[llvm-dev] [IR][AsmPrinter][MCJIT]: ensure every x64 "CALL" to Jit function uses relative address

Vivien Millet via llvm-dev llvm-dev at lists.llvm.org
Mon Feb 25 08:03:36 PST 2019

Hi Lang,
That would be great ! :)
For now I found a hack which consist of loading a "full of NOP" DLL, use my
own DllMemMgr which allocates JIT sections inside the DLL virtual space,
backup the memory inside some buffers, unload the DLL, copy the buffer
inside the DLL file, create the PDB, reload the DLL file which hopefully in
99% is reloaded at the same virtual address (this allow me to debug with
PDB after reload). If not reloaded at same address I restart the process
from the beginning until it does (quite dirty but it really does the job
until I have a better option).

Keep me in touch with the progress of your work !
Thanks !


Le jeu. 21 févr. 2019 à 01:02, Lang Hames <lhames at gmail.com> a écrit :

> Hi Vivien,
> RuntimeDyld (the JIT-linker currently underlying MCJIT and ORC) requires
> large code model, which I believe renders this discussion moot since all
> jumps are via registers.
> I am working on a replacement linker that will support the small code
> model. I hope to have prototypes out for review next week. In the new model
> you will be able to register a callback to inspect the linkers data
> structures: this could be used to identify the location of each jump stub
> and jump-stub pointer.
> Kind Regards,
> Lang.
> On Sat, Feb 9, 2019 at 12:05 PM Vivien Millet via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> Sorry I meant 0xE8, 0xFF was in my head because of disassembling and
>> seeing it in use, my bad.
>> Ok I didn't thought about PIC,  that's a good idea !  ... I forgot it
>> existed at the wrong moment I guess ...
>> How can I modify the GOT then ?
>> Is there an api somewhere in the execution engine / MCJIT ?
>> Or is it somewhere else ?
>> Or I need to accept the idea of hacking stuff inside the LLVM code myself
>> ?
>> Le sam. 9 févr. 2019 à 20:38, Tim Northover <t.p.northover at gmail.com> a
>> écrit :
>>> On Sat, 9 Feb 2019 at 17:38, Vivien Millet via llvm-dev
>>> <llvm-dev at lists.llvm.org> wrote:
>>> > Is it possible to tell LLVM somewhere that we prefer the asm printer
>>> to use x64 relative CALL  (0xFF) instead of absolute one?
>>> 0xff is always absolute, isn't it? And for JITs the relative variants
>>> are problematic because you normally can't guarantee your mmapped
>>> region will be within  2GB of of what it's calling, so the offset may
>>> be too big.
>>> > The goal is to be able to move the entire JIT program memory somewhere
>>> else and still be able to run the program.
>>> It sounds like you need to configure the ExecutionEngine to use PIC
>>> mode with setRelocationModel. With that, LLVM do some GOT magic to get
>>> the right address to jump to and then use an absolute jump to that
>>> register. You still have to arrange for the GOT to contain the right
>>> value (looks like it's the address of the function relative to the
>>> start of the GOT at first glance), but that's more tractable than
>>> monkey-patching all the callsites.
>>> Cheers.
>>> Tim.
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>> llvm-dev at lists.llvm.org
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