RFR JDK-6321472: Add CRC-32C API
Peter Levart
peter.levart at gmail.com
Wed Oct 22 05:56:47 UTC 2014
On 10/21/2014 11:34 PM, Staffan Friberg wrote:
> I believe it must be <, as it is in the tail loop as well, because end
> is (off+len or limit) so end is exclusive, similar to
> subString(begin,end).
>
> Makes sense?
>
> //Staffan
>
> On 10/21/2014 01:46 PM, Peter Levart wrote:
>> Sorry Staffan, another nit...
>>
>> 212 for (; off < (end - Long.BYTES); off += Long.BYTES) {
>> and
>>
>> 286 for (; off < (end - Long.BYTES); off += Long.BYTES) {
>>
>>
>> I think you could change "off < (end - Long.BYTES)" to "off <= (end -
>> Long.BYTES)". Am I right?
The tail loop has < :
319 for (; off < end; off++) {
...but it could be written as:
for (; off <= (end - Byte.BYTES); off += Byte.BYTES) { ...
;-)
In other words, when off == end - Long.BYTES, you can still read
Long.BYTES starting at 'off' .
Regards, Peter
>>
>> Regards, Peter
>>
>>
>> On 10/21/2014 10:30 PM, Peter Levart wrote:
>>>
>>> On 10/21/2014 08:49 PM, Staffan Friberg wrote:
>>>> Hi,
>>>>
>>>> Got an offline comment that the package.html should be update as
>>>> well to cover CRC-32C.
>>>>
>>>> Otherwise there are no code changes in this new webrev.
>>>>
>>>> http://cr.openjdk.java.net/~sfriberg/JDK-6321472/webrev.04
>>>
>>>
>>> Hi Staffan,
>>>
>>> 198 if (end - off >= 8 && Unsafe.ARRAY_BOOLEAN_INDEX_SCALE
>>> == 1) {
>>>
>>> ARRAY_BOOLEAN_INDEX_SCALE -> ARRAY_BYTE_INDEX_SCALE
>>>
>>>
>>> Otherwise looks good now.
>>>
>>> Regards, Peter
>>>
>>> P.S.
>>>
>>> I think (by looking at DirectByteBuffer.asIntBuffer()
>>> implementation) that when doing 32 bit (4 byte) reads using Unsafe,
>>> the address only has to be aligned to 4 bytes (8 is necessary
>>> alignment for 64 bit reads). So updateDirectByteBuffer could make
>>> this alignment on 4 bytes as it's only using 32 bit reads (with
>>> additional check on ADDRESS_SIZE, you could do that for updateBytes
>>> too).
>>>
>>> You don't get much out of it, so you decide if it's worth complication.
>>>
>>>
>>>>
>>>> //Staffan
>>>>
>>>> On 10/21/2014 10:28 AM, Staffan Friberg wrote:
>>>>> 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
>>>>
>>>
>>
>
>
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