[RFC][MLIR] OpenMP Loop Transformation : tile and unroll directive operation support for omp.dialect

[kiranchandramohan]

@ftynse

In this light, since we don’t have the op equivalent of canonical loop nest in the dialect, we can’t really attach the directives to something using attributes or regions because the foreign-dialect attributes are discardable and the own-dialect attributes or regions are intrusive in the client dialect.

I did not fully understand this point. From your original proposal, the attributes are always on the OpenMP dialect operations. For the workshare-loop it is on omp.wsloop, and so is it for the omp.transform_loop standalone operation. There are no foreign-dialect attributes here.

@ftynse

An alternative approach using modern infrastructure would be to introduce an OpenMPCanonicalLoopOpInterface that gives the OpenMP dialect and lowering access to the loop aspects they need like the iteration variable and the body. SCF ForOp can then have an external implementation of the interface registered by the OpenMP dialect, FIR can take a similar route if it doesn’t want to depend on OpenMP with a fully separate registration. (Note that this was not technically possible when we added wsloop.)

When we discussed this before, there were some strong objections against the nested approach and there was a requirement that worksharing loop be represented as a loop. Some of it is captured in this comment. Would adding the OpenMPLoopInterface to the scf.for or fir.do_loop be sufficient to prevent transformations of these loops by other passes?