[clang] [Clang][CodeGen] Start migrating away from assuming the Default AS is 0 (PR #88182)

[Jessica Clarke via cfe-commits]

jrtc27 wrote:

> > I'm not quite sure how to parse this comment, could you explain what you have in mind here? The problem is precisely that the FE assumes 0 is fine / picks it by default, which ends up into dangerzones when e.g. a target happened to use 0 to point to private (stack). I feel as if I'm missing the core of your comment though, so apologies in advance.
> 
> I'm just saying that I don't think it makes any sense to add a concept of a default AS to LLVM. The "default" AS is a frontend concept, not a middle-end / back-end concept. LLVM would only need a default AS if it were inventing a memory allocation/operation from whole cloth, which is generally not something LLVM should ever be doing; the only legitimate counter-example I can think of would be something like materializing a constant into constant global memory, in which case LLVM needs to assign the new constant an AS.

Libcall emission needs to know what AS to use for pointer arguments to things like __sync/__atomic implementations and various string-y mem*/str* functions. That's the main one that comes to mind from our experience in CHERI LLVM, and current LLVM just assumes that's AS0.

https://github.com/llvm/llvm-project/pull/88182