Optimize CRC32 calculation on arm64

[zorrorffm]

ARM64 provides crc32 instructions for accelerating crc32 calculation.
This patch is optimization for linux under aarch64

The comparision of performance is as below

old
crc32c : 5.670 micros/op; 688.9 MB/s (4K per op)

new
crc32c : 0.451 micros/op; 8663.4.7 MB/s (4K per op)

Change-Id: I51d25ca19688fc95a57b84ee0c1493c18a288087

The name I signed CLA with is [email protected]
The issue number is #478

[pwnall]

@zorrorffm Thank you very much for the contribution!

Due to some infrastructure issues, I won't be able to act on this very quickly. That being said, I am very grateful for your PR, and I look forward to improving the ARM performance.

[zorrorffm]

@pwnall Thanks you.

If you need ARM machine to run some tests, I would be pleased to do that.

[pwnall]

@zorrorffm I ran tests on a Google Pixel C tablet, and the optimized version seems to be 4-10x faster. If this is reasonably representative hardware, I don't think I'll need another machine. I'll take you up on your offer if it turns out I really need a machine that can easily run a GNU/Linux system.

[pwnall]

@zorrorffm The LevelDB work hasn't happened yet, but I used your code in https://github.com/google/crc32c/blob/master/src/crc32c_arm64.cc -- thank you very much!

[pwnall]

@zorrorffm We deployed the version of this patch at https://github.com/google/crc32c/blob/master/src/crc32c_arm64.cc in Chrome, and we got crashes when trying to execute the vmull_p64 instruction on MSM8916 boards.

I have a hunch that we should also be checking that the flags returned by hwcap include HWCAP_PMULL, which is (1 << 4), before enabling the AArch64 accelerated code. Is this correct?

[pwnall]

@zorrorffm On the issue above, can you please take a look and comment on google/crc32c#6?

[zorrorffm]

@pwnall Yes, your fix is correct. Thank you for your findings.

[zorrorffm]

Update the patch with detection of CPU capabilities. Acceleration is available only if pmull and crc32c instructions are supported

[cmumford]

@googlebot rescan

[cmumford]

Thx for the PR @zorrorffm. The optimized crc32c code was moved out into https://github.com/google/crc32c (which supports armv8-a+crc+crypto) in 5c39524.