[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
Sat Feb 9 12:07:07 PST 2019

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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