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8234544: ObjectSynchronizer::FastHashCode() cleanups from Async Monitor Deflation project

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Reviewed-by: dholmes
This commit is contained in:
Daniel D. Daugherty 2019-11-22 09:06:35 -05:00
parent 8333ea85fa
commit 008bdefad4
1 changed files with 31 additions and 26 deletions
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57
src/hotspot/share/runtime/synchronizer.cpp
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@ -694,35 +694,39 @@ intptr_t ObjectSynchronizer::FastHashCode(Thread* self, oop obj) {
// object should remain ineligible for biased locking // object should remain ineligible for biased locking
assert(!mark.has_bias_pattern(), "invariant"); assert(!mark.has_bias_pattern(), "invariant");
if (mark.is_neutral()) { if (mark.is_neutral()) { // if this is a normal header
hash = mark.hash(); // this is a normal header hash = mark.hash();
if (hash != 0) { // if it has hash, just return it if (hash != 0) { // if it has a hash, just return it
return hash; return hash;
} }
hash = get_next_hash(self, obj); // allocate a new hash code hash = get_next_hash(self, obj); // get a new hash
temp = mark.copy_set_hash(hash); // merge the hash code into header temp = mark.copy_set_hash(hash); // merge the hash into header
// use (machine word version) atomic operation to install the hash // try to install the hash
test = obj->cas_set_mark(temp, mark); test = obj->cas_set_mark(temp, mark);
if (test == mark) { if (test == mark) { // if the hash was installed, return it
return hash; return hash;
} }
// If atomic operation failed, we must inflate the header // Failed to install the hash. It could be that another thread
// into heavy weight monitor. We could add more code here // installed the hash just before our attempt or inflation has
// for fast path, but it does not worth the complexity. // occurred or... so we fall thru to inflate the monitor for
// stability and then install the hash.
} else if (mark.has_monitor()) { } else if (mark.has_monitor()) {
monitor = mark.monitor(); monitor = mark.monitor();
temp = monitor->header(); temp = monitor->header();
assert(temp.is_neutral(), "invariant: header=" INTPTR_FORMAT, temp.value()); assert(temp.is_neutral(), "invariant: header=" INTPTR_FORMAT, temp.value());
hash = temp.hash(); hash = temp.hash();
if (hash != 0) { if (hash != 0) { // if it has a hash, just return it
return hash; return hash;
} }
// Skip to the following code to reduce code size // Fall thru so we only have one place that installs the hash in
// the ObjectMonitor.
} else if (self->is_lock_owned((address)mark.locker())) { } else if (self->is_lock_owned((address)mark.locker())) {
temp = mark.displaced_mark_helper(); // this is a lightweight monitor owned // This is a stack lock owned by the calling thread so fetch the
// displaced markWord from the BasicLock on the stack.
temp = mark.displaced_mark_helper();
assert(temp.is_neutral(), "invariant: header=" INTPTR_FORMAT, temp.value()); assert(temp.is_neutral(), "invariant: header=" INTPTR_FORMAT, temp.value());
hash = temp.hash(); // by current thread, check if the displaced hash = temp.hash();
if (hash != 0) { // header contains hash code if (hash != 0) { // if it has a hash, just return it
return hash; return hash;
} }
// WARNING: // WARNING:
@ -735,29 +739,30 @@ intptr_t ObjectSynchronizer::FastHashCode(Thread* self, oop obj) {
// may not propagate to other threads correctly. // may not propagate to other threads correctly.
} }
// Inflate the monitor to set hash code // Inflate the monitor to set the hash.
monitor = inflate(self, obj, inflate_cause_hash_code); monitor = inflate(self, obj, inflate_cause_hash_code);
// Load displaced header and check it has hash code // Load ObjectMonitor's header/dmw field and see if it has a hash.
mark = monitor->header(); mark = monitor->header();
assert(mark.is_neutral(), "invariant: header=" INTPTR_FORMAT, mark.value()); assert(mark.is_neutral(), "invariant: header=" INTPTR_FORMAT, mark.value());
hash = mark.hash(); hash = mark.hash();
if (hash == 0) { if (hash == 0) { // if it does not have a hash
hash = get_next_hash(self, obj); hash = get_next_hash(self, obj); // get a new hash
temp = mark.copy_set_hash(hash); // merge hash code into header temp = mark.copy_set_hash(hash); // merge the hash into header
assert(temp.is_neutral(), "invariant: header=" INTPTR_FORMAT, temp.value()); assert(temp.is_neutral(), "invariant: header=" INTPTR_FORMAT, temp.value());
uintptr_t v = Atomic::cmpxchg(temp.value(), (volatile uintptr_t*)monitor->header_addr(), mark.value()); uintptr_t v = Atomic::cmpxchg(temp.value(), (volatile uintptr_t*)monitor->header_addr(), mark.value());
test = markWord(v); test = markWord(v);
if (test != mark) { if (test != mark) {
// The only non-deflation update to the ObjectMonitor's // The attempt to update the ObjectMonitor's header/dmw field
// header/dmw field is to merge in the hash code. If someone // did not work. This can happen if another thread managed to
// adds a new usage of the header/dmw field, please update // merge in the hash just before our cmpxchg().
// this code. // If we add any new usages of the header/dmw field, this code
// will need to be updated.
hash = test.hash(); hash = test.hash();
assert(test.is_neutral(), "invariant: header=" INTPTR_FORMAT, test.value()); assert(test.is_neutral(), "invariant: header=" INTPTR_FORMAT, test.value());
assert(hash != 0, "Trivial unexpected object/monitor header usage."); assert(hash != 0, "should only have lost the race to a thread that set a non-zero hash");
} }
} }
// We finally get the hash // We finally get the hash.
return hash; return hash;
} }