javac Test.java
java -XX:-TieredCompilation -Xbatch -XX:CompileCommand=compileonly,*::test Test

# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/workspace/open/src/hotspot/share/opto/node.hpp:921), pid=1308573, tid=1308587
# assert(is_Loop()) failed: invalid node class: Region
#
# JRE version: Java(TM) SE Runtime Environment (23.0+10) (fastdebug build 23-ea+10-652)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 23-ea+10-652, mixed mode, sharing, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x12bceee] Node::as_Loop() const [clone .part.0]+0xe

[...]

Current CompileTask:
C2:95 1 % b Test::test @ 46 (79 bytes)

Stack: [0x00007f25a292b000,0x00007f25a2a2c000], sp=0x00007f25a2a26c70, free space=1007k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x12bceee] Node::as_Loop() const [clone .part.0]+0xe (node.hpp:921)
V [libjvm.so+0x12c1774] PhaseIdealLoop::remix_address_expressions(Node*)+0x7c4
V [libjvm.so+0x12c37be] PhaseIdealLoop::split_if_with_blocks_pre(Node*)+0xee
V [libjvm.so+0x12c86c7] PhaseIdealLoop::split_if_with_blocks(VectorSet&, Node_Stack&)+0x1d7
V [libjvm.so+0x12bb2fd] PhaseIdealLoop::build_and_optimize()+0xf2d
V [libjvm.so+0x9f66b8] PhaseIdealLoop::optimize(PhaseIterGVN&, LoopOptsMode)+0x388
V [libjvm.so+0x9f0fd3] Compile::Optimize()+0x4c3
V [libjvm.so+0x9f51a5] Compile::Compile(ciEnv*, ciMethod*, int, Options, DirectiveSet*)+0x1c35
V [libjvm.so+0x848ca5] C2Compiler::compile_method(ciEnv*, ciMethod*, int, bool, DirectiveSet*)+0x1d5
V [libjvm.so+0xa00e9c] CompileBroker::invoke_compiler_on_method(CompileTask*)+0x92c
V [libjvm.so+0xa01b28] CompileBroker::compiler_thread_loop()+0x468
V [libjvm.so+0xec458c] JavaThread::thread_main_inner()+0xcc
V [libjvm.so+0x17ae2d6] Thread::call_run()+0xb6
V [libjvm.so+0x14ba117] thread_native_entry(Thread*)+0x127

The root cause is a missing '_head->is_Loop()' check to handle the case when the loop head is not a LoopNode but a RegionNode because the loop is irreducible:

(rr) p n_loop->_head->dump(1)
  412 IfTrue === 411 [[ 345 ]] #1 !jvms: Test$Class1::Class1_method0 @ bci:286 (line 119)
  339 SafePoint === 336 1 340 1 1 344 290 1 1 1 1 291 1 1 293 294 295 1 320 297 1 298 299 300 301 302 1 303 1 [[ 345 ]] SafePoint !jvms: Test$Class1::Class1_method0 @ bci:307 (line 122)
  345 Region === 345 339 412 [[ 345 456 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 1861 1863 1865 1867 1882 1888 ]] #irreducible !jvms: Test$Class1::Class1_method0 @ bci:138 (line 101)

We then crash when invoking LoopNode::skip_strip_mined on a RegionNode or assert in debug.

This is a regression from JDK-8303511 in JDK 21 b13. All later JDK versions are affected. The fix was also backported to older releases but these are not affected, probably because JDK-8280126 was not backported.