This assert sometimes triggers when running in a loop:

intelsdv01:/export/twisti/jruby$ bin/jruby.sh --server -Xcompile.invokedynamic=true -J-XX:-PrintVMOptions -J-XX:+UseSerialGC test/test_thread.rb
Loaded suite test/test_thread
Started
............=============== DEBUG MESSAGE: illegal bytecode sequence - method not verified ================

# To suppress the following error report, specify this argument
# after -XX: or in .hotspotrc: SuppressErrorAt=/assembler_x86.cpp:5093
#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/home/ct232829/hotspot-comp/6817525/src/cpu/x86/vm/assembler_x86.cpp:5093), pid=16856, tid=1126
# assert(false) failed: DEBUG MESSAGE
#
# JRE version: 7.0-b135
# Java VM: Java HotSpot(TM) Server VM (21.0-b05-internal-fastdebug mixed mode solaris-x86 )
# Failed to write core dump. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
#
# An error report file with more information is saved as:
# /export/twisti/jruby/hs_err_pid16856.log
#
# If you would like to submit a bug report, please visit:
# http://java.sun.com/webapps/bugreport/crash.jsp
#
Current thread is 1126
Dumping core ...
Abort
--------------- T H R E A D ---------------

Current thread (0x08f02400): JavaThread "RubyThread-1115: /export/twisti/jruby/lib/ruby/1.8/test/unit/testcase.rb:78" daemon [_thread_in_vm, id=1126, stack(0xd1eae000,0xd1fae000)]

Stack: [0xd1eae000,0xd1fae000], sp=0xd1facd80, free space=1019k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x1ce44f6] void VMError::report(outputStream*)+0x902
V [libjvm.so+0x1ce56a9] void VMError::report_and_die()+0x56d
V [libjvm.so+0xaf7b6a] void report_vm_error(const char*,int,const char*,const char*)+0x53a
V [libjvm.so+0x4f1ca4] void MacroAssembler::debug32(int,int,int,int,int,int,int,int,int,char*)+0x40c
j test.test_thread.method__37$RUBY$call_to_s(Ltest/test_thread;Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/Block;)Lorg/jruby/runtime/builtin/IRubyObject;+10
j test$test_thread#method__37$RUBY$call_to_s.call(Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/RubyModule;Ljava/lang/String;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/Block;)Lorg/jruby/runtime/builtin/IRubyObject;+19
j test$test_thread#method__37$RUBY$call_to_s.call(Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/RubyModule;Ljava/lang/String;Lorg/jruby/runtime/builtin/IRubyObject;)Lorg/jruby/runtime/builtin/IRubyObject;+11
j org.jruby.runtime.invokedynamic.InvokeDynamicSupport.fallback(Lorg/jruby/runtime/invokedynamic/InvokeDynamicSupport$JRubyCallSite;Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/builtin/IRubyObject;Ljava/lang/String;Lorg/jruby/runtime/builtin/IRubyObject;)Lorg/jruby/runtime/builtin/IRubyObject;+79
j test.test_thread.block_39$RUBY$test_poly_calls_thread_safe(Ltest/test_thread;Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/Block;)Lorg/jruby/runtime/builtin/IRubyObject;+42
j test$test_thread#block_39$RUBY$test_poly_calls_thread_safe.call(Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/Block;)Lorg/jruby/runtime/builtin/IRubyObject;+9
j org.jruby.runtime.CompiledBlock.yield(Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/Binding;Lorg/jruby/runtime/Block$Type;Lorg/jruby/runtime/Block;)Lorg/jruby/runtime/builtin/IRubyObject;+49
j org.jruby.runtime.CompiledBlock.yield(Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/Binding;Lorg/jruby/runtime/Block$Type;)Lorg/jruby/runtime/builtin/IRubyObject;+9
j org.jruby.runtime.Block.yield(Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/builtin/IRubyObject;)Lorg/jruby/runtime/builtin/IRubyObject;+14
j org.jruby.RubyFixnum.times(Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/Block;)Lorg/jruby/runtime/builtin/IRubyObject;+49
j org.jruby.RubyFixnum$i$0$0$times.call(Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/RubyModule;Ljava/lang/String;Lorg/jruby/runtime/Block;)Lorg/jruby/runtime/builtin/IRubyObject;+7
j org.jruby.runtime.invokedynamic.InvokeDynamicSupport.fallback(Lorg/jruby/runtime/invokedynamic/InvokeDynamicSupport$JRubyCallSite;Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/builtin/IRubyObject;Ljava/lang/String;Lorg/jruby/runtime/Block;)Lorg/jruby/runtime/builtin/IRubyObject;+88
j test.test_thread.block_38$RUBY$test_poly_calls_thread_safe(Ltest/test_thread;Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/Block;)Lorg/jruby/runtime/builtin/IRubyObject;+49
j test$test_thread#block_38$RUBY$test_poly_calls_thread_safe.call(Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/Block;)Lorg/jruby/runtime/builtin/IRubyObject;+9
j org.jruby.runtime.CompiledBlock.yield(Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/RubyModule;ZLorg/jruby/runtime/Binding;Lorg/jruby/runtime/Block$Type;Lorg/jruby/runtime/Block;)Lorg/jruby/runtime/builtin/IRubyObject;+70
j org.jruby.runtime.BlockBody.call(Lorg/jruby/runtime/ThreadContext;[Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/Binding;Lorg/jruby/runtime/Block$Type;Lorg/jruby/runtime/Block;)Lorg/jruby/runtime/builtin/IRubyObject;+27
j org.jruby.runtime.Block.call(Lorg/jruby/runtime/ThreadContext;[Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/Block;)Lorg/jruby/runtime/builtin/IRubyObject;+15
j org.jruby.RubyProc.call(Lorg/jruby/runtime/ThreadContext;[Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/builtin/IRubyObject;Lorg/jruby/runtime/Block;)Lorg/jruby/runtime/builtin/IRubyObject;+59
j org.jruby.RubyProc.call(Lorg/jruby/runtime/ThreadContext;[Lorg/jruby/runtime/builtin/IRubyObject;)Lorg/jruby/runtime/builtin/IRubyObject;+7
j org.jruby.internal.runtime.RubyRunnable.run()V+94
j java.lang.Thread.run()V+11
v ~StubRoutines::call_stub
V [libjvm.so+0xf181f8] void JavaCalls::call_helper(JavaValue*,methodHandle*,JavaCallArguments*,Thread*)+0x894
V [libjvm.so+0x185033c] void os::os_exception_wrapper(void(*)(JavaValue*,methodHandle*,JavaCallArguments*,Thread*),JavaValue*,methodHandle*,JavaCallArguments*,Thread*)+0x18
V [libjvm.so+0xf15e90] void JavaCalls::call_virtual(JavaValue*,Handle,KlassHandle,Symbol*,Symbol*,Thread*)+0x458
V [libjvm.so+0x118f35e] void thread_entry(JavaThread*,Thread*)+0x266
V [libjvm.so+0x1bdc509] void JavaThread::thread_main_inner()+0x179
V [libjvm.so+0x1bdc1b9] void JavaThread::run()+0x619
V [libjvm.so+0x183feeb] java_start+0x6fb
C [libc.so.1+0xa71d0] _thr_setup+0x4e
C [libc.so.1+0xa74c0] __moddi3+0x60