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

[Matthew Maurer via cfe-commits]

maurer wrote:

This is not a Rust concern, but re-reading the initial post, it *looks* like your own statistics suggest that consuming 3 bits for arity costs more than it buys you. As stated, (didn't check your math, just going off what you said) prior to your change, we expect 0.01383765 collisions in your sample environment. After your change, we expect to have the *sum* of your right hand column in collisions, which comes out to 0.0266774 - nearly double the rate of collisions we have with the basic implementation. In fact, I think that any scheme like this will always going to increase the number of overall collisions, given that the arity is implicitly hashed into the representation already.

The main reason I could see to consider this is if for some reason a cross-arity collision is more dangerous than a same-arity collision in terms of exploitability, which I can't immediately argue, but perhaps you have something for this that was just assumed in the initial post?

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