[Mlir-commits] [mlir] [MLIR][OpenMP] Add omp.simd operation (PR #79843)

[Jan Leyonberg]

jsjodin wrote:

Using wrapper ops seems to be a better option imo since it should be easier to extend and we avoid the combinatorial explosion. I have one question about if there are cases where writing something on two separate omp lines vs a single line would change the semantics, e.g. in one case it is a combined construct but the other it isn't? I believe another issue with not using wrapper ops is that it may not be possible to create the combined ops directly if there are loop transformation ops present, which means the wrapper ops will have to be used anyway. To clarify, if the loop transformations are handled by the OpenMPIRBuilder, then there is no point in the compilation where combined ops could be created. I'm not convinced this is the right approach though, it would make sense to me to to the loop transformations first and then do the lowering.

There is another problem which @DominikAdamski was mentioning and that I've been trying to get some clarity on, which is how to communicate information between op lowerings since there are dependencies between e.g. distribute and parallel/wsloop, this will have to be solved. In the case of loops there is a proposal to use CLIs, but this is only one kind of information that needs to be communicated, another is reduction information, and there are other cases as well (collapse?). I think it makes sense that the OpenMPIRBuilder keeps track of these things. @kiranchandramohan suggested using block arguments for CLIs, which probably fits better with this approach, and the solution would be more uniform compared to special handling for CLIs. Another option would be to have other kind of lowering information represented in MLIR as values like CLIs, but it might degenerate if CLIs get expanded to hold other information into having a single value linking all the omp ops, which would just represent the state in the OpenMPIRBuilder. This is why I"m leaning towards wrapper ops with a simple recursive traversal and the OpenMPIRBuilder keeping the information that needs to be passed between the op lowerings. 

https://github.com/llvm/llvm-project/pull/79843