[Mlir-commits] [mlir] [mlir][OpenMP] inscan reduction modifier and scan op mlir support (PR #114737)

[Kiran Chandramohan]

================
@@ -1560,6 +1560,26 @@ def CancellationPointOp : OpenMP_Op<"cancellation_point", clauses = [
   let hasVerifier = 1;
 }
 
+def ScanOp : OpenMP_Op<"scan", [
----------------
kiranchandramohan wrote:

The 2-region option proposed by @skatrak would be easier to interface with the OpenMPIRBuilder during translation. 

The 1-region option might be easier for the lowering in Flang (parse-tree to MLIR).

But just want to understand whether the no-region option will definitely lead to an issue.

> However, MLIR transformation passes are able to move operations within a region in a way that would potentially break scan.
Are these Operations moving across the scan operation? Would these be only constants or something else? Assuming the scan operation is side-effecting will operations cross it?

Generally, from a flang point of view, we typically have loading and storing from memory/variables. Since all variables are from outside the loop region these will be visible in both `pre` and `post`. This will change if we have performed load-store forwarding or mem2reg. Also, I wonder about the case where all the contents of the block are inside  a block construct. In this case the alloca for `j` will not be visible in the `post` region and would need yielding.
```
program mn
  do i=1,10
  block
    integer :: j

   scan directive
    = j
  end block
  end do
end program
```

https://github.com/llvm/llvm-project/pull/114737