[llvm] BPF: Ensure __sync_fetch_and_add() always generate atomic_fetch_add insn (PR #101428)

[via llvm-commits]

yonghong-song wrote:

> > I think we should report an error when __sync_fetch_and_add is used for cpu v1.
> 
> datadog is not the only user of (void)__sync_fetch_and_add(&counter, ...); cilium has the same pattern and they also support old kernels. If new llvm errors with -mcpu=v1 the users wouldn't upgrading llvm. So I think for -mcpu=v1 and (void)__sync_fetch_and_add() llvm should generate old bpf xadd insn, since that's the only insn it can generate. For pattern "var = __sync_fetch_and_add()" and -mcpu=v1 llvm should error. I believe it started doing this already some time back.
> 
> For (void)__sync_fetch_and_add() and -mcpu=v3 we should generate fetch_and_add() insn.

Sounds good, I will fix the issue for this in llvm20.

https://github.com/llvm/llvm-project/pull/101428