RFR JDK-6321472: Add CRC-32C API

[Bernd Eckenfels]

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? Something below the typical G1
Humongous sinze or even something which keeps it in young (as those
arrays are known to be recycled immediatelly).

Gruss
Bernd


 Am Tue, 21 Oct 2014 10:28:50 -0700
schrieb Staffan Friberg <staffan.friberg at 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