On 2021-02-02 Brett Okken wrote:
> Thus far I have only tested on jdk 11 64bit windows, but the fairly
> clear winner is:
>
> public void update(byte[] buf, int off, int len) {
> final int end = off + len;
> int i=off;
> if (len > 3) {
> switch (i & 3) {
> case 3:
> crc = TABLE[0][(buf[i++] ^ (int) crc) & 0xFF] ^
> (crc >>> 8);
> case 2:
> crc = TABLE[0][(buf[i++] ^ (int) crc) & 0xFF] ^
> (crc >>> 8);
> case 1:
> crc = TABLE[0][(buf[i++] ^ (int) crc) & 0xFF] ^
> (crc >>> 8);
> }
To ensure (i & 3) == 0 when entering the main loop, the case-labels
should be 1-2-3, not 3-2-1. This may have messed up your tests. :-(
With a very quick test I didn't see much difference if I changed the
case-label order.
On 2021-02-02 Brett Okken wrote:
> I tested jdk 15 64bit and jdk 11 32bit, client and server and the
> above implementation is consistently quite good.
> The alternate in running does not do the leading alignment. This
> version is really close in 64 bit testing and slightly faster for 32
> bit. The differences are pretty small, and both are noticeably better
> than my original proposal (and all 3 are significantly faster than
> current). I think I would lead towards the simplicity of not doing the
> leading alignment, but I do not have a strong opinion.
Let's go with the simpler option.
> switch (len & 3) {
> case 3:
> crc = TABLE[0][(buf[i++] ^ (int) crc) & 0xFF] ^
> (crc >>> 8);
I suppose this should use the same (faster) array indexing style as the
main loop:
crc = TABLE[0][(buf[off++] & 0xFF) ^ ((int)crc & 0xFF)]
^ (crc >>> 8);
Also, does it really help to unroll the loop? With 8191-byte buffers I
see no significant difference (in a quick not-very-accurate test) if
the switch-statement is replaced with a while-loop.
With these two changes the code becomes functionally identical to the
version I posted with the name "Modified slicing-by-4". Is that an OK
version to commit?
Is the following fine to you as the file header? Your email address can
be omitted if you prefer that. I will mention in the commit message
that you adapted the code from XZ Utils and benchmarked it.
/*
* CRC64
*
* Authors: Brett Okken <EMAIL>
* Lasse Collin <EMAIL>
*
* This file has been put into the public domain.
* You can do whatever you want with this file.
*/
Thanks!
--
Lasse Collin | IRC: Larhzu @ IRCnet & Freenode
