From hs_err log:

#
# A fatal error has been detected by the Java Runtime Environment:
#
# SIGSEGV (0xb) at pc=0xffffffdd5a5aeff0, pid=2341, tid=53
#
# JRE version: Java(TM) SE Runtime Environment (12.0) (fastdebug build 12-internal+0-jdk12-jdk.372)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 12-internal+0-jdk12-jdk.372, mixed mode, tiered, compressed oops, g1 gc, solaris-sparc)
# Problematic frame:
# V [libjvm.so+0x12aeff0] bool java_lang_String::is_instance(oop)+0x90
#
# Core dump will be written. Default location: /scratch/opt/mach5/mesos/work_dir/5bfc0868-ce37-4ceb-9ccf-40d87d74cb70/testoutput/jtreg/JTwork/scratch/1/core or core.2341
#
# If you would like to submit a bug report, please visit:
# http://bugreport.java.com/bugreport/crash.jsp
#

--------------- S U M M A R Y ------------

Command Line: -Xlog:module=trace --module-path=/scratch/opt/mach5/mesos/work_dir/5bfc0868-ce37-4ceb-9ccf-40d87d74cb70/testoutput/jtreg/JTwork/classes/4/runtime/modules/ModuleStress/ModuleStressGC.d/mods -Djdk.module.main=jdk.test jdk.test/test.MainGC

Host: sca00moz, Sparcv9 64 bit 3600 MHz, 56 cores, 58G, Oracle Solaris 11.2 SPARC
Time: Sat Sep 8 02:33:28 2018 UTC elapsed time: 5 seconds (0d 0h 0m 5s)

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

Current thread (0x00000001010d4000): JavaThread "pool-1-thread-32" [_thread_in_vm, id=53, stack(0xffffffff2f500000,0xffffffff2f600000)]

Stack: [0xffffffff2f500000,0xffffffff2f600000], sp=0xffffffff2f5fd9c0, free space=1014k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x12aeff0] bool java_lang_String::is_instance(oop)+0x90
V [libjvm.so+0x1b7d0a0] void oopDesc::print_value_on(outputStream*)const+0x70
V [libjvm.so+0xccdae0] void ClassLoaderData::print_value_on(outputStream*)const+0x100
V [libjvm.so+0x1aeabf4] void Modules::define_module(_jobject*,unsigned char,_jstring*,_jstring*,const char*const*,int,Thread*)+0x1734
V [libjvm.so+0x14a4e08] JVM_DefineModule+0x298
C [libjava.so+0x10b00] Java_java_lang_Module_defineModule0+0x100
j java.lang.Module.defineModule0(Ljava/lang/Module;ZLjava/lang/String;Ljava/lang/String;[Ljava/lang/String;)V+-906317187 java.base@12-internal
j java.lang.Module.defineModule0(Ljava/lang/Module;ZLjava/lang/String;Ljava/lang/String;[Ljava/lang/String;)V+0 java.base@12-internal
j java.lang.Module.<init>(Ljava/lang/ModuleLayer;Ljava/lang/ClassLoader;Ljava/lang/module/ModuleDescriptor;Ljava/net/URI;)V+104 java.base@12-internal
j java.lang.Module.defineModules(Ljava/lang/module/Configuration;Ljava/util/function/Function;Ljava/lang/ModuleLayer;)Ljava/util/Map;+296 java.base@12-internal
j java.lang.ModuleLayer.<init>(Ljava/lang/module/Configuration;Ljava/util/List;Ljava/util/function/Function;)V+34 java.base@12-internal
j java.lang.ModuleLayer.defineModulesWithOneLoader(Ljava/lang/module/Configuration;Ljava/util/List;Ljava/lang/ClassLoader;)Ljava/lang/ModuleLayer$Controller;+55 java.base@12-internal
j java.lang.ModuleLayer.defineModulesWithOneLoader(Ljava/lang/module/Configuration;Ljava/lang/ClassLoader;)Ljava/lang/ModuleLayer;+6 java.base@12-internal
j test.MainGC$1.call()Ljava/lang/Void;+11 jdk.test
j test.MainGC$1.call()Ljava/lang/Object;+1 jdk.test
j java.util.concurrent.FutureTask.run()V+39 java.base@12-internal
j java.util.concurrent.ThreadPoolExecutor.runWorker(Ljava/util/concurrent/ThreadPoolExecutor$Worker;)V+92 java.base@12-internal
j java.util.concurrent.ThreadPoolExecutor$Worker.run()V+5 java.base@12-internal
j java.lang.Thread.run()V+11 java.base@12-internal
v ~StubRoutines::call_stub
V [libjvm.so+0x12ab57c] void JavaCalls::call_helper(JavaValue*,const methodHandle&,JavaCallArguments*,Thread*)+0x5bc
V [libjvm.so+0x12a9a60] void JavaCalls::call_virtual(JavaValue*,Klass*,Symbol*,Symbol*,JavaCallArguments*,Thread*)+0x1e0
V [libjvm.so+0x12a9b68] void JavaCalls::call_virtual(JavaValue*,Handle,Klass*,Symbol*,Symbol*,Thread*)+0xb8
V [libjvm.so+0x14cf5f8] void thread_entry(JavaThread*,Thread*)+0x1e8
V [libjvm.so+0x1f9b9f4] void JavaThread::thread_main_inner()+0x2e4
V [libjvm.so+0x1f9b6e0] void JavaThread::run()+0x650
V [libjvm.so+0x1bcfdbc] thread_native_entry+0x2dc