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

[Dmitrij Pochepko]

Hi,

as part of JEP “Improve performance of String and Array operations on 
AArch64” I wanted to send out a pre-review for some of the improved 
intrinsics to get early feedback. This is the first in a row.

Please pre-review patch for 8187472 - “AARCH64: array_equals intrinsic 
doesn't use prefetch for large arrays” which improves large array 
handling (small arrays are unaffected).

In short, this patch uses large (64 byte) loop with prefetch instruction 
to handle large arrays, which is done in a stub. I can observe 
performance boost on systems without h/w prefetcher up to x6. System 
with hardware prefetching (Cortex A53 and some very modern ones) also 
benefit from this patch (15% improvement).

I've tried a number of different versions (attached to JDK-8187472) with 
different load instructions (ldr/ldp/<simd>), slightly different code 
shapes, different data dependencies across registers, alignments, e.t.c. 
Version presented in webrev (version 2.6d from JDK-8187472 attachments) 
is the simplest from the fast ones (as measured on 3 systems available 
for testing).

I've used this simple benchmark to measure performance: 
http://cr.openjdk.java.net/~dpochepk/8187472/ArrayEqualsBench.java

Chart for ThunderX: 
http://cr.openjdk.java.net/~dpochepk/8187472/ThunderX.png

Chart for Cortex A53(R-Pi): 
http://cr.openjdk.java.net/~dpochepk/8187472/R-Pi.png

Raw numbers for ThunderX: 
http://cr.openjdk.java.net/~dpochepk/8187472/ThunderX.results.txt

Raw numbers for R-Pi: 
http://cr.openjdk.java.net/~dpochepk/8187472/R-Pi.results.txt

webrev: http://cr.openjdk.java.net/~dpochepk/8187472/webrev.01/

Testing: I've run existing jtreg test 
(java/util/Arrays/ArraysEqCmpTest.java) in both Xmixed and Xcomp and 
found no regressions.

Any additional numbers on other systems are welcome, as well as early 
feedback on the code.

Thanks,

Dmitrij