On 10/17/2014 01:46 AM, Peter Levart wrote:
On 10/17/2014 03:42 AM, Staffan Friberg wrote:
Hi,
This RFE adds a CRC-32C class. It implements Checksum so it will have
the same API CRC-32, but use a different polynomial when calculating
the CRC checksum.
CRC-32C implementation uses slicing-by-8 to achieve high performance
when calculating the CRC value.
A part from adding the new class, java.util.zip.CRC32C, I have also
added two default methods to Checksum. These are methods that were
added to Adler32 and CRC32 in JDK 8 but before default methods were
added, which was why they were only added to the implementors and not
the interface.
Bug: https://bugs.openjdk.java.net/browse/JDK-6321472
Webrev: http://cr.openjdk.java.net/~sfriberg/JDK-6321472/webrev.00
I have started a CCC request for the changes, but was asked to get
feedback from the core libs group before finalizing the request in
case there are any API or Javadoc changes suggested.
Thanks,
Staffan
Hi Staffan,
I can see CRC32C.reflect(int) method reverses the bits in 32 bit int
value. You could use Integer.reverse(int) instead.
The CRC32C.swap32(int) method is (almost) exactly the same as
Integer.reverseBytes(int) and equivalent.
I wonder if handling ByteBuffer could be simplified. You could
leverage it's own byte order manipulation by temporarily setting (and
resetting afterwards) ByteBuffer.order() and then use
ByteBuffer.getInt() to extract 32 bits at a time for your algorithm.
This could get you the optimal variant of algorithm for both kinds of
buffers (direct or byte[] based). Perhaps even the byte[] based
variant of algorithm could be implemented by wrapping the array with
ByteBuffer, passing it to common private method, and relying on the
escape analysis of Hotspot to allocate the HeapByteBuffer wrapper
object on stack.
Regards, Peter
Hi Peter,
Thanks for reviewing.
I have switched to the Integer methods. Was looking through that API but
I was too stuck with the reflect and swap names so I missed the reverse
methods... :)
As Vitaly noted in his email the wrapped case runs much slower. Going
through the generated code it looks like the getInt method actually read
four bytes and then builds and int from them, unless we have some
intrinsic replacing that code.
Bits.java
static int getIntL(long a) {
return makeInt(_get(a + 3),
_get(a + 2),
_get(a + 1),
_get(a ));
}
static private int makeInt(byte b3, byte b2, byte b1, byte b0) {
return (((b3 ) << 24) |
((b2 & 0xff) << 16) |
((b1 & 0xff) << 8) |
((b0 & 0xff) ));
}
It looks like the same holds true for DirectByteBuffers unless you are
on x86 which supports unaligned reads. So I think aligning and using
Unsafe is the best option here for performance.
DirectByteBuffer.java
private int getInt(long a) {
if (unaligned) {
int x = unsafe.getInt(a);
return (nativeByteOrder ? x : Bits.swap(x));
}
return Bits.getInt(a, bigEndian);
}
Bits.java
static boolean unaligned() {
if (unalignedKnown)
return unaligned;
String arch = AccessController.doPrivileged(
new sun.security.action.GetPropertyAction("os.arch"));
unaligned = arch.equals("i386") || arch.equals("x86")
|| arch.equals("amd64") || arch.equals("x86_64");
unalignedKnown = true;
return unaligned;
}
Regards,
Staffan