[clang] [lld] [llvm] [mlir] [IR] Introduce `T<address space>` to `DataLayout` to represent flat address space if a target supports it (PR #108786)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> Both in InferAddressSpaces, and in Attributor, you don't really care about whether a flat address-space exists. 

Right, this is more of an undesirable address space. Optimizations don't need to know anything about its behavior beyond that.


> In reply to your question above re whether this is a DL or a Target property, I don't have a strong opinion there (it appears @shiltian and @arsenm might). 

I don't really like putting this in the DataLayout. My original idea was to move it to TargetMachine, but we want to avoid the dependence on CodeGen. The DataLayout is just the other place we have that defines module level target information. The simple solution is just have a switch over the target architecture in Attributor.

> I do believe that this is a necessary bit of query-able information, especially from a Clang, for correctness reasons (more on that below).

I don't think this buys frontends much. Clang still needs to understand the full language address space -> target address space mapping. This would just allow populating one entry generically


> Ah, this is part of the challenge - we do indeed assume that 0 is flat, but Targets aren't bound by LangRef to use 0 to denote flat (and some, like SPIR / SPIR-V) do not

As I mentioned above, SPIRV can just work its way out of this problem for its IR. SPIR's only reason for existence is bitcode compatibility, so doing anything with there will be quite a lot of work which will never realistically happen. 


> I'm fine with adding the enforcement in LLVM that AS0 needs to be the flat AS, if a target has it, but the definition of a flat AS still needs to be set. If we do that, how will SPIR/SPIR-V work?
> This is the most generic wording I can come up with so far. Happy to hear more feedbacks.

I would like to avoid adding additional special properties to AS0, or defining the flat concept. 



https://github.com/llvm/llvm-project/pull/108786