[foreign-memaccess] RFR 8237082: Workaround C2 limitations when working with long loops

[Maurizio Cimadamore]

Hi,
both C2 and Graal do not like for loops with long computations inside - 
in C2, the bound-check-elimination code (BCE) is extremely sensitive to 
which opcodes are used when putting together offsets in a linear 
expression [1]. So, whenever C2 sees a long opcode being used (e.g. 
LMUL, or LADD) the BCE logic bails out, which means that in such cases 
we get no bounds check hoisting.

The medium/long term solution is, obviously, to fix C2 [2], so that it 
works on long loops as well as with int loops - after all, this is where 
many of the new APIs we are designing are headed.

As a short term boost, I've put together a patch which classifies 
segments as either small or large; if a segment is small, then we can 
use some trickery to force int opcodes to be used in offset computations 
instead of their long counterparts. Doing so removes most of the 
performance bottlenecks associated with indexed var handle access, some 
of which were visible in the synthetic benchmarks (see 
LoopOverNonConstant [3]).

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8237082_v2/

Cheers
Maurizio

[1] - 
http://hg.openjdk.java.net/panama/dev/file/b94889c7e153/src/hotspot/share/opto/loopnode.cpp
[2] - https://bugs.openjdk.java.net/browse/JDK-8223051
[3] - 
http://hg.openjdk.java.net/panama/dev/file/fb3c9c52fdff/test/micro/org/openjdk/bench/jdk/incubator/foreign/LoopOverNonConstant.java