Bad combination of:
-XX:+PostLoopMultiversioning -XX:-DoReserveCopyInSuperWord

Tested with AVX512.

Without PostLoopMultiversioning we would not create vector mask.
With DoReserveCopyInSuperWord we would just bail out of SuperWord::output.

I see that PostLoopMultiversioning is generally not very stable, given these issues, I guess that is why it is an experimental flag: JDK-8304774 JDK-8301904 JDK-8301657

JDK17 SIGSEGV with this:
java -XX:CompileCommand=compileonly,Test::test -Xbatch -XX:+UnlockExperimentalVMOptions -XX:+PostLoopMultiversioning -XX:-DoReserveCopyInSuperWord Test.java


Master hits assert with this:
./java -XX:CompileCommand=compileonly,Test::test -XX:+TraceNewVectors -XX:+TraceSuperWord -XX:-Verbose -Xbatch -XX:+TraceLoopOpts -XX:+UnlockExperimentalVMOptions -XX:+PostLoopMultiversioning -XX:-DoReserveCopyInSuperWord Test.java


# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/oracle-work/jdk-fork2/open/src/hotspot/share/opto/superword.cpp:2708), pid=1527892, tid=1527905
# assert(vmask != nullptr) failed: vector mask should be generated
#
# JRE version: Java(TM) SE Runtime Environment (21.0) (slowdebug build 21-internal-LTS-2023-05-24-0919094.emanuel...)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (slowdebug 21-internal-LTS-2023-05-24-0919094.emanuel..., mixed mode, tiered, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x1472d09] SuperWord::output()+0x689