[llvm] [NVPTX] Make GlobalUniqueCallSite a member of NVPTXISelLowering (PR #130212)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

> Can you elaborate on the problem you're trying to solve? What kind of race conditions do you see while updating an atomic variable?
> 
> Moving the counter into NVPTXTargetLowering, instead of relying on a global counter, is fine in principle, I just want to understand what's going on.

The problem is that if a process is doing multiple distinct compilations of different modules they will share GlobalUniqueCallSite. As a result, depending on the order in which these compilations occur, the emitted PTX will be different. With this change the emitted PTX of one compilation will not be changed by whatever compilations were run before. Of course this won't be an issue for llc but we have encountered this internally. 

Even setting that aside, I think it is probably a little bit cleaner/preferable to use a class member over a global variable.  

https://github.com/llvm/llvm-project/pull/130212