[lld] [llvm] [lld][llvm/mc] Add support for sdata8 for FDE CIE (PR #174508)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> What should the discussion be about in the ABI group? I can kickstart it but I am not sure the topic. The format itself supports multiple byte-encodings so it seems like purely a lack of implementation support. (This is not adding a new byte-encoding type)
> 
> Appreciate any guidance on how to make progress here. I have a few more contributions in this direction (I sent you an email to maskray.me) about it 🙇

Current and future proposals:

* We propose moving sdata4 to sdata8 for a variant of the medium code model. While the x86-64 code model specifications do not explicitly mandate a specific DW_EH_PE (DWARF Exception Handling) encoding, coordinating with GCC/binutils ensures cross-toolchain compatibility and alignment on encoding expectations.
* Looking ahead, we are exploring the implementation of range extension thunks. This will allow the linker to handle branch targets that exceed the 32-bit reach of CALL/JUMP instructions, without incurring overhead on all call instructions (many will be in-reach, so generating the multi-instruction sequence on the compiler side is wasteful).

These extensions need ABI discussions and toolchain coordination.

https://github.com/llvm/llvm-project/pull/174508