[llvm-dev] ORC JIT - Incorrect support for COFF files?

Mon Aug 24 00:57:08 PDT 2020

Hey Lang and Stefan,

Using dllimport on my “planschiValue” actually worked! But I have no idea why, because the relocation is still a REL32 if I use dumpbin… So how is it possible for that to work?
However… when I load an COFF object file, am I able to change the relocations to dllimport somehow? I honestly can’t imagine how this would work since the machine code is probably already adjusted to use a REL32… or something… Just wanted to ask…

@Stefan Gränitz<mailto:stefan.graenitz at gmail.com> I remember your patch! How does it deal with extern variables though? As far as I understood from the code – not that I understood much – it fixes function calls by having a trampoline right?

Thank you guys!

Kind greetings
Björn

From: Lang Hames <lhames at gmail.com>
Sent: 22 August 2020 19:50
To: Stefan Gränitz <stefan.graenitz at gmail.com>
Cc: Gaier, Bjoern <Bjoern.Gaier at horiba.com>; LLVM Developers Mailing List <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] ORC JIT - Incorrect support for COFF files?

Hi Stefan, Bjoern,

For calls across object boundaries __dllimport now supported in RuntimeDyld: https://github.com/llvm/llvm-project/commit/337e131ca7de48072def7729df69434c37a66eb7<https://hes32-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2fgithub.com%2fllvm%2fllvm%2dproject%2fcommit%2f337e131ca7de48072def7729df69434c37a66eb7&umid=9d785513-e9cd-44e5-a111-cb77401a12f6&auth=b6f66d00f8195cc5198eee21f0dbabe6af0a3180-2bd41aad154bd7e67313fcb36c401a0c65d55652>. You may just be able to mark your externs as __dllimport for this to work. I don't recall whether __dllimport works for data symbols too, but I suspect so -- I think it's basically an explicit GOT entry.

If anyone is ever interested in writing a JITLink COFF implementation I will be happy to help out or review patches -- I'd love to get better COFF support in tree.

-- Lang.

On Fri, Aug 21, 2020 at 2:09 PM Stefan Gränitz <stefan.graenitz at gmail.com<mailto:stefan.graenitz at gmail.com>> wrote:
Hi Björn

I made a workaround for this specific issue a long time ago for the Projucer C++ JIT Engine. It basically forwards the call to another stub that provides enough space to encode a full 64-bit address. The patch is based on LLVM 3.9, so I guess it won't work out-of-the-box on a recent release, but it may give you enough hints to figure it out on your own:
https://github.com/weliveindetail/pj-llvm/commit/97cd336d458ae9c73232d1b539ceefcdb2f5eb0f<https://hes32-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2fgithub.com%2fweliveindetail%2fpj%2dllvm%2fcommit%2f97cd336d458ae9c73232d1b539ceefcdb2f5eb0f&umid=9d785513-e9cd-44e5-a111-cb77401a12f6&auth=b6f66d00f8195cc5198eee21f0dbabe6af0a3180-dffa61903884c3a0176d5f66111470dc6e9b9d2b>

Alternatively, you could consider a memory manager that makes sure all calls are within a 32-bit range. For the code you JIT yourself, this should be no problem. In case you have to link against an old MSVCRT though, this is not possible, because it has a fixed load address that's far beyond the 32-bit range from any heap allocation you can make for your JITed code. That's been our issue back then and so this patch was the last resort.

Please also note: this is for freestanding function only. I didn't consider member function calls, etc. (addend is always 0) because it was not relevant for the specific issue.

Hope it helps
Stefan
On 21/08/2020 13:22, Gaier, Bjoern via llvm-dev wrote:
I figured out that this problem is caused because “planschiValue” has a REL32 relocation and the addresses between the code and the variable overflows 32bit.
Is there any workaround for this kind of issue?

From: llvm-dev <llvm-dev-bounces at lists.llvm.org><mailto:llvm-dev-bounces at lists.llvm.org> On Behalf Of Gaier, Bjoern via llvm-dev
Sent: 20 August 2020 12:15
To: LLVM Developers Mailing List <llvm-dev at lists.llvm.org><mailto:llvm-dev at lists.llvm.org>; Lang Hames <lhames at gmail.com><mailto:lhames at gmail.com>
Subject: [llvm-dev] ORC JIT - Incorrect support for COFF files?

Hey LLVM-Mailing-List and Lang,

I’m still learning how to use the ORC JIT but I finally reached the point to JIT and execute some code. For this purpose I created a test file (TestModule.cpp) and compiled it with Clang, generating two different files, one in the LLVM IR format and one in the Microsoft COFF format.

The JIT resolves all undefined references, including “extern int planschiValue;” and “void externFunction();”. Using the IR Module I will get the correct address for “planschiValue” and the correct value – however this is not the case for the object file.
For the object file I get (via printf from the module) the address 0x000002295D6B003C while the actual address is 0x00007FF71D9959A4.

I’m really surprised about this, because the IR module works with no problem. I attach the source code, the IR file and the resulting object file (including its assembly file).

Any ideas what I’m doing wrong?

Thank you for the help in advance and kind greetings
Björn

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