llvm dev - Feb 2019 - [IR][AsmPrinter][MCJIT]: ensure every x64 "CALL" to Jit function uses relative address

Hi,
Is it possible to tell LLVM somewhere that we prefer the asm printer to use
x64 relative CALL  (0xFF) instead of absolute one ? The goal is to be able
to move the entire JIT program memory somewhere else and still be able to
run the program.

Thanks !
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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.

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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