MCJIT + Win64 SEH handling problem (missing unwinding info)

Hi all,

I’m using MCJIT for running the manually generated code under both Win32 and Win64 environments. Unfortunately catching exceptions under Win64 doesn’t work. As I understand the main problem is the lack of function table that is needed to unwind a call stack.

Microsoft says
Function tables are used on 64-bit Windows to determine how to unwind or walk the stack. These tables are usually generated by the compiler and stored as part of the image. However, applications must provide the function table for dynamically generated code.

As I can see LLVM usually creates Function Table when generating COFF (Win64EH::UnwindEmitter). But MCJIT can load only ELF files.

Is there any way to workaround the problem? Maybe it is already planned to implement COFF loading in MCJIT?

Best regards,

Hi Aliaksei,

The MCJIT code is structured in such a way as to abstract the details of object file formats. What is primarily lacking is a COFF implementation of the RuntimeDyld interface. When we first implemented MCJIT on Windows using the ELF object format was sufficient for the applications we were aware of that intended to use MCJIT on Windows, so we used ELF-on-Windows as a sort of shortcut.

I’m not aware of anything that would prevent MCJIT from working with COFF files if someone was willing to invest the effort to implement the RuntimeDyld support for COFF. It’s likely that one or two small issues would arise, but I don’t think there would be any major design issues. However, even after basic MCJIT execution of COFF images is in place there may be additional work to be done to make exception handling work correctly. I don’t know one way or the other about that.

If you are interested in trying to implement the RuntimeDyld support for COFF I can provide you with some details of what is involved.