Re: What does volatile guarantee?

Lew <>
Sun, 14 Feb 2010 21:35:49 -0500
Lew quoted or indirectly quoted someone who said :

Actually, it does. The particular argument is an extension of the observation
that += and ++ both represent a read followed by a write, with an increment or
addition in there somewhere. Those are inherently separate operations.

Roedy Green wrote:

I have forgotten what the machine was, but I once used a machine that

I believe it's called "the x86 family of processors".

Contains special support for atomic instructions (XCHG, CMPXCHG(8B),
XADD, and integer instructions which [sic] combine with the LOCK prefix)

Roedy Green wrote:

had an atomic ++ to memory machine instruction. You used it for
implementing locks rather that Test and Set. It is a plausible
question to ask if ++ is atomic.

Of course.

The post I answered that Andreas Leitgeb wrote:

So, what to say to those whining^H^H^H^H^H^H^Hsuggesting
making just "++" atomic?

wasn't about how to answer a question whether ++ is atomic, but a whine that
it isn't.

To a simple question about whether ++ in Java is atomic, a simple answer is,
"No, but you can use 'AtomicInteger##getAndIncrement()', et al., or

I would not be astounded to learn that the 'AtomicInteger' method is
implemented in terms of an atomic instruction where available (e.g., on x86

... the JVM improvements in JDK 5.0, ... exposed (to the class
libraries, but not to user classes) an interface to access
hardware-level synchronization primitives. The atomic variable
classes, and other classes in java.util.concurrent, in turn[,]
expose these features to user classes.


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