The following test failed in the JDK15 CI:

java/util/Calendar/CalendarTestScripts/JapaneseRollTests.java

Here's a snippet from the log file:

----------System.out:(31/1821)----------
Starting tz_japan.cts...
Completed tz_japan.cts
Starting lenient.cts...
Completed lenient.cts
Starting rolldow.cts...
Test#1: Showa 63
Test#2: Showa 64
# To suppress the following error report, specify this argument
# after -XX: or in .hotspotrc:  SuppressErrorAt=/regmask.hpp:204
#
# A fatal error has been detected by the Java Runtime Environment:
#
#  Internal Error (/scratch/mesos/slaves/b0d836b1-c68c-4dbd-8b78-5085890ddd4c-S17/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/180f4b9b-5412-452a-8a1e-e2b74b06d823/runs/db56e059-8738-4f13-b82e-fa13230fb696/workspace/open/src/hotspot/share/opto/regmask.hpp:204), pid=89294, tid=22787
#  assert(_A[i] == 0) failed: _hwm too low: 0 regs at: 7
#
# JRE version: Java(TM) SE Runtime Environment (15.0+13) (fastdebug build 15-ea+13-491)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 15-ea+13-491, mixed mode, tiered, g1 gc, bsd-amd64)
# Core dump will be written. Default location: core.89294
#
# An error report file with more information is saved as:
# /mesos/work_dir/slaves/7cfa21b9-afde-4338-a459-d3e0a13ec793-S1176/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/fbb4379e-f417-42f6-8ac6-9dbcb4ed5a94/runs/404be7cf-a3d5-4933-a3a4-cc19b49a5667/testoutput/test-support/jtreg_open_test_jdk_jdk_util/scratch/1/hs_err_pid89294.log
Test#3: Heisei 2
#
# Compiler replay data is saved as:
# /mesos/work_dir/slaves/7cfa21b9-afde-4338-a459-d3e0a13ec793-S1176/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/fbb4379e-f417-42f6-8ac6-9dbcb4ed5a94/runs/404be7cf-a3d5-4933-a3a4-cc19b49a5667/testoutput/test-support/jtreg_open_test_jdk_jdk_util/scratch/1/replay_pid89294.log
#
# If you would like to submit a bug report, please visit:
#   https://bugreport.java.com/bugreport/crash.jsp
#
Current thread is 22787
Dumping core ...
----------System.err:(0/0)----------


Here's the crashing thread's stack:

---------------  T H R E A D  ---------------

Current thread (0x00007faa4e0e5800):  JavaThread "C2 CompilerThread0" daemon [_thread_in_native, id=22787, stack(0x0000700007815000,0x0000700007915000)]


Current CompileTask:
C2:   9539 1012       4       java.text.DecimalFormat::applyPattern (1645 bytes)

Stack: [0x0000700007815000,0x0000700007915000],  sp=0x0000700007910b00,  free space=1006k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V  [libjvm.dylib+0xb449c7]  VMError::report_and_die(int, char const*, char const*, __va_list_tag*, Thread*, unsigned char*, void*, void*, char const*, int, unsigned long)+0x6e5
V  [libjvm.dylib+0xb450e3]  VMError::report_and_die(Thread*, void*, char const*, int, char const*, char const*, __va_list_tag*)+0x47
V  [libjvm.dylib+0x3382fc]  report_vm_error(char const*, int, char const*, char const*, ...)+0x145
V  [libjvm.dylib+0x25f51a]  RegMask::valid_watermarks() const+0x10e
V  [libjvm.dylib+0x2e5338]  RegMask::overlap(RegMask const&) const+0x12
V  [libjvm.dylib+0x9c0642]  RegMask::is_UP() const+0x16
V  [libjvm.dylib+0x4f8064]  PhaseChaitin::raise_pressure(Block*, LRG&, PhaseChaitin::Pressure&, PhaseChaitin::Pressure&)+0x20
V  [libjvm.dylib+0x4f8a84]  PhaseChaitin::add_input_to_liveout(Block*, Node*, IndexSet*, double, PhaseChaitin::Pressure&, PhaseChaitin::Pressure&)+0x122
V  [libjvm.dylib+0x4f9212]  PhaseChaitin::build_ifg_physical(ResourceArea*)+0x5a6
V  [libjvm.dylib+0x276316]  PhaseChaitin::Register_Allocate()+0x75c
V  [libjvm.dylib+0x30457a]  Compile::Code_Gen()+0x2ba
V  [libjvm.dylib+0x3021cb]  Compile::Compile(ciEnv*, C2Compiler*, ciMethod*, int, bool, bool, bool, DirectiveSet*)+0xbf1
V  [libjvm.dylib+0x2541fa]  C2Compiler::compile_method(ciEnv*, ciMethod*, int, DirectiveSet*)+0xe8
V  [libjvm.dylib+0x31446b]  CompileBroker::invoke_compiler_on_method(CompileTask*)+0x555
V  [libjvm.dylib+0x313d0b]  CompileBroker::compiler_thread_loop()+0x283
V  [libjvm.dylib+0xabb8e5]  JavaThread::thread_main_inner()+0x193
V  [libjvm.dylib+0xab80bf]  Thread::call_run()+0x11b
V  [libjvm.dylib+0x9317ec]  thread_native_entry(Thread*)+0xe0
C  [libsystem_pthread.dylib+0x3661]  _pthread_body+0x154
C  [libsystem_pthread.dylib+0x350d]  _pthread_body+0x0
C  [libsystem_pthread.dylib+0x2bf9]  thread_start+0xd