[Mlir-commits] [flang] [mlir] [Flang][MLIR][OpenMP] - Add support for firstprivate when translating omp.target ops from MLIR to LLVMIR (PR #131213)

[Pranav Bhandarkar]

================
@@ -161,9 +161,10 @@ def PrivateClauseOp : OpenMP_Op<"private", [IsolatedFromAbove, RecipeInterface]>
     /// needsMap returns true if the value being privatized should additionally
     /// be mapped to the target region using a MapInfoOp. This is most common
     /// when an allocatable is privatized. In such cases, the descriptor is used
-    /// in privatization and needs to be mapped on to the device.
+    /// in privatization and needs to be mapped on to the device. The use of
+    /// firstprivate also creates the need to map the host variable to the device.
     bool needsMap() {
-      return initReadsFromMold();
+      return readsFromMold();
----------------
bhandarkar-pranav wrote:

`readsFromMold` factors in the copy region too. When offloading, we have to ensure that any host variable needed by the target is mapped to the target. The `MoldArg` of the `copy` region is one such host variable needed on the target. Without this change something as simple as this doesn't compile
```
program test0
  implicit none
  integer :: a
  !$omp target firstprivate(a)
  !$omp end target
end program test0
```
Failing in the following manner during MLIR to LLVMIR translation. 
```
Referring to an instruction in another function!
  %0 = load i32, ptr %1, align 4
While deleting: ptr %
Use still stuck around after Def is destroyed:  %0 = load i32, ptr <badref>, align 4
```

Note that in the above, the `init` region is not generated and so the host `a` is never mapped to the target because `needsMap` looks at the `init` region only.
This is the MLIR for the above (Showing only the relevant parts
```
 omp.private {type = firstprivate} @_QFEa_firstprivate_i32 : i32 copy {
  ^bb0(%arg0: !fir.ref<i32>, %arg1: !fir.ref<i32>):
    %0 = fir.load %arg0 : !fir.ref<i32>
    hlfir.assign %0 to %arg1 : i32, !fir.ref<i32>
    omp.yield(%arg1 : !fir.ref<i32>)
  }
  func.func @_QQmain() attributes {fir.bindc_name = "test0"} {
   ...
  omp.target private(@_QFEa_firstprivate_i32 %1#0 -> %arg0 : !fir.ref<i32>) {
      %11:2 = hlfir.declare %arg0 {uniq_name = "_QFEa"} : (!fir.ref<i32>) -> (!fir.ref<i32>, !fir.ref<i32>)
      omp.terminator
    }
```

https://github.com/llvm/llvm-project/pull/131213