[clang] [llvm] [X86] Enhance kCFI type IDs with a 3-bit arity indicator. (PR #117121)

[Phoebe Wang via llvm-commits]

phoebewang wrote:

> > > @phoebewang and @lvwr I also noticed that there is this code in LLVM:
> > > https://github.com/llvm/llvm-project/blob/9ba6672b9f0e82a1f6d4100dc832c84447ea545c/llvm/lib/Transforms/Utils/ModuleUtils.cpp#L202-L214
> > > 
> > > . As far as I can tell, this code is not triggered when I build the Linux kernel with `-fsanitize=kcfi`.
> > > When is this code triggered? And do you think it is necessary to additionally implement the arity-enhancement to this code?
> > 
> > 
> > I'm not familar with KCFI. I find it's added by @samitolvanen in [e1c36bd](https://github.com/llvm/llvm-project/commit/e1c36bde0551977d4b2efae032af6dfc4b2b3936). I think you should triger it with attached test case.
> 
> It looks to me like this code might be triggered in some LTO configurations, and/or when linking code compiled from multiple source languages with the expectation that the KCFI type IDs will be compatible. Is my understanding correct?

Looks like the latter, see https://github.com/llvm/llvm-project/commit/71c7313f42d2b6063fea09854cf4fc46fd0627e1

https://github.com/llvm/llvm-project/pull/117121