We have two versions after private discussion with Aleksey.
BigInteger getRadixConversionCache(int radix, int exponent) {
BigInteger[][] pc = powerCache; // volatile read
BigInteger[] cacheLine = pc[radix];
if (exponent < cacheLine.length)
return cacheLine[exponent];
int oldSize = cacheLine.length;
cacheLine = Arrays.copyOf(cacheLine, exponent + 1);
for (int i = oldSize; i < exponent + 1; i++)
cacheLine[i] = cacheLine[i - 1].square();
pc = Arrays.copyOf(powerCache);
pc[radix] = cacheLine;
powerCache = pc; // volatile write, publish
return cacheLine[exponent];
}
BigInteger getRadixConversionCache(int radix, int exponent) {
BigInteger[][] pc = powerCache; // volatile read
BigInteger[] cacheLine = pc[radix];
if (exponent < cacheLine.length)
return cacheLine[exponent];
int oldSize = cacheLine.length;
cacheLine = Arrays.copyOf(cacheLine, exponent + 1);
for (int i = oldSize; i < exponent + 1; i++)
cacheLine[i] = cacheLine[i - 1].square();
synchronized (BigInteger.class) {
if (powerCache[radix].length < cacheLine.length) {
pc = Arrays.copyOf(powerCache);
pc[radix] = cacheLine;
powerCache = pc; // volatile write, publish
}
}
return powerCache[radix][exponent];
}
The first version is simpler but sometimes a thread may discard powers with
larger exponents computed by another thread.
A thread may discard only powers computed by itself In the second version.
On Tue, Jun 25, 2013 at 11:53 PM, Aleksey Shipilev <
aleksey.shipi...@oracle.com> wrote:
> On 06/25/2013 11:36 PM, Dmitry Nadezhin wrote:
> > What about such code ?
> >
> > BigInteger getRadixConversionCache(int radix, int exponent) {
> > BigInteger retVal = null;
> > BigInteger[][] pc = powerCache; // volatile read
> > BigInteger[] cacheLine = pc[radix];
> > int oldSize = cacheLine.length;
> > if (exponent >= oldSize) {
> > cacheLine = Arrays.copyOf(cacheLine, exponent + 1);
> > for (int i = oldSize; i <= exponent; i++) {
> > retVal = cacheLine[i - 1].square();
> > cacheLine[i] = retVal;
> > }
> > synchronized (pc) {
> > if (pc[radix].length < cacheLine.length) {
> > pc[radix] = cacheLine;
> > powerCache = pc; // volatile write, publish
> > }
> > }
> > } else {
> > retVal = cacheLine[exponent];
> > }
> > return retVal;
> > }
>
> Still the same race. The reader thread sees the full-length cacheLine
> early, but the elements are not yet there.
>
> > Is dummy volatile write necessary inside synchronized block ?
> > powerCache = pc; // volatile write, publish
> >
>
> Yes, since you need to have the matched synchronized(pc) otherwise to
> produce the pairwise "acquire".
>
> -Aleksey.
>
>
