llvm dev - Oct 2014 - [LLVMdev] 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
<http://msdn.microsoft.com/en-us/library/windows/desktop/ms680595%28v=vs.85%29.aspx>
*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,
Aliaksei
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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.

-Andy


From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Aliaksei Zasenka
Sent: Tuesday, October 28, 2014 5:26 AM
To: LLVMdev at cs.uiuc.edu
Subject: [LLVMdev] 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
<http://msdn.microsoft.com/en-us/library/windows/desktop/ms680595%28v=vs.85%29.aspx>
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,
Aliaksei

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