On 9/01/2013 9:04 PM, Aleksey Shipilev wrote:
Thanks, David.
On 01/09/2013 02:55 PM, David Holmes wrote:
Not sure where this is now but FYI you needed two CRs for this: one for
hotspot and one for the JDK. They have different target versions (hs25
vs 8) and depending on the path you use to integrate could end up in
different builds - hence distinct CRs for accurate tracking purposes.
Too late, since these are already committed? I can submit another CR if
we need that.
I'll fix it up - it is easier to do than explain. :) As it is, the fact
these both went in through hotspot repo at least means they appeared in
jdk8/jdk8 at the same time.
I have a concern that the Long versions of these methods may be used
directly without there being any check for supports_cx8
I actually have the question about this. What is the usual pattern for
using AtomicLong.VM_SUPPORTS_LONG_CAS? AtomicLong seems to use Unsafe
directly without the checks. AtomicLongFieldUpdater does the checks.
Something is fishy about this whole thing.
I had forgotten at what levels this operates too. As I think is now
clear(er) there is a Java level check (and fallback to locks) for the
AtomicLongFieldUpdater based on supports_cx8. Then there are further
checks of supports_cx8 in unsafe.cpp. Critically in
Unsafe_CompareAndSwapLong. (Still needed on some platforms)
Also note that Unsafe_get/setLongVolatile are also gated, unnecessarily,
on supports_cx8. We have to have atomic 64-bit read/write for direct
Java volatile accesses anyway. There's also an open CR to define
supports_atomic_long_ops so that supports_cx8 is only used for the CAS,
where it is needed, rather than simple read/write ops.
David
-----
I think this one tries to explain something, but I fail to grasp the intent:
/**
* Records whether the underlying JVM supports lockless
* compareAndSwap for longs. While the Unsafe.compareAndSwapLong
* method works in either case, some constructions should be
* handled at Java level to avoid locking user-visible locks.
*/
static final boolean VM_SUPPORTS_LONG_CAS = VMSupportsCS8();
Doug?
-Aleksey.
