Re: What does volatile guarantee?

From:
Eric Sosman <esosman@ieee-dot-org.invalid>
Newsgroups:
comp.lang.java.programmer
Date:
Mon, 15 Feb 2010 10:35:02 -0500
Message-ID:
<hlbpjc$jpj$1@news.eternal-september.org>
On 2/14/2010 4:30 PM, Andreas Leitgeb wrote:

Lew <noone@lewscanon.com> quoted:

Eric Sosman wrote in this thread on 2/12:

Before anybody whines^H^H^H^H^H^Hsuggests that making +=
atomic would be easy, let him ponder
volatile int a,b,c,...,z;
a += b += c+= ... += z;


I'm not whining (nor suggesting) for += nor ++ being atomic,
but that particular argument does not extend to the "++"
operator.

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

Just curious.


     Could be (or have been) done, I guess. Nowadays most
machines have compare-and-swap or something of the kind, and
these can be repeated in a loop until successful:

    loop: load r0,x
          load r1,r0
          inc r1
          cas r0,r1,x // if x==r0, set x=r1 and r1==x
          cmp r0,r1 // did the swap happen?
          jneq loop // failed: retry

Such instructions tend to be fairly expensive, though, since
they need to do things like flush a CPU's store buffers and
maybe bypass caches to go all the way to (s-l-o-w) RAM. The
expense isn't fatal in and of itself (`synchronized' needs to
do similar things) -- but from a language perspective it means
you can no longer say "++ is shorthand for +=1" unless you also
promise atomicity for +=, and then comes the slippery slope.

     Atomic x++ when x is Double.NaN could be *really* fun ...

--
Eric Sosman
esosman@ieee-dot-org.invalid

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