[clang] [llvm] [WIP][LLVM] Add `__builtin_readfixedtimer` intrinsic and buiiltin (PR #81331)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

This is a draft, I'm trying to get this to map to either `s_memtime` or `s_sendmsg_rtn(0x83)` depending on the target for AMDGPU. Currently, it will recognize the new intrinsic but just lower it to `i64 0`. I'm not overly familiar with the backend, so any suggestions would be greatly appreciated.

https://github.com/llvm/llvm-project/pull/81331