[9] RFR(S): 8183103: Post loop vectorization produces incorrect results

[Tobias Hartmann]

Hi,

please review the following patch:
https://bugs.openjdk.java.net/browse/JDK-8183103
http://cr.openjdk.java.net/~thartmann/8183103/webrev.00/

Post loop vectorization (-XX:+PostLoopMultiversioning) produces incorrect results on machines that have AVX-512 (-XX:UseAVX=3) support. Please find my detailed analysis in the bug comments.

Taking into account that we are very late in the JDK 9 release cycle, a full fix being very complex and requiring testing on AVX-512 supporting platforms and also due to other problems with AVX=3 in the past [1], we decided to make PostLoopMultiversioning and UseAVX=3 experimental in JDK 9. It will be disabled by default but still available if the user sets -XX:+UnlockExperimentalVMOptions on the command line.

I filed JDK-8183390 to investigate a full fix in JDK 10 (or a JDK 9 update release).

Thanks,
Tobias

[1]
https://bugs.openjdk.java.net/browse/JDK-8178800
https://bugs.openjdk.java.net/browse/JDK-8178033
https://bugs.openjdk.java.net/browse/JDK-8159016