[Mlir-commits] [mlir] [mlir][SCF] Add `RecursiveMemoryEffects` to `scf.reduce` (PR #75314)

[Ivan Butygin]

Hardcode84 wrote:

> Why is this a problem to fold it away?

`scf.reduce` has a weird semantics, it describes how values should be reduced between `scf.parallel` loop interations. `scf.reduce` ops count must match reduction variables count and they should never be removed.

```
  %0:2 = scf.parallel (%i0, %i1) = (%c1, %c3) to (%c2, %c6) step (%c1, %c3) init(%A, %B) -> (index, index) {
    scf.reduce(%i0) : index {
    ^bb0(%lhs: index, %rhs: index):
      %1 = arith.addi %lhs, %rhs : index
      scf.reduce.return %1 : index
    }
    scf.reduce(%i1) : index {
    ^bb0(%lhs: index, %rhs: index):
      %2 = arith.muli %lhs, %rhs : index
      scf.reduce.return %2 : index
    }
    scf.yield
  }
```

https://github.com/llvm/llvm-project/pull/75314