From bdfd1097572dd0f7334515d77cfc8b5984ff873c Mon Sep 17 00:00:00 2001
From: Michel Wortmann <michel.wortmann@ecmwf.int>
Date: Mon, 17 Nov 2025 14:11:13 +0000
Subject: [PATCH 1/2] Prototypes of MPL_CREATE_GROUP_COMMUNICATOR and
 MPL_CREATE_UNION_COMMUNICATOR subroutines.

---
 src/fiat/mpl/internal/mpl_groups.F90 | 31 ++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/src/fiat/mpl/internal/mpl_groups.F90 b/src/fiat/mpl/internal/mpl_groups.F90
index 2b992311..3abd2aca 100644
--- a/src/fiat/mpl/internal/mpl_groups.F90
+++ b/src/fiat/mpl/internal/mpl_groups.F90
@@ -31,6 +31,7 @@ MODULE MPL_GROUPS
 USE MPL_MPIF
 USE MPL_DATA_MODULE
 USE MPL_MESSAGE_MOD
+USE MPL_ABORT_MOD
 
 IMPLICIT NONE
 PRIVATE
@@ -143,6 +144,36 @@ SUBROUTINE MPL_CART_COORDS(KPROC, KPROCW, KPROCV)
 
 END SUBROUTINE MPL_CART_COORDS
 
+SUBROUTINE MPL_CREATE_GROUP_COMMUNICATOR(NPROC, OFFSET, NEW_GROUP, NEW_COMM)
+    INTEGER, INTENT(IN) :: NPROC, OFFSET
+    INTEGER, INTENT(OUT) :: NEW_GROUP, NEW_COMM
+    INTEGER, ALLOCATABLE :: RANKS(:)
+    INTEGER :: I, SIZE, IERR, WORLD_GROUP
+
+    CALL MPI_COMM_SIZE(MPI_COMM_WORLD, SIZE, IERR)
+
+    IF (OFFSET+NPROC > SIZE) THEN
+        CALL MPL_ABORT("MPL_CREATE_GROUP_COMMUNICATOR: OFFSET+NPROC exceeds MPI_COMM_SIZE")
+    END IF
+
+    ALLOCATE(RANKS(NPROC))
+    RANKS = [(I, I=OFFSET, OFFSET+NPROC-1)]
+
+    CALL MPI_COMM_GROUP(MPI_COMM_WORLD, WORLD_GROUP, IERR)
+    CALL MPI_GROUP_INCL(WORLD_GROUP, NPROC, RANKS, NEW_GROUP, IERR)
+    CALL MPI_COMM_CREATE(MPI_COMM_WORLD, NEW_GROUP, NEW_COMM, IERR)
+
+END SUBROUTINE MPL_CREATE_GROUP_COMMUNICATOR
+
+SUBROUTINE MPL_CREATE_UNION_COMMUNICATOR(GROUP1, GROUP2, UNION_COMM)
+    INTEGER, INTENT(IN) :: GROUP1, GROUP2
+    INTEGER, INTENT(OUT) :: UNION_COMM
+    INTEGER :: IERR, UNION_GROUP
+
+    CALL MPI_GROUP_UNION(GROUP1, GROUP2, UNION_GROUP, IERR)
+    CALL MPI_COMM_CREATE(MPI_COMM_WORLD, UNION_GROUP, UNION_COMM, IERR)
+
+END SUBROUTINE MPL_CREATE_UNION_COMMUNICATOR
 ! ------------------------------------------------------------------
 
 END MODULE MPL_GROUPS

From 1d1a6638ebc89af70c3d42c8abf3e60eaa5f8535 Mon Sep 17 00:00:00 2001
From: Michel Wortmann <michel.wortmann@ecmwf.int>
Date: Thu, 27 Nov 2025 14:35:33 +0000
Subject: [PATCH 2/2] Add create group and union communicator routines to
 public.

---
 src/fiat/mpl/internal/mpl_groups.F90 | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/fiat/mpl/internal/mpl_groups.F90 b/src/fiat/mpl/internal/mpl_groups.F90
index 3abd2aca..e7868f78 100644
--- a/src/fiat/mpl/internal/mpl_groups.F90
+++ b/src/fiat/mpl/internal/mpl_groups.F90
@@ -36,7 +36,8 @@ MODULE MPL_GROUPS
 IMPLICIT NONE
 PRIVATE
 PUBLIC MPL_COMM_GRID, MPL_ALL_LEVS_COMM, MPL_ALL_MS_COMM, &
-     & MPL_GROUPS_CREATE, MPL_CART_RANK, MPL_CART_COORDS
+     & MPL_GROUPS_CREATE, MPL_CART_RANK, MPL_CART_COORDS, MPL_CREATE_UNION_COMMUNICATOR, &
+     & MPL_CREATE_GROUP_COMMUNICATOR
 
 INTEGER(KIND=JPIM) :: MPL_COMM_GRID, MPL_ALL_LEVS_COMM, MPL_ALL_MS_COMM, &
            & MPL_GP_GRID