On Thu, Oct 23, 2014 at 12:10 AM, Bernd Eckenfels <e...@zusammenkunft.net>
wrote:
> Hello,
>
> just a question in the default impl:
>
> + } else {
> + byte[] b = new byte[rem];
> + buffer.get(b);
> + update(b, 0, b.length);
> + }
>
> would it be a good idea to actually put a ceiling on the size of the
> array which is processed at once?
This is an excellent catch.
Should not be too large, probably 4k or so.
Stanimir
> Am Tue, 21 Oct 2014 10:28:50 -0700
> schrieb Staffan Friberg <staffan.frib...@oracle.com>:
>
> > Hi Peter,
> >
> > Thanks for the comments..
> > >
> > > 217 if (Unsafe.ADDRESS_SIZE == 4) {
> > > 218 // On 32 bit platforms read two ints
> > > instead of a single 64bit long
> > >
> > > When you're reading from byte[] using Unsafe (updateBytes), you
> > > have the option of reading 64bit values on 64bit platforms. When
> > > you're reading from DirectByteBuffer memory
> > > (updateDirectByteBuffer), you're only using 32bit reads.
> > I will add a comment in the code for this decision. The reason is
> > that read a long results in slightly worse performance in this case,
> > in updateBytes it is faster. I was able to get it to run slightly
> > faster by working directly with the address instead of always adding
> > address + off, but this makes things worse in the 32bit case since
> > all calculation will now be using long variables. So using the getInt
> > as in the current code feels like the best solution as it strikes the
> > best balance between 32 and 64bit. Below is how updateByteBuffer
> > looked with the rewrite I mentioned.
> >
> >
> > ong address = ((DirectBuffer) buffer).address();
> > crc = updateDirectByteBuffer(crc, address + pos, address + limit);
> >
> >
> > private static int updateDirectByteBuffer(int crc, long adr,
> > long end) {
> >
> > // Do only byte reads for arrays so short they can't be
> > aligned if (end - adr >= 8) {
> >
> > // align on 8 bytes
> > int alignLength = (8 - (int) (adr & 0x7)) & 0x7;
> > for (long alignEnd = adr + alignLength; adr < alignEnd;
> > adr++) { crc = (crc >>> 8)
> > ^ byteTable[(crc ^ UNSAFE.getByte(adr)) &
> > 0xFF]; }
> >
> > if (ByteOrder.nativeOrder() == ByteOrder.BIG_ENDIAN) {
> > crc = Integer.reverseBytes(crc);
> > }
> >
> > // slicing-by-8
> > for (; adr < (end - Long.BYTES); adr += Long.BYTES) {
> > int firstHalf;
> > int secondHalf;
> > if (Unsafe.ADDRESS_SIZE == 4) {
> > // On 32 bit platforms read two ints instead of
> > a single 64bit long firstHalf = UNSAFE.getInt(adr);
> > secondHalf = UNSAFE.getInt(adr + Integer.BYTES);
> > } else {
> > long value = UNSAFE.getLong(adr);
> > if (ByteOrder.nativeOrder() ==
> > ByteOrder.LITTLE_ENDIAN) { firstHalf = (int) value;
> > secondHalf = (int) (value >>> 32);
> > } else { // ByteOrder.BIG_ENDIAN
> > firstHalf = (int) (value >>> 32);
> > secondHalf = (int) value;
> > }
> > }
> > crc ^= firstHalf;
> > if (ByteOrder.nativeOrder() ==
> > ByteOrder.LITTLE_ENDIAN) { crc = byteTable7[crc & 0xFF]
> > ^ byteTable6[(crc >>> 8) & 0xFF]
> > ^ byteTable5[(crc >>> 16) & 0xFF]
> > ^ byteTable4[crc >>> 24]
> > ^ byteTable3[secondHalf & 0xFF]
> > ^ byteTable2[(secondHalf >>> 8) & 0xFF]
> > ^ byteTable1[(secondHalf >>> 16) & 0xFF]
> > ^ byteTable0[secondHalf >>> 24];
> > } else { // ByteOrder.BIG_ENDIAN
> > crc = byteTable0[secondHalf & 0xFF]
> > ^ byteTable1[(secondHalf >>> 8) & 0xFF]
> > ^ byteTable2[(secondHalf >>> 16) & 0xFF]
> > ^ byteTable3[secondHalf >>> 24]
> > ^ byteTable4[crc & 0xFF]
> > ^ byteTable5[(crc >>> 8) & 0xFF]
> > ^ byteTable6[(crc >>> 16) & 0xFF]
> > ^ byteTable7[crc >>> 24];
> > }
> > }
> >
> > if (ByteOrder.nativeOrder() == ByteOrder.BIG_ENDIAN) {
> > crc = Integer.reverseBytes(crc);
> > }
> > }
> >
> > // Tail
> > for (; adr < end; adr++) {
> > crc = (crc >>> 8)
> > ^ byteTable[(crc ^ UNSAFE.getByte(adr)) & 0xFF];
> > }
> >
> > return crc;
> > }
> >
> >
> > >
> > > Also, in updateBytes, the usage of
> > > Unsafe.ARRAY_INT_INDEX_SCALE/ARRAY_LONG_INDEX_SCALE to index a byte
> > > array sounds a little scary. To be ultra portable you could check
> > > that ARRAY_BYTE_INDEX_SCALE == 1 first and refuse to use Unsafe for
> > > byte arrays if it is not 1. Then use Integer.BYTES/Long.BYTES to
> > > manipulate 'offsets' instead. In updateDirectByteBuffer it would be
> > > more appropriate to use Integer.BYTES/Long.BYTES too.
> > Good idea. Added a check in the initial if statement and it will get
> > automatically optimized away.
> >
> > > 225 firstHalf = (int) (value &
> > > 0xFFFFFFFF); 226 secondHalf = (int) (value
> > > >>> 32); 227 } else { // ByteOrder.BIG_ENDIAN
> > > 228 firstHalf = (int) (value >>> 32);
> > > 229 secondHalf = (int) (value &
> > > 0xFFFFFFFF);
> > >
> > > firstHalf = (int) value; // this is equivalent for line 225
> > > secondHalf = (int) value; // this is equivalent for line 229
> > Done.
> >
> > Here is the latest webrev,
> > http://cr.openjdk.java.net/~sfriberg/JDK-6321472/webrev.03
> >
> > Cheers,
> > Staffan
>
>
