[LLVMdev] VmKit - Error in j3/precompiler after a big surgery to MMTk part

Yi Lin qinsoon at gmail.com
Thu Sep 12 03:43:23 PDT 2013

Hi Gaël,

Since I am replacing Java MMTk with C MMTk, accordingly I also changed 
the build process.

On 12/09/13 19:53 , Gaël Thomas wrote:
> Hi Yi,
> The precompiler is in charge of translating the most common classes of
> the Java runtime library to llvm code (typically, java.lang.Object),
> in order to natively compile these classes and decrease the bootstrap
> time. So, if you have a segmentation fault at this stage, it means
> that you have broken something in the interface between MMTk and
> VMKit. If it can helps you:
> * during a first stage, we compile VMKit and J3 with a fake MMTk (we
> replace gcmalloc by a simple malloc)
> * we use the generated VMKit/J3 to natively compile MMTk
> * we link this code with VMKit and replace malloc by the gcmalloc of
> MMTk. At this stage, you have VMKit that use MMTk. In your case, the
> compilation is correct up to this stage, meaning that VMKit is still
> able to compile MMTk.
The above stages seem no longer needed. I also removed the fake MMTk 
The error happened in precompiling, so I assume the compilation of C 
MMTk and linking with J3 appears to be fine (though there might be problems)
> * we use VMKit/J3 (linked with MMTk) to precompile the rt.jar. So,
> your compilation process breaks at this stage with a segmentation
> fault caused by an arithmetic exception. As you have a segmentation
> fault, I think that your C code of MMTk probably corrupt the memory,
> either because you have a bug in your code, or because something goes
> wrong in your modifications in the glue between VMKit and MMTk
> (probably this second case).
> As you have modified the object layout (in MMTkObject.h?), it probably
> means that you use a different object layout? What kind of
> modifications have you introduced? Because if you have changed the
> object layout of Java objects, it can potentially impact all the code
> of J3 and VMKit...
Since MMTk is no longer generated by j3, but by my AOT compiler, the 
object layout is different. I didn't modify MMTkObject.h, but I added my 
own header to allow glue code correctly (hopefully) access fields and 
construct 'objects' for MMTk. I noticed that vmkit/j3 holds reference to 
MMTk objects, but my guess was they wouldn't access MMTk objects without 
going through the glue code. So I only modified the glue code to adapt 
to new layout, not the other parts. But still, the 'objects' in C MMTk 
are not Java objects, and do not follow j3 Java object layout. I am not 
sure if j3 implies that MMTk objects are its own java object. Is this 
problematic? If so, I need to reconsider my solution.

And thanks for the reply, its very helpful.

> Gaël
> 2013/9/12 Yi Lin <qinsoon at gmail.com>:
>> Hi,
>> I replaced the MMTk code with a C-version MMTk code (I have been working on
>> such AOT compiler for a while). The compiler should deal with most of the
>> MMTk code base correctly - at least I get marksweep running with a tiny
>> client). So I am trying to put the C version MMTk back with VmKit. I tried
>> not to change much existing code except where necessary, such as name
>> mangling and MMTkObject layout.
>> I am having the following error during building VmKit:
>> [vmkit ./tools/trainer]: Pre-compiling bootstrap code
>> Thread 0x110000000 received a SIGSEGV: either the VM code or an external
>> native method is bogus. Aborting...
>> Aborted
>> make[1]: ***
>> [/home/yilin/vmkit/vmkit/tools/trainer/Release+Asserts/Precompiled.bc] Error
>> 134
>> [vmkit .]: abort with error in ./tools/trainer
>> make: *** [all] Error 1
>> The stacktrace from gdb is:
>> Program received signal SIGSEGV, Segmentation fault.
>> [Switching to Thread 0x1100ff700 (LWP 21670)]
>> 0x000000000107b180 in llvm::BumpPtrAllocator::Allocate(unsigned long,
>> unsigned long) ()
>> (gdb) bt
>> #0  0x000000000107b180 in llvm::BumpPtrAllocator::Allocate(unsigned long,
>> unsigned long) ()
>> #1  0x00000000005ae35d in
>> j3::JavaMethod::marshalArguments(vmkit::ThreadAllocator&, __va_list_tag*) ()
>> #2  0x00000000005b04e4 in j3::JavaObject*
>> j3::JavaMethod::invokeStaticAP<j3::JavaObject*, j3::JavaObject*
>> (*)(j3::JavaConstantPool*, void*, void*)>(j3::Jnjvm*, j3::Class*,
>> __va_list_tag*) ()
>> #3  0x00000000005af054 in j3::JavaMethod::invokeJavaObjectStatic(j3::Jnjvm*,
>> j3::Class*, ...) ()
>> #4  0x00000000005cb662 in j3::Jnjvm::asciizToStr(char const*) ()
>> #5  0x00000000005cbde4 in j3::Jnjvm::CreateArithmeticException() ()
>> #6  0x00007ffff7f6316d in ?? ()
>> #7  0x0000000000000000 in ?? ()
>> The main thread was waiting in "vmkit::VirtualMachine::waitForExit() ()", so
>> the application (HelloWorld) should have been started.
>> 1. To me, the stack trace seems not showing where the original error
>> happened. It seems an arithmetic error happened somewhere, VM was trying to
>> throw an exception, but met another error. Is this correct? And how can I
>> track where that actual error happened?
>> 2. What does the precompiler do? I dont quite understand why it is needed
>> here.
>> Any information would be very helpful. Thanks very much.
>> Regards,
>> Yi
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