[llvm-branch-commits] [flang] [flang][OpenMP] Handle "loop-local values" in `do concurrent` nests (PR #127635)

[Sergio Afonso via llvm-branch-commits]

================
@@ -202,6 +202,57 @@ variables: `i` and `j`. These are locally allocated inside the parallel/target
 OpenMP region similar to what the single-range example in previous section
 shows.
 
+### Data environment
+
+By default, variables that are used inside a `do concurrent` loop nest are
+either treated as `shared` in case of mapping to `host`, or mapped into the
+`target` region using a `map` clause in case of mapping to `device`. The only
+exceptions to this are:
+  1. the loop's iteration variable(s) (IV) of **perfect** loop nests. In that
+     case, for each IV, we allocate a local copy as shown by the mapping
+     examples above.
+  1. any values that are from allocations outside the loop nest and used
+     exclusively inside of it. In such cases, a local privatized
+     copy is created in the OpenMP region to prevent multiple teams of threads
+     from accessing and destroying the same memory block, which causes runtime
+     issues. For an example of such cases, see
+     `flang/test/Transforms/DoConcurrent/locally_destroyed_temp.f90`.
+
+Implicit mapping detection (for mapping to the target device) is still quite
+limited and work to make it smarter is underway for both OpenMP in general 
+and `do concurrent` mapping.
----------------
skatrak wrote:

Nit: There's no mapping support at this stage, so maybe state that to avoid misleading anyone reading it.

https://github.com/llvm/llvm-project/pull/127635