[concurrency-interest] AtomicXXX.lazySet and happens-before reasoning

Ruslan Cheremin cheremin at gmail.com
Mon Oct 10 09:50:30 EDT 2011

> I'm aware that the type of instruction and whether a fence is a no-op is
> arch specific, but I was curious how the compilers (c1/c2) make decisions
> around these things,  such as if they track pairs of read/write of same
> volatile across method bounds ( I believe they don't at the moment).

As far, as I understand, JIT does not need to track pairs -- since
StoreLoad is required to
keep volatiles access visible in same order for any thread (in
synchronization order), so
you can't remove it anyway.

I totally agree what it'll be very helpfull and interesting to see
some kind of "improved
JSR-133 cookbook" with slightely more detailed description about
ordering fences
issued for supporting varios kinds of JMM requirements, and their roles. Keeping
in mind forecoming Fences it can help to answer many arising questions like
"what Fence should I add to lazySet to get exactly volatile store and why".

> On Oct 10, 2011 9:14 AM, "Doug Lea" <dl at cs.oswego.edu> wrote:
>> On 10/10/11 08:56, Vitaly Davidovich wrote:
>>> I agree that the way StoreLoad is implemented ensures that volatile reads
>>> of
>>>  different memory location don't move before the store, but I think the
>>> JMM
>>> only talks about needing this for loads of same memory location (see
>>> Doug's
>>> cookbook web page description).
>> Additionally, the "synchronization order" (roughly, any trace of
>> all volatile accesses) is required to be a total order, which
>> is the main constraint that forces the Dekker example to work.
>> This is not made clear enough in cookbook, which should be improved.
>>> By the way, would be great if someone from the Hotspot runtime/compiler
>>> team
>>> could shed some light on how Hotspot handles these, with the caveat that
>>> people shouldn't necessarily base their code on it if it makes stronger
>>> guarantees than the JMM :).
>> The main visible cases are processor- not JVM- based, and
>> most are only visible in giving you more consistency than required
>> for non-volatile accesses. In general, x86 and sparc are
>> stronger than POWER and ARM, with a few others
>> (Azul, IA64) in the middle.
>> -Doug
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