Acquire/Release vs Volatile in VarHandle

[Gianluca Stivan]

Hi everyone,

apologies if this is not the correct mailing list, it seemed the most
relevant :).

I've been experimenting with the new VarHandle APIs and especially the
Acquire/Release and Opaque modes. To my understanding they should map
pretty much 1-1 to the memory_order_{acquire,release} and
memory_order_relaxed on C++11.

As i was benchmarking, I didn't notice any significant change in
performance from relaxing my memory constraints from Volatile to a
Acquire/Release. I am using ARMv7, as it's hardware with a weaker memory
model.

I am just a beginner in this topic, but I started digging into it and
noticed that it appears that all the calls to Acquire/Release just become
calls to Volatile.

// X-VarHandle.java.template
@ForceInline
static $type$ getAcquire(FieldInstanceReadOnly handle, Object holder) {
  return
UNSAFE.get$Type$Acquire(Objects.requireNonNull(handle.receiverType.cast(holder)),
handle.fieldOffset);
}

// Unsafe
@HotSpotIntrinsicCandidate
public final Object getObjectAcquire(Object o, long offset) {
  return getObjectVolatile(o, offset);
}

Am I correct in that? Why is that?

I then thought, if I can't lower my constraints using the VarHandle APIs,
maybe I could use fences. But again the results are similar. After some
more digging, it appears that release/acquire fences compile to a full
fence (at least on linux_arm).

// orderAccess_linux_arm.inline.hpp
inline void OrderAccess::acquire()    { dmb_ld(); }
inline void OrderAccess::release()    { dmb_sy(); }
inline void OrderAccess::fence()      { dmb_sy(); }

`dmb_ld()` is just `dbm_sy()` unless architecture is AARCH64 (which is ARM
>= v8, as I understand)

As I mentioned, I'm new to all of this, but am I correct in understanding
that there seems to be no way to actually get true release/acquire/relaxed
behavior (as you would in C++) on the current build of JDK9?
Similar algorithms on C++ do show some differences in performance when I
relax memory constraints from seq_cst to a mix of relaxed and
acquire/release.

Thank you very much for taking the time to read this!

Best regards,

Gianluca