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

[Maurizio Cimadamore]

Addressed all comments:

http://cr.openjdk.java.net/~mcimadamore/panama/8237082_v3

I've also added a new benchmark (LoopOverNew), as well as fixed the JNI 
code which went out of sync after the package renaming.

Maurizio

On 14/01/2020 15:06, Maurizio Cimadamore wrote:
> 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
>
>