Rewriting pointer arithmetic: Newbie question

Dear Clang Devs:

I’m working on an extension to C, and using clang to do it. I’ve been able to successfully create a type attribute, which is associated with pointer types. Now, these extensions require that I rewrite pointer arithmetic to use specific runtime calls from a library that I’m linking into these files. It’s not clear to me where to even begin looking for the right place to do these rewrites inside of clang. I’d appreciate some help in where the appropriate place to rewrite this arithmetic would be.