Please help me how to add a test for it, in the current state this patch won't do anything because the standard build for libunwind does not add -mbranch-protection=standard (or bti).

Rebased and address review comments.

Fix review comments from Tamas.

Rebased.

The patched is rebased and the hopefully the logic is now simpler.

just a nit, LGTM otherwise.

ping

Patch is rebased, test is updated.

Thanks for the comments, patch is improved
isStringAttribute() check removed, the attribute is always a string in this case or "null" so the check is not needed.
Function level the attribute is now only change when needed, so as the function level attribute is expected to be rare I hope the performance won't be impacted by the patch.
I kept the "tri-state" logic because of the emitted functions. Introducing a "branch-target-enforcement-disabled" attribute seems even more confusing for me.

Previous version of the patch handled only the functions that are created in clang. Sanitizers can't see the codegen options therefore they also disables BTI.
This version of the patch is less invasive in my opinion, effects only aarch64.
branch-target-enforcement(BTI) and sign-return-address(PAC) are added to all function that comes from clang frontend.
If the backend encounters with the function without BTI attribute but the module is compiled with BTI then it assumes it should be made BTI compatible.

rebased and test is updated.

Could you submit it on my behalf?

Rebased to 105a270028ab and rename pacPlt to zPacPlt is applied.

LGTM

I don't have commit right, could you please commit it on my behalf?

Patch is rebased due to conflict with the upstream changes.

Thanks for the review, I have no push rights, could you please submit it on my behalf?