[CodeGen][RISCV] Change Shadow Call Stack Register to X3

ShadowCallStack implementation uses s2 register on RISC-V, but that
choice is problematic for reasons described in:

https://lists.riscv.org/g/sig-toolchains/message/544,
https://github.com/riscv-non-isa/riscv-elf-psabi-doc/issues/370, and
https://github.com/google/android-riscv64/issues/72

The concern over the register choice was also brought up in
https://reviews.llvm.org/D84414.

https://reviews.llvm.org/D84414#2228666 said:

```
  "If the register choice is the only concern about this work, then I think
  we can probably land it as-is and fixup the register choice if we see
  major drawbacks later. Yes, it's an ABI issue, but on the other hand the
  shadow call stack is not a standard ABI anyway.""
```

Since we have now found a sufficient reason to fixup the register
choice, we should go ahead and update the implementation. We propose
using x3(gp) which is now the platform register in the RISC-V ABI.

Reviewed By: asb, hiraditya, mcgrathr, craig.topper

Differential Revision: https://reviews.llvm.org/D146463

0 files changed, 0 insertions, 0 deletions