Simply running java -XX:-UseLoopSafepoints already crashes the VM:

# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/oracle/jdk/open/src/hotspot/share/opto/cfgnode.cpp:1893), pid=1368883, tid=1368900
# assert(r->in(0) == __null || !r->in(0)->is_Root()) failed: not a specially hidden merge
#
# JRE version: Java(TM) SE Runtime Environment (17.0) (fastdebug build 17-internal+0-LTS-2021-05-04-1333554.tobias...)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 17-internal+0-LTS-2021-05-04-1333554.tobias..., mixed mode, tiered, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x8b1d54] PhiNode::Ideal(PhaseGVN*, bool)+0x6e4

Running more sophisticated testing fails with all kinds of different asserts and crashes.

JDK-6232281 made an attempt of fixing some simple cases.