// hashCode() generation : // // Possibilities: // * MD5Digest of {obj,stwRandom} // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function. // * A DES- or AES-style SBox[] mechanism // * One of the Phi-based schemes, such as: // 2654435761 = 2^32 * Phi (golden ratio) // HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ; // * A variation of Marsaglia's shift-xor RNG scheme. // * (obj ^ stwRandom) is appealing, but can result // in undesirable regularity in the hashCode values of adjacent objects // (objects allocated back-to-back, in particular). This could potentially // result in hashtable collisions and reduced hashtable efficiency. // There are simple ways to "diffuse" the middle address bits over the // generated hashCode values:
staticinlineintptr_tget_next_hash(Thread * Self, oop obj){ intptr_t value = 0; if (hashCode == 0) { // This form uses global Park-Miller RNG. // On MP system we'll have lots of RW access to a global, so the // mechanism induces lots of coherency traffic. value = os::random(); } elseif (hashCode == 1) { // This variation has the property of being stable (idempotent) // between STW operations. This can be useful in some of the 1-0 // synchronization schemes. intptr_t addrBits = cast_from_oop<intptr_t>(obj) >> 3; value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom; } elseif (hashCode == 2) { value = 1; // for sensitivity testing } elseif (hashCode == 3) { value = ++GVars.hcSequence; } elseif (hashCode == 4) { value = cast_from_oop<intptr_t>(obj); } else { // Marsaglia's xor-shift scheme with thread-specific state // This is probably the best overall implementation -- we'll // likely make this the default in future releases. unsigned t = Self->_hashStateX; t ^= (t << 11); Self->_hashStateX = Self->_hashStateY; Self->_hashStateY = Self->_hashStateZ; Self->_hashStateZ = Self->_hashStateW; unsigned v = Self->_hashStateW; v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)); Self->_hashStateW = v; value = v; }
value &= markOopDesc::hash_mask; if (value == 0) value = 0xBAD; assert(value != markOopDesc::no_hash, "invariant"); TEVENT(hashCode: GENERATE); return value; }
intptr_tObjectSynchronizer::FastHashCode(Thread * Self, oop obj){ if (UseBiasedLocking) { // NOTE: many places throughout the JVM do not expect a safepoint // to be taken here, in particular most operations on perm gen // objects. However, we only ever bias Java instances and all of // the call sites of identity_hash that might revoke biases have // been checked to make sure they can handle a safepoint. The // added check of the bias pattern is to avoid useless calls to // thread-local storage. if (obj->mark()->has_bias_pattern()) { // Handle for oop obj in case of STW safepoint Handle hobj(Self, obj); // Relaxing assertion for bug 6320749. assert(Universe::verify_in_progress() || !SafepointSynchronize::is_at_safepoint(), "biases should not be seen by VM thread here"); BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current()); obj = hobj(); assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); } }
// object should remain ineligible for biased locking assert(!mark->has_bias_pattern(), "invariant");
if (mark->is_neutral()) { hash = mark->hash(); // this is a normal header if (hash) { // if it has hash, just return it return hash; } hash = get_next_hash(Self, obj); // allocate a new hash code temp = mark->copy_set_hash(hash); // merge the hash code into header // use (machine word version) atomic operation to install the hash test = obj->cas_set_mark(temp, mark); if (test == mark) { return hash; } // If atomic operation failed, we must inflate the header // into heavy weight monitor. We could add more code here // for fast path, but it does not worth the complexity. } elseif (mark->has_monitor()) { monitor = mark->monitor(); temp = monitor->header(); assert(temp->is_neutral(), "invariant"); hash = temp->hash(); if (hash) { return hash; } // Skip to the following code to reduce code size } elseif (Self->is_lock_owned((address)mark->locker())) { temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned assert(temp->is_neutral(), "invariant"); hash = temp->hash(); // by current thread, check if the displaced if (hash) { // header contains hash code return hash; } // WARNING: // The displaced header is strictly immutable. // It can NOT be changed in ANY cases. So we have // to inflate the header into heavyweight monitor // even the current thread owns the lock. The reason // is the BasicLock (stack slot) will be asynchronously // read by other threads during the inflate() function. // Any change to stack may not propagate to other threads // correctly. }
// Inflate the monitor to set hash code monitor = ObjectSynchronizer::inflate(Self, obj, inflate_cause_hash_code); // Load displaced header and check it has hash code mark = monitor->header(); assert(mark->is_neutral(), "invariant"); hash = mark->hash(); if (hash == 0) { hash = get_next_hash(Self, obj); temp = mark->copy_set_hash(hash); // merge hash code into header assert(temp->is_neutral(), "invariant"); test = Atomic::cmpxchg(temp, monitor->header_addr(), mark); if (test != mark) { // The only update to the header in the monitor (outside GC) // is install the hash code. If someone add new usage of // displaced header, please update this code hash = test->hash(); assert(test->is_neutral(), "invariant"); assert(hash != 0, "Trivial unexpected object/monitor header usage."); } } // We finally get the hash return hash; }
