[LLVMdev] Thumb call relocation for the Runtime dynamic linker (RuntimeDyldELF.cpp)

Jonas Zaddach zaddach at eurecom.fr
Fri May 24 09:34:23 PDT 2013

Hi Tim,

thanks for the advice, I will rework my patch to be conformant to Thumb2
specifications, and have a look at the testing, and then resubmit.


On Friday, May 24, 2013, Tim Northover wrote:

> Hi Jonas,
> > here is a patch to add Thumb call relocation to the dynamic linker. I
> would be happy if you could commit it to the SVN.
> Thanks very much for working on this. It looks like a good starting-point,
> but there are a couple of issues with the patch at the moment.
> First, it only handles RelValue up to 22 bits (depending on how you count)
> in size. But on ARMv6T2 onwards the J1 and J2 fields of the instruction
> form part of the immediate. Technically, I think a linker is allowed to do
> what you've done so it's probably good enough for now as long as we put an
> assertion into the code that RelType isn't too big.
> Second, I don't think it handles BLX correctly. When used on a BLX the
> instruction is assumed to be at "Align(PC, 4)" rather than just "PC". I
> *think* this means that if the BLX instruction was at an address "== 2 (mod
> 4)" then your code would set the 'H' bit and create an undefined
> instruction.
> Finally, it would be very good to have some tests for the patch. Or does
> it help out on normal regression tests when you set the MCJIT's default
> triple to thumbv7?
> Tim.
> (P.S. we normally send all patches to the llvm-commits mailing list with
> something like "[PATCH]" in the subject. Odd, but just how LLVM evolved I
> suppose. I've added them on CC).
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