as 'shared', though how to tell the hardware

// to do the tagging is an interesting problem), or

//

// 2. there never are such cache lines in the issuing processor's D$, which

// means all references to shared data (however identified: see above)

// bypass the D$ (i.e., are satisfied from the E$).

//

// If your machine doesn't have an E$, substitute 'main memory' for 'E$'.

//

// Either of these alternatives is a pain, so no current machine we know of

// has incoherent caches.

//

// If loadload didn't have these properties, the store-release sequence for

// publishing a shared data structure wouldn't work, because a processor

// trying to read data newly published by another processor might go to

// its own incoherent caches to satisfy the read instead of to the newly

// written shared memory.

//

//

//                NOTE WELL!!

//

//                A Note on MutexLocker and Friends

//

// See mutexLocker.hpp.  We assume throughout the VM that MutexLocker's

// and friends' constructors do a fence, a lock and an acquire *in that

// order*.  And that their destructors do a release and unlock, in *that*

// order.  If their implementations change such that these assumptions

// are violated, a whole lot of code will break.

class OrderAccess : AllStatic {

 public:

  static void     loadload();

  static void     storestore();

  static void     loadstore();

  static void     storeload();

  static void     acquire();

  static void     release();

  static void     fence();

  static jbyte    load_acquire(volatile jbyte*   p);

  static jshort   load_acquire(volatile jshort*  p);

  static jint     load_acquire(volatile jint*    p);

  static jlong    load_acquire(volatile jlong*   p);

  static jubyte   load_acquire(volatile jubyte*  p);

  static jushort  load_acquire(volatile jushort* p);

  static juint    load_acquire(volatile juint*   p);

  static julong   load_acquire(volatile julong*  p);

  static jfloat   load_acquire(volatile jfloat*  p);

  static jdouble  load_acquire(volatile jdouble* p);

  static intptr_t load_ptr_acquire(volatile intptr_t*   p);

  static void*    load_ptr_acquire(volatile void*       p);

  static void*    load_ptr_acquire(const volatile void* p);

  static void     release_store(volatile jbyte*   p, jbyte   v);

  static void     release_store(volatile jshort*  p, jshort  v);

  static void     release_store(volatile jint*    p, jint    v);

  static void     release_store(volatile jlong*   p, jlong   v);

  static void     release_store(volatile jubyte*  p, jubyte  v);

  static void     release_store(volatile jushort* p, jushort v);

  static void     release_store(volatile juint*   p, juint   v);

  static void     release_store(volatile julong*  p, julong  v);

  static void     release_store(volatile jfloat*  p, jfloat  v);

  static void     release_store(volatile jdouble* p, jdouble v);

  static void     release_store_ptr(volatile intptr_t* p, intptr_t v);

  static void     release_store_ptr(volatile void*     p, void*    v);

  static void     store_fence(jbyte*   p, jbyte   v);

  static void     store_fence(jshort*  p, jshort  v);

  static void     store_fence(jint*    p, jint    v);

  static void     store_fence(jlong*   p, jlong   v);

  static void     store_fence(jubyte*  p, jubyte  v);

  static void     store_fence(jushort* p, jushort v);

  static void     store_fence(juint*   p, juint   v);

  static void     store_fence(julong*  p, julong  v);

  static void     store_fence(jfloat*  p, jfloat  v);

  static void     store_fence(jdouble* p, jdouble v);

  static void     store_ptr_fence(intptr_t* p, intptr_t v);

  static void     store_ptr_fence(void**    p, void*    v);

  static void     release_store_fence(volatile jbyte*   p, jbyte   v);

  static void     release_store_fence(volatile jshort*  p, jshort  v);

  static void     release_store_fence(volatile jint*    p, jint    v);

  static void     release_store_fence(volatile jlong*   p, jlong   v);

  static void     release_store_fence(volatile jubyte*  p, jubyte  v);

  static void     release_store_fence(volatile jushort* p, jushort v);

  static void     release_store_fence(volatile juint*   p, juint   v);

  static void     release_store_fence(volatile julong*  p, julong  v);

  static void     release_store_fence(volatile jfloat*  p, jfloat  v);

  static void     release_store_fence(volatile jdouble* p, jdouble v);

  static void     release_store_ptr_fence(volatile intptr_t* p, intptr_t v);

  static void     release_store_ptr_fence(volatile void*     p, void*    v);

 private:

  // This is a helper that invokes the StubRoutines::fence_entry()

  // routine if it exists, It should only be used by platforms that

  // don't another way to do the inline eassembly.

  static void StubRoutines_fence();

};