$ CONF=linux-arm-server-fastdebug make run-test TEST=runtime/Monitor/SyncOnValueBasedClassTest.java
...

#  Internal Error (/home/pi/jdk/src/hotspot/cpu/arm/register_arm.hpp:155), pid=3793, tid=3808
#  assert(is_valid()) failed: invalid register
#
# JRE version: OpenJDK Runtime Environment (17.0) (fastdebug build 17-internal+0-adhoc.pi.jdk)
# Java VM: OpenJDK Server VM (fastdebug 17-internal+0-adhoc.pi.jdk, compiled mode, emulated-client, g1 gc, linux-arm)
# Problematic frame:
# V  [libjvm.so+0xe1a6a8]  MacroAssembler::load_klass(RegisterImpl*, RegisterImpl*, AsmCondition)+0xa4

Current CompileTask:
C1:    318    2   !b        java.lang.Class::desiredAssertionStatus (54 bytes)

Stack: [0x72580000,0x72600000],  sp=0x725fe170,  free space=504k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V  [libjvm.so+0xe1a6a8]  MacroAssembler::load_klass(RegisterImpl*, RegisterImpl*, AsmCondition)+0xa4
V  [libjvm.so+0x43b6b4]  C1_MacroAssembler::lock_object(RegisterImpl*, RegisterImpl*, RegisterImpl*, RegisterImpl*, Label&)+0xcf8
V  [libjvm.so+0x3d731c]  LIR_Assembler::emit_lock(LIR_OpLock*)+0x160
V  [libjvm.so+0x3b1a00]  LIR_OpLock::emit_code(LIR_Assembler*)+0x1c
V  [libjvm.so+0x3c3624]  LIR_Assembler::emit_lir_list(LIR_List*)+0xf0
V  [libjvm.so+0x3c3c08]  LIR_Assembler::emit_block(BlockBegin*)+0x98
V  [libjvm.so+0x3c3e0c]  LIR_Assembler::emit_code(BlockList*)+0x74
V  [libjvm.so+0x363e9c]  Compilation::emit_code_body()+0x130
V  [libjvm.so+0x3646c8]  Compilation::compile_java_method()+0x5e0
V  [libjvm.so+0x365278]  Compilation::compile_method()+0x1ec


The problem is in this code:

  if (DiagnoseSyncOnValueBasedClasses != 0) {
    load_klass(tmp1, obj);
    ldr_u32(tmp1, Address(tmp1, Klass::access_flags_offset()));
    tst(tmp1, JVM_ACC_IS_VALUE_BASED_CLASS);
    b(slow_case, ne);
  }

tmp1 is noreg when !BiasedLocking, because c1_LIRGenerator_arm.cpp provides it only when UseBiasedLocking is enabled.

void LIRGenerator::do_MonitorEnter(MonitorEnter* x) {
  ...
  // Need a scratch register for biased locking on arm
  LIR_Opr scratch = LIR_OprFact::illegalOpr;
  if(UseBiasedLocking) {
    scratch = new_pointer_register();
  } else {
    scratch = atomicLockOpr(); // <--- actually illegalOpr
  }

  ...

  monitor_enter(obj.result(), lock, hdr, scratch,
                x->monitor_no(), info_for_exception, info);
}