JVM 6u18 repeatedly dumped core when -XX:+OptimizeStringConcat was used.
The compiled Java method affected was always:
   
J java.lang.Throwable.printStackTrace(Ljava/io/PrintWriter;)V


#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (nmethod.cpp:1836), pid=10048, tid=59
# Error: guarantee(cont_offset != 0,"unhandled implicit exception in compiled code")
#
# JRE version: 6.0_18-b07
# Java VM: Java HotSpot(TM) Server VM (16.0-b13 mixed mode solaris-sparc )
# If you would like to submit a bug report, please visit:
# http://java.sun.com/webapps/bugreport/crash.jsp
#

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

Current thread (0x01ea1000): JavaThread "TP-Processor4" daemon [_thread_in_Java, id=59, stack(0xb1580000,0xb1600000)]

Stack: [0xb1580000,0xb1600000], sp=0xb15fb670, free space=1edfee3b5e4k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x84badc] void VMError::report_and_die()+0x764
V [libjvm.so+0x445b1c] void report_fatal(const char*,int,const char*)+0x2c
V [libjvm.so+0x22d680] unsigned char*nmethod::continuation_for_implicit_exception(unsigned char*)+0x5c
V [libjvm.so+0x20f990] unsigned char*SharedRuntime::continuation_for_implicit_exception(JavaThread*,unsigned char*,SharedRuntime::ImplicitExceptionKind)+0x2a0
V [libjvm.so+0x1c079c] JVM_handle_solaris_signal+0x5fc
C [libc.so.1+0xc01e4] __sighndlr
C [libc.so.1+0xb52dc] call_user_handler
J java.lang.Throwable.printStackTrace(Ljava/io/PrintWriter;)V
j org.apache.log4j.spi.ThrowableInformation.getThrowableStrRep()[Ljava/lang/String;+43
j org.apache.log4j.spi.LoggingEvent.getThrowableStrRep()[Ljava/lang/String;+13
j org.apache.log4j.WriterAppender.subAppend(Lorg/apache/log4j/spi/LoggingEvent;)V+26
j org.apache.log4j.DailyRollingFileAppender.subAppend(Lorg/apache/log4j/spi/LoggingEvent;)V+54
j org.apache.log4j.WriterAppender.append(Lorg/apache/log4j/spi/LoggingEvent;)V+10
j org.apache.log4j.AppenderSkeleton.doAppend(Lorg/apache/log4j/spi/LoggingEvent;)V+106
j org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(Lorg/apache/log4j/spi/LoggingEvent;)I+41
j org.apache.log4j.Category.callAppenders(Lorg/apache/log4j/spi/LoggingEvent;)V+26
j org.apache.log4j.Category.forcedLog(Ljava/lang/String;Lorg/apache/log4j/Priority;Ljava/lang/Object;Ljava/lang/Throwable;)V+14
j org.apache.log4j.Category.error(Ljava/lang/Object;Ljava/lang/Throwable;)V+37
[ ... ]