RFR: 8187472 - AARCH64: array_equals intrinsic doesn't use prefetch for large arrays

[Dmitrij Pochepko]

Hi all,

please review patch for JDK-8187472 - AARCH64: array_equals intrinsic 
doesn't use prefetch for large arrays

this patch adds logic with prefetch instruction usage, which is helping 
most of CPUs. It also introduce 2 ways of loading data: via loading into 
general-purpose registers and via SIMD registers(controlled by newly 
introduced vm key: UseSIMDForArrayEquals.

I've measured performance on few CPUs (ThunderX, ThunderX2(not yet 
released), Cortex A53, Cortex A73) and arranged defaults based on 
measurement results(so, SIMD is default for everything except ThunderX 
and A53. Also, prefetch instruction slows down A73 - I've disabled it 
there).


In general, this patch changes very short arrays handling(performing 
8-byte read instead of few smaller reads, using the fact of 8-byte 
alignment) and jumping into stub with large 64-byte read loop for larger 
arrays).

Measurements(measured array length 7,64,128,256,512,1024,100000. 
Improvement in %. 80% improvement means that new version is 80% faster, 
i.e. 5 times.):


ThunderX: 2%, -4%, 0%, 2%, 32%, 55%, 80%

ThunderX2: 0%, -3%, 17%, 19%, 29%, 31%, 47%

Cortex A53 at 533MHz: 8%, -1%, -2%, 4%, 6%, 5%, 3%

Cortex A73 at 903MHz: 8%, -3%, 0%, 7%, 8%, 9%, 8%

Note: medium sizes are a bit slower because of additional branch 
added(which checks size and jumps to stub).

Note2: Cortex CPUs were downclocked to have stable frequency because of 
overheat on device I was using(frequency was randomly jumping producing 
unstable results otherwise), so, results might be a bit different on 
devices with higher frequency.


webrev: http://cr.openjdk.java.net/~dpochepk/8187472/webrev.07/

CR: https://bugs.openjdk.java.net/browse/JDK-8187472

Testing: I've run all hotspot jtreg tests with and without patch on 
aarch64 and found no new failures

Benchmark: 
http://cr.openjdk.java.net/~dpochepk/8187472/webrev.07/ArrayEqualsBench.java


Thanks,

Dmitrij

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