[llvm] [clang-tools-extra] [clang] [InstCombine] Convert or concat to fshl if opposite or concat exists (PR #68502)

[Nikita Popov via cfe-commits]

nikic wrote:

Thanks for the updated example!

To explain what I meant in first comment using this example: We would perform the transform https://alive2.llvm.org/ce/z/nllcB_, which does not depend at all on how `%yx` is constructed, and whether there is any way to form the `fshl` separately. If the `%yx` is appropriately constructed, the `fshl` can be removed (https://alive2.llvm.org/ce/z/B_KOwv, another missing transform).

Is this not a viable approach? Is there a concern here that generating both fshl and bitreverse may be non-profitable for targets without bitreverse? Or maybe supporting this makes the matching too expensive?

https://github.com/llvm/llvm-project/pull/68502