[PATCH] D78397: [mlir][Transforms] Add pass to perform sparse conditional constant propagation

[River Riddle via Phabricator via llvm-commits]

rriddle added a comment.

In D78397#1990753 <https://reviews.llvm.org/D78397#1990753>, @fhahn wrote:

> Conceptually this looks very similar to LLVM's SCCP. Do you anticipate using any MLIR specific properties to cover more cases than the LLVM version? I am not really up-to-speed with the latest on MLIR dialects, but to me it seems like there is a set of transformations where there is currently no additional information to use from MLIR dialects compared to LLVM IR, like SCCP or GVN.
>
> Viewed very simplistically, they only require the following from an IR:  SSA, a way to traverse a function along CFG edges, a way to simplify instructions and a way to replace values. Do you think it would be feasible to provide some kind of interface to abstract those and then work towards sharing the implementations of the underlying algorithms between LLVM & MLIR? Or is the plan to duplicate various passes from LLVM also in MLIR?


Thanks for the comment Florian! I would be very +1 on sharing common implementations of the more generalized algorithms if we can. From a technical perspective, the major problems I've encountered with bridging the gap are a few impedance mismatches between LLVM and MLIR that arise from different implementation decisions:

- The multi-level aspect leads to special handling for various constructs in MLIR(e.g. regions/structured control flow/etc.) that aren't present in LLVM.
- MLIR globals(like functions) are not SSA values, and require specific handling because they do not use or interact with the traditional SSA use list.
- Block arguments vs PHIs

When adding things in MLIR that already exist in LLVM, at least for me, it is a cost-benefit computation between the amount of work it would take to refactor the thing in LLVM to be usable by MLIR. By amount of work I mean not only the technical aspects, but also the effort to convince the community that it is the right thing to do. I'd love to share as much as possible(as we do for things like ADT/Support/etc) and I'm happy to collaborate in that direction, but if I'm driving it alone the cost has often out-weighed the benefits.


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