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

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HaohaiWen 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?

It's absolutely a feasible solution.
Solution1:
First optimize bitreverse then eliminate redundant fshl: https://alive2.llvm.org/ce/z/g_gWf3
This requires
a) First teach collectBitParts to not only search until unknown opcode, but also try to use itself as root.
b) Teach recognizeBSwapOrBitReverseIdiom to recognize bit pattern [n/2 -1, ..., 1, 0, n-1, n-2, .... n/2]. Then insert bitreverse and fshl.
c) Teach instcombine to remove redundant fshl if opposite concat exists. This requires to scan def-users chains.




To eliminate fshl, we also need to scan def->users chains.

Solution2:
First optimize or to fshl then optimize bitreverse: https://alive2.llvm.org/ce/z/WbzJVo



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