From 9ad3754ed74fec808a283fb895276a9cdb8ed983 Mon Sep 17 00:00:00 2001
From: Mark Olesen <Mark.Olesen@esi-group.com>
Date: Wed, 24 Jan 2024 12:58:20 +0100
Subject: [PATCH] ENH: remove obsolete Pstream functions (#3087)

- the old Pstream::scatter routines (which were largely a misnomer)
  have been superseded by various broadcast routines, but were left in
  the code with #ifndef/#ifdef Foam_Pstream_scatter_nobroadcast
  guards. Now noisily deprecate them, and remove the old manual tree
  communication in favour of MPI broadcast and/or
  serialize/de-serialize with wrapped Pstream::broadcast

- consolidate various gather methods to include the communication
  structure directly. No functional change, but reduces the number of
  methods.

ENH: add parallel guard to UPstream::whichCommunication() method

- returns List::null() as the schedule for non-parallel instead
  of an inappropriate linear or tree schedule

ENH: Pstream::listGatherValues, Pstream::listScatterValues

- like the existing UPstream versions but supporting non-contiguous
---
 applications/test/gatherValues1/Make/files    |   2 +-
 ...gatherValues1.C => Test-gatherValues1.cxx} |  72 +--
 src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H  | 179 +++-----
 .../db/IOstreams/Pstreams/PstreamBroadcast.C  |  26 +-
 .../IOstreams/Pstreams/PstreamCombineGather.C | 414 +-----------------
 .../db/IOstreams/Pstreams/PstreamGather.C     | 236 ++++++----
 .../db/IOstreams/Pstreams/PstreamGatherList.C |  26 +-
 .../db/IOstreams/Pstreams/PstreamReduceOps.H  |  35 +-
 src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C |   1 +
 src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H |  23 +-
 .../db/IOstreams/Pstreams/UPstreamTemplates.C | 105 ++---
 11 files changed, 389 insertions(+), 730 deletions(-)
 rename applications/test/gatherValues1/{Test-gatherValues1.C => Test-gatherValues1.cxx} (81%)

diff --git a/applications/test/gatherValues1/Make/files b/applications/test/gatherValues1/Make/files
index 31bbb718f0..b5fc79cbe0 100644
--- a/applications/test/gatherValues1/Make/files
+++ b/applications/test/gatherValues1/Make/files
@@ -1,3 +1,3 @@
-Test-gatherValues1.C
+Test-gatherValues1.cxx
 
 EXE = $(FOAM_USER_APPBIN)/Test-gatherValues1
diff --git a/applications/test/gatherValues1/Test-gatherValues1.C b/applications/test/gatherValues1/Test-gatherValues1.cxx
similarity index 81%
rename from applications/test/gatherValues1/Test-gatherValues1.C
rename to applications/test/gatherValues1/Test-gatherValues1.cxx
index 69ce09ce12..02bb8597ed 100644
--- a/applications/test/gatherValues1/Test-gatherValues1.C
+++ b/applications/test/gatherValues1/Test-gatherValues1.cxx
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2021-2023 OpenCFD Ltd.
+    Copyright (C) 2021-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -52,7 +52,7 @@ int main(int argc, char *argv[])
 
     const labelList localValues
     (
-        identity(2 *(Pstream::myProcNo()+1), -5*Pstream::myProcNo())
+        identity(2 *(UPstream::myProcNo()+1), -5*UPstream::myProcNo())
     );
 
     // Test resize
@@ -76,8 +76,8 @@ int main(int argc, char *argv[])
         // One-sided sizing!  master only
         const globalIndex allProcAddr
         (
-            sendData.size(),
-            globalIndex::gatherOnly{}
+            globalIndex::gatherOnly{},
+            sendData.size()
         );
 
         Pout<< "listGather sizes: " << flatOutput(allProcAddr.sizes()) << nl;
@@ -98,8 +98,8 @@ int main(int argc, char *argv[])
         // One-sided sizing!  master only
         const globalIndex allProcAddr
         (
-            sendData.size(),
-            globalIndex::gatherOnly{}
+            globalIndex::gatherOnly{},
+            sendData.size()
         );
 
         Pout<< "listGather sizes: " << flatOutput(allProcAddr.sizes()) << nl;
@@ -116,7 +116,7 @@ int main(int argc, char *argv[])
     {
         const labelList::subList& sendData =
         (
-            Pstream::master()
+            UPstream::master()
           ? SubList<label>(localValues, 0)  // exclude
           : SubList<label>(localValues)
         );
@@ -147,11 +147,11 @@ int main(int argc, char *argv[])
             << UPstream::listScatterValues(subProcAddr.offsets()) << nl;
 
 
-        Pout<< endl << "local list [" << Pstream::myProcNo() << "] "
+        Pout<< endl << "local list [" << UPstream::myProcNo() << "] "
             << flatOutput(localValues) << nl;
 
 
-        Pout<< endl << "local send [" << Pstream::myProcNo() << "] "
+        Pout<< endl << "local send [" << UPstream::myProcNo() << "] "
             << sendSize << nl;
 
 
@@ -163,7 +163,7 @@ int main(int argc, char *argv[])
 
         Pout<< "off-proc: " << allValues << endl;
 
-        if (Pstream::master())
+        if (UPstream::master())
         {
             Info<< "master: " << flatOutput(localValues) << nl;
 
@@ -196,7 +196,7 @@ int main(int argc, char *argv[])
             {
                 globalIndex glob
                 (
-                    globalIndex:gatherNone{},
+                    globalIndex::gatherNone{},
                     labelList(Foam::one{}, 0)
                 );
                 Info<< "single:" << nl;
@@ -208,35 +208,37 @@ int main(int argc, char *argv[])
         }
     }
 
-    // This will likely fail - not declared as is_contiguous
-    // Cannot even catch since it triggers an abort()
-
-    #if 0
+    // Non-contiguous gather - use Pstream, not UPstream!
     {
-        std::pair<label,vector> sendData(Pstream::myProcNo(), vector::one);
+        typedef std::pair<label,vector> valueType;
 
-        const bool oldThrowingError = FatalError.throwing(true);
+        valueType sendData(UPstream::myProcNo(), vector::one);
 
-        try
-        {
-            List<std::pair<label,vector>> countValues
-            (
-                UPstream::listGatherValues<std::pair<label, vector>>
-                (
-                    sendData
-                )
-            );
+        List<valueType> countValues
+        (
+            Pstream::listGatherValues(sendData)
+        );
 
-            Pout<< "listGather: " << flatOutput(countValues) << nl;
-        }
-        catch (const Foam::error& err)
-        {
-            Info<< err.message().c_str() << nl;
-        }
-
-        FatalError.throwing(oldThrowingError);
+        Pout<< "listGather: " << flatOutput(countValues) << nl;
+    }
+
+    // Non-contiguous scatter - use Pstream, not UPstream!
+    {
+        List<fileName> allValues;
+
+        if (UPstream::master())
+        {
+            allValues.resize(UPstream::nProcs());
+            forAll(allValues, proci)
+            {
+                allValues[proci] = "processor" + Foam::name(proci);
+            }
+        }
+
+        fileName procName = Pstream::listScatterValues(allValues);
+
+        Pout<< "listScatter: " << procName << nl;
     }
-    #endif
 
     Info<< "\nEnd\n" << endl;
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
index dbcfe69c56..fe251cbf91 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
@@ -47,9 +47,6 @@ SourceFiles
 #include "UPstream.H"
 #include "DynamicList.H"
 
-// Legacy
-// #define Foam_Pstream_scatter_nobroadcast
-
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
 namespace Foam
@@ -130,22 +127,9 @@ public:
 
     // Gather
 
-        //- Gather (reduce) data, appyling \c bop to combine \c value
-        //- from different processors. The basis for Foam::reduce().
-        //  Uses the specified communication schedule.
-        template<class T, class BinaryOp>
-        static void gather
-        (
-            const List<commsStruct>& comms,
-            T& value,
-            const BinaryOp& bop,
-            const int tag,
-            const label comm
-        );
-
         //- Gather (reduce) data, applying \c bop to combine \c value
         //- from different processors. The basis for Foam::reduce().
-        //  Uses linear/tree communication.
+        //  Uses linear/tree communication (with parallel guard).
         template<class T, class BinaryOp>
         static void gather
         (
@@ -155,6 +139,34 @@ public:
             const label comm = UPstream::worldComm
         );
 
+        //- Gather individual values into list locations.
+        //  On master list length == nProcs, otherwise zero length.
+        //  \n
+        //  For \b non-parallel :
+        //  the returned list length is 1 with localValue.
+        template<class T>
+        static List<T> listGatherValues
+        (
+            const T& localValue,
+            const label comm = UPstream::worldComm,
+            //! Only used for non-contiguous types
+            const int tag = UPstream::msgType()
+        );
+
+        //- Scatter individual values from list locations.
+        //  On master input list length == nProcs, ignored on other procs.
+        //  \n
+        //  For \b non-parallel :
+        //  returns the first list element (or default initialized).
+        template<class T>
+        static T listScatterValues
+        (
+            const UList<T>& allValues,
+            const label comm = UPstream::worldComm,
+            //! Only used for non-contiguous types
+            const int tag = UPstream::msgType()
+        );
+
 
     // Gather/combine data
     // Inplace combine values from processors.
@@ -162,39 +174,10 @@ public:
 
         //- Gather data, applying \c cop to inplace combine \c value
         //- from different processors.
-        //  Uses the specified communication schedule.
+        //  Uses linear/tree communication (with parallel guard).
         template<class T, class CombineOp>
         static void combineGather
         (
-            const List<commsStruct>& comms,
-            T& value,
-            const CombineOp& cop,
-            const int tag,
-            const label comm
-        );
-
-        //- Gather data, applying \c cop to inplace combine \c value
-        //- from different processors.
-        //  Uses linear/tree communication.
-        template<class T, class CombineOp>
-        static void combineGather
-        (
-            T& value,
-            const CombineOp& cop,
-            const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm
-        );
-
-        //- Reduce inplace (cf. MPI Allreduce)
-        //- applying \c cop to inplace combine \c value
-        //- from different processors.
-        //- After completion all processors have the same data.
-        //  Uses the specified communication schedule.
-        //  Wraps combineGather/broadcast (may change in the future).
-        template<class T, class CombineOp>
-        static void combineReduce
-        (
-            const List<commsStruct>& comms,
             T& value,
             const CombineOp& cop,
             const int tag = UPstream::msgType(),
@@ -232,17 +215,8 @@ public:
 
     // Combine variants working on whole List at a time.
 
-        template<class T, class CombineOp>
-        static void listCombineGather
-        (
-            const List<commsStruct>& comms,
-            List<T>& values,
-            const CombineOp& cop,
-            const int tag,
-            const label comm
-        );
-
-        //- Like above but switches between linear/tree communication
+        //- Combines List elements.
+        //  Uses linear/tree communication (with parallel guard).
         template<class T, class CombineOp>
         static void listCombineGather
         (
@@ -252,7 +226,9 @@ public:
             const label comm = UPstream::worldComm
         );
 
+        //- Combines List elements.
         //- After completion all processors have the same data.
+        //  Uses linear/tree communication (with parallel guard).
         template<class T, class CombineOp>
         static void listCombineReduce
         (
@@ -279,17 +255,8 @@ public:
     // Combine variants working on whole map at a time.
     // Container needs iterators, find() and insert methods defined.
 
-        template<class Container, class CombineOp>
-        static void mapCombineGather
-        (
-            const List<commsStruct>& comms,
-            Container& values,
-            const CombineOp& cop,
-            const int tag,
-            const label comm
-        );
-
-        //- Like above but switches between linear/tree communication
+        //- Combine Map elements.
+        //  Uses linear/tree communication (with parallel guard).
         template<class Container, class CombineOp>
         static void mapCombineGather
         (
@@ -355,7 +322,7 @@ public:
         );
 
         //- Gather data, but keep individual values separate.
-        //- Uses linear/tree communication.
+        //- Uses MPI_Allgather or manual linear/tree communication.
         //  After completion all processors have the same data.
         //  Wraps gatherList/scatterList (may change in the future).
         template<class T>
@@ -369,91 +336,43 @@ public:
 
         // Scatter
 
-            //- Broadcast data: Distribute without modification.
-            //  \note comms and tag parameters only used when
-            //  Foam_Pstream_scatter_nobroadcast is defined
-            template<class T>
-            static void scatter
-            (
-                const List<commsStruct>& comms,
-                T& value,
-                const int tag,
-                const label comm
-            );
-
-            //- Broadcast data: Distribute without modification.
-            //  \note tag parameter only used when
-            //  Foam_Pstream_scatter_nobroadcast is defined
+            //- Broadcast data
             template<class T>
+            FOAM_DEPRECATED_FOR(2024-01, "broadcast()")
             static void scatter
             (
                 T& value,
-                const int tag = UPstream::msgType(),
+                const int tag = UPstream::msgType(),  //!< ignored
                 const label comm = UPstream::worldComm
             );
 
-            //- Broadcast data: Distribute without modification.
-            //  \note tag parameter only used when
-            //  Foam_Pstream_scatter_nobroadcast is defined
-            template<class T>
-            static void combineScatter
-            (
-                const List<commsStruct>& comms,
-                T& value,
-                const int tag,
-                const label comm
-            );
-
-            //- Broadcast data: Distribute without modification.
-            //  \note tag parameter only used when
-            //  Foam_Pstream_scatter_nobroadcast is defined
+            //- Broadcast data
             template<class T>
+            FOAM_DEPRECATED_FOR(2024-01, "broadcast()")
             static void combineScatter
             (
                 T& value,
-                const int tag = UPstream::msgType(),
+                const int tag = UPstream::msgType(),  //!< ignored
                 const label comm = UPstream::worldComm
             );
 
-            //- Broadcast data: Distribute without modification.
-            //  \note comms and tag parameters only used when
-            //  Foam_Pstream_scatter_nobroadcast is defined
-            template<class T>
-            static void listCombineScatter
-            (
-                const List<commsStruct>& comms,
-                List<T>& value,
-                const int tag,
-                const label comm
-            );
-
-            //- Broadcast data: Distribute without modification.
-            //  \note comms and tag parameters only used when
-            //  Foam_Pstream_scatter_nobroadcast is defined
+            //- Broadcast data
             template<class T>
+            FOAM_DEPRECATED_FOR(2024-01, "broadcast()")
             static void listCombineScatter
             (
                 List<T>& value,
-                const int tag = UPstream::msgType(),
+                const int tag = UPstream::msgType(),  //!< ignored
                 const label comm = UPstream::worldComm
             );
 
-            //- Broadcast data: Distribute without modification.
-            template<class Container>
-            static void mapCombineScatter
-            (
-                const List<commsStruct>& comms,
-                Container& values,
-                const int tag,
-                const label comm
-            );
-
-            //- Like above but switches between linear/tree communication
+            //- Broadcast data
             template<class Container>
+            FOAM_DEPRECATED_FOR(2024-01, "broadcast()")
             static void mapCombineScatter
             (
                 Container& values,
-                const int tag = UPstream::msgType(),
+                const int tag = UPstream::msgType(),  //!< ignored
                 const label comm = UPstream::worldComm
             );
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.C
index 1f39cca693..e9e2ce98bc 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2022-2023 OpenCFD Ltd.
+    Copyright (C) 2022-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -137,4 +137,28 @@ void Foam::Pstream::broadcastList(ListType& list, const label comm)
 }
 
 
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+// Convenience wrappers - defined after all specialisations are known
+
+namespace Foam
+{
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+//- Return a broadcasted value (uses a copy internally)
+template<class Type>
+Type returnBroadcast(const Type& value, const label comm)
+{
+    Type work(value);
+    Pstream::broadcast(work, comm);
+    return work;
+}
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+} // End namespace Foam
+
+
 // ************************************************************************* //
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamCombineGather.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamCombineGather.C
index 41d427635b..4696d00592 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamCombineGather.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamCombineGather.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2019-2023 OpenCFD Ltd.
+    Copyright (C) 2019-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -25,7 +25,7 @@ License
     along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
 
 Description
-    Variant of gather, scatter.
+    Variant of gather.
     Normal gather uses:
     - default construct and read (>>) from Istream
     - binary operator and assignment operator to combine values
@@ -46,7 +46,6 @@ Description
 template<class T, class CombineOp>
 void Foam::Pstream::combineGather
 (
-    const List<UPstream::commsStruct>& comms,
     T& value,
     const CombineOp& cop,
     const int tag,
@@ -55,8 +54,10 @@ void Foam::Pstream::combineGather
 {
     if (UPstream::is_parallel(comm))
     {
-        // My communication order
-        const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
+        // Communication order
+        const auto& comms = UPstream::whichCommunication(comm);
+        // if (comms.empty()) return;  // extra safety?
+        const auto& myComm = comms[UPstream::myProcNo(comm)];
 
         // Receive from my downstairs neighbours
         for (const label belowID : myComm.below())
@@ -89,7 +90,7 @@ void Foam::Pstream::combineGather
                 (
                     UPstream::commsTypes::scheduled,
                     belowID,
-                    0,
+                    0,  // bufsize
                     tag,
                     comm
                 );
@@ -132,7 +133,7 @@ void Foam::Pstream::combineGather
                 (
                     UPstream::commsTypes::scheduled,
                     myComm.above(),
-                    0,
+                    0,  // bufsize
                     tag,
                     comm
                 );
@@ -143,144 +144,6 @@ void Foam::Pstream::combineGather
 }
 
 
-template<class T>
-void Foam::Pstream::combineScatter
-(
-    const List<UPstream::commsStruct>& comms,
-    T& value,
-    const int tag,
-    const label comm
-)
-{
-    #ifndef Foam_Pstream_scatter_nobroadcast
-    Pstream::broadcast(value, comm);
-    #else
-    if (UPstream::is_parallel(comm))
-    {
-        // My communication order
-        const UPstream::commsStruct& myComm = comms[UPstream::myProcNo(comm)];
-
-        // Receive from up
-        if (myComm.above() != -1)
-        {
-            if (is_contiguous<T>::value)
-            {
-                UIPstream::read
-                (
-                    UPstream::commsTypes::scheduled,
-                    myComm.above(),
-                    reinterpret_cast<char*>(&value),
-                    sizeof(T),
-                    tag,
-                    comm
-                );
-            }
-            else
-            {
-                IPstream fromAbove
-                (
-                    UPstream::commsTypes::scheduled,
-                    myComm.above(),
-                    0,
-                    tag,
-                    comm
-                );
-                value = T(fromAbove);
-            }
-        }
-
-        // Send to my downstairs neighbours
-        forAllReverse(myComm.below(), belowI)
-        {
-            const label belowID = myComm.below()[belowI];
-
-            if (is_contiguous<T>::value)
-            {
-                UOPstream::write
-                (
-                    UPstream::commsTypes::scheduled,
-                    belowID,
-                    reinterpret_cast<const char*>(&value),
-                    sizeof(T),
-                    tag,
-                    comm
-                );
-            }
-            else
-            {
-                OPstream toBelow
-                (
-                    UPstream::commsTypes::scheduled,
-                    belowID,
-                    0,
-                    tag,
-                    comm
-                );
-                toBelow << value;
-            }
-        }
-    }
-    #endif
-}
-
-
-template<class T, class CombineOp>
-void Foam::Pstream::combineGather
-(
-    T& value,
-    const CombineOp& cop,
-    const int tag,
-    const label comm
-)
-{
-    Pstream::combineGather
-    (
-        UPstream::whichCommunication(comm),
-        value,
-        cop,
-        tag,
-        comm
-    );
-}
-
-
-template<class T>
-void Foam::Pstream::combineScatter
-(
-    T& value,
-    const int tag,
-    const label comm
-)
-{
-    #ifndef Foam_Pstream_scatter_nobroadcast
-    Pstream::broadcast(value, comm);
-    #else
-    Pstream::combineScatter
-    (
-        UPstream::whichCommunication(comm),
-        value,
-        tag,
-        comm
-    );
-    #endif
-}
-
-
-template<class T, class CombineOp>
-void Foam::Pstream::combineReduce
-(
-    const List<UPstream::commsStruct>& comms,
-    T& value,
-    const CombineOp& cop,
-    const int tag,
-    const label comm
-)
-{
-    Pstream::combineGather(comms, value, cop, tag, comm);
-    Pstream::broadcast(value, comm);
-}
-
-
 template<class T, class CombineOp>
 void Foam::Pstream::combineReduce
 (
@@ -292,9 +155,7 @@ void Foam::Pstream::combineReduce
 {
     if (UPstream::is_parallel(comm))
     {
-        const auto& comms = UPstream::whichCommunication(comm);
-
-        Pstream::combineGather(comms, value, cop, tag, comm);
+        Pstream::combineGather(value, cop, tag, comm);
         Pstream::broadcast(value, comm);
     }
 }
@@ -305,7 +166,6 @@ void Foam::Pstream::combineReduce
 template<class T, class CombineOp>
 void Foam::Pstream::listCombineGather
 (
-    const List<UPstream::commsStruct>& comms,
     List<T>& values,
     const CombineOp& cop,
     const int tag,
@@ -314,8 +174,10 @@ void Foam::Pstream::listCombineGather
 {
     if (UPstream::is_parallel(comm))
     {
-        // My communication order
-        const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
+        // Communication order
+        const auto& comms = UPstream::whichCommunication(comm);
+        // if (comms.empty()) return;  // extra safety?
+        const auto& myComm = comms[UPstream::myProcNo(comm)];
 
         // Receive from my downstairs neighbours
         for (const label belowID : myComm.below())
@@ -351,7 +213,7 @@ void Foam::Pstream::listCombineGather
                 (
                     UPstream::commsTypes::scheduled,
                     belowID,
-                    0,
+                    0,  // bufsize
                     tag,
                     comm
                 );
@@ -397,7 +259,7 @@ void Foam::Pstream::listCombineGather
                 (
                     UPstream::commsTypes::scheduled,
                     myComm.above(),
-                    0,
+                    0,  // bufsize
                     tag,
                     comm
                 );
@@ -408,129 +270,6 @@ void Foam::Pstream::listCombineGather
 }
 
 
-template<class T>
-void Foam::Pstream::listCombineScatter
-(
-    const List<UPstream::commsStruct>& comms,
-    List<T>& values,
-    const int tag,
-    const label comm
-)
-{
-    #ifndef Foam_Pstream_scatter_nobroadcast
-    Pstream::broadcast(values, comm);
-    #else
-    if (UPstream::is_parallel(comm))
-    {
-        // My communication order
-        const UPstream::commsStruct& myComm = comms[UPstream::myProcNo(comm)];
-
-        // Receive from up
-        if (myComm.above() != -1)
-        {
-            if (is_contiguous<T>::value)
-            {
-                UIPstream::read
-                (
-                    UPstream::commsTypes::scheduled,
-                    myComm.above(),
-                    values.data_bytes(),
-                    values.size_bytes(),
-                    tag,
-                    comm
-                );
-            }
-            else
-            {
-                IPstream fromAbove
-                (
-                    UPstream::commsTypes::scheduled,
-                    myComm.above(),
-                    0,
-                    tag,
-                    comm
-                );
-                fromAbove >> values;
-            }
-        }
-
-        // Send to my downstairs neighbours
-        forAllReverse(myComm.below(), belowI)
-        {
-            const label belowID = myComm.below()[belowI];
-
-            if (is_contiguous<T>::value)
-            {
-                UOPstream::write
-                (
-                    UPstream::commsTypes::scheduled,
-                    belowID,
-                    values.cdata_bytes(),
-                    values.size_bytes(),
-                    tag,
-                    comm
-                );
-            }
-            else
-            {
-                OPstream toBelow
-                (
-                    UPstream::commsTypes::scheduled,
-                    belowID,
-                    0,
-                    tag,
-                    comm
-                );
-                toBelow << values;
-            }
-        }
-    }
-    #endif
-}
-
-
-template<class T, class CombineOp>
-void Foam::Pstream::listCombineGather
-(
-    List<T>& values,
-    const CombineOp& cop,
-    const int tag,
-    const label comm
-)
-{
-    Pstream::listCombineGather
-    (
-        UPstream::whichCommunication(comm),
-        values,
-        cop,
-        tag,
-        comm
-    );
-}
-
-
-template<class T>
-void Foam::Pstream::listCombineScatter
-(
-    List<T>& values,
-    const int tag,
-    const label comm
-)
-{
-    #ifndef Foam_Pstream_scatter_nobroadcast
-    Pstream::broadcast(values, comm);
-    #else
-    Pstream::listCombineScatter
-    (
-        UPstream::whichCommunication(comm),
-        values,
-        tag,
-        comm
-    );
-    #endif
-}
-
-
 template<class T, class CombineOp>
 void Foam::Pstream::listCombineReduce
 (
@@ -542,9 +281,7 @@ void Foam::Pstream::listCombineReduce
 {
     if (UPstream::is_parallel(comm))
     {
-        const auto& comms = UPstream::whichCommunication(comm);
-
-        Pstream::listCombineGather(comms, values, cop, tag, comm);
+        Pstream::listCombineGather(values, cop, tag, comm);
         Pstream::broadcast(values, comm);
     }
 }
@@ -555,7 +292,6 @@ void Foam::Pstream::listCombineReduce
 template<class Container, class CombineOp>
 void Foam::Pstream::mapCombineGather
 (
-    const List<UPstream::commsStruct>& comms,
     Container& values,
     const CombineOp& cop,
     const int tag,
@@ -564,8 +300,10 @@ void Foam::Pstream::mapCombineGather
 {
     if (UPstream::is_parallel(comm))
     {
-        // My communication order
-        const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
+        // Communication order
+        const auto& comms = UPstream::whichCommunication(comm);
+        // if (comms.empty()) return;  // extra safety?
+        const auto& myComm = comms[UPstream::myProcNo(comm)];
 
         // Receive from my downstairs neighbours
         for (const label belowID : myComm.below())
@@ -576,7 +314,7 @@ void Foam::Pstream::mapCombineGather
             (
                 UPstream::commsTypes::scheduled,
                 belowID,
-                0,
+                0,  // bufsize
                 tag,
                 comm
             );
@@ -623,7 +361,7 @@ void Foam::Pstream::mapCombineGather
             (
                 UPstream::commsTypes::scheduled,
                 myComm.above(),
-                0,
+                0,  // bufsize
                 tag,
                 comm
             );
@@ -633,110 +371,6 @@ void Foam::Pstream::mapCombineGather
 }
 
 
-template<class Container>
-void Foam::Pstream::mapCombineScatter
-(
-    const List<UPstream::commsStruct>& comms,
-    Container& values,
-    const int tag,
-    const label comm
-)
-{
-    #ifndef Foam_Pstream_scatter_nobroadcast
-    Pstream::broadcast(values, comm);
-    #else
-    if (UPstream::is_parallel(comm))
-    {
-        // My communication order
-        const UPstream::commsStruct& myComm = comms[UPstream::myProcNo(comm)];
-
-        // Receive from up
-        if (myComm.above() != -1)
-        {
-            IPstream fromAbove
-            (
-                UPstream::commsTypes::scheduled,
-                myComm.above(),
-                0,
-                tag,
-                comm
-            );
-            fromAbove >> values;
-
-            if (debug & 2)
-            {
-                Pout<< " received from "
-                    << myComm.above() << " data:" << values << endl;
-            }
-        }
-
-        // Send to my downstairs neighbours
-        forAllReverse(myComm.below(), belowI)
-        {
-            const label belowID = myComm.below()[belowI];
-
-            if (debug & 2)
-            {
-                Pout<< " sending to " << belowID << " data:" << values << endl;
-            }
-
-            OPstream toBelow
-            (
-                UPstream::commsTypes::scheduled,
-                belowID,
-                0,
-                tag,
-                comm
-            );
-            toBelow << values;
-        }
-    }
-    #endif
-}
-
-
-template<class Container, class CombineOp>
-void Foam::Pstream::mapCombineGather
-(
-    Container& values,
-    const CombineOp& cop,
-    const int tag,
-    const label comm
-)
-{
-    Pstream::mapCombineGather
-    (
-        UPstream::whichCommunication(comm),
-        values,
-        cop,
-        tag,
-        comm
-    );
-}
-
-
-template<class Container>
-void Foam::Pstream::mapCombineScatter
-(
-    Container& values,
-    const int tag,
-    const label comm
-)
-{
-    #ifndef Foam_Pstream_scatter_nobroadcast
-    Pstream::broadcast(values, comm);
-    #else
-    Pstream::mapCombineScatter
-    (
-        UPstream::whichCommunication(comm),
-        values,
-        tag,
-        comm
-    );
-    #endif
-}
-
-
 template<class Container, class CombineOp>
 void Foam::Pstream::mapCombineReduce
 (
@@ -748,9 +382,7 @@ void Foam::Pstream::mapCombineReduce
 {
     if (UPstream::is_parallel(comm))
     {
-        const auto& comms = UPstream::whichCommunication(comm);
-
-        Pstream::mapCombineGather(comms, values, cop, tag, comm);
+        Pstream::mapCombineGather(values, cop, tag, comm);
         Pstream::broadcast(values, comm);
     }
 }
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGather.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGather.C
index daeb052876..f879f91283 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGather.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGather.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2019-2022 OpenCFD Ltd.
+    Copyright (C) 2019-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -41,7 +41,6 @@ Description
 template<class T, class BinaryOp>
 void Foam::Pstream::gather
 (
-    const List<UPstream::commsStruct>& comms,
     T& value,
     const BinaryOp& bop,
     const int tag,
@@ -50,8 +49,10 @@ void Foam::Pstream::gather
 {
     if (UPstream::is_parallel(comm))
     {
-        // My communication order
-        const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
+        // Communication order
+        const auto& comms = UPstream::whichCommunication(comm);
+        // if (comms.empty()) return;  // extra safety?
+        const auto& myComm = comms[UPstream::myProcNo(comm)];
 
         // Receive from my downstairs neighbours
         for (const label belowID : myComm.below())
@@ -76,7 +77,7 @@ void Foam::Pstream::gather
                 (
                     UPstream::commsTypes::scheduled,
                     belowID,
-                    0,
+                    0,  // bufsize
                     tag,
                     comm
                 );
@@ -107,7 +108,7 @@ void Foam::Pstream::gather
                 (
                     UPstream::commsTypes::scheduled,
                     myComm.above(),
-                    0,
+                    0,  // bufsize
                     tag,
                     comm
                 );
@@ -119,110 +120,181 @@ void Foam::Pstream::gather
 
 
 template<class T>
-void Foam::Pstream::scatter
+Foam::List<T> Foam::Pstream::listGatherValues
 (
-    const List<UPstream::commsStruct>& comms,
-    T& value,
-    const int tag,
-    const label comm
+    const T& localValue,
+    const label comm,
+    const int tag
 )
 {
-    #ifndef Foam_Pstream_scatter_nobroadcast
-    Pstream::broadcast(value, comm);
-    #else
+    // OR
+    // if (is_contiguous<T>::value)
+    // {
+    //     return UPstream::listGatherValues(localValue, comm);
+    // }
+
+    List<T> allValues;
+
     if (UPstream::is_parallel(comm))
     {
-        // My communication order
-        const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
+        const label numProc = UPstream::nProcs(comm);
 
-        // Receive from up
-        if (myComm.above() != -1)
+        if (UPstream::master(comm))
         {
-            if (is_contiguous<T>::value)
-            {
-                UIPstream::read
-                (
-                    UPstream::commsTypes::scheduled,
-                    myComm.above(),
-                    reinterpret_cast<char*>(&value),
-                    sizeof(T),
-                    tag,
-                    comm
-                );
-            }
-            else
-            {
-                IPstream fromAbove
-                (
-                    UPstream::commsTypes::scheduled,
-                    myComm.above(),
-                    0,
-                    tag,
-                    comm
-                );
-                fromAbove >> value;
-            }
+            allValues.resize(numProc);
         }
 
-        // Send to my downstairs neighbours. Note reverse order (compared to
-        // receiving). This is to make sure to send to the critical path
-        // (only when using a tree schedule!) first.
-        forAllReverse(myComm.below(), belowI)
+        if (is_contiguous<T>::value)
         {
-            const label belowID = myComm.below()[belowI];
+            UPstream::mpiGather
+            (
+                reinterpret_cast<const char*>(&localValue),
+                allValues.data_bytes(),
+                sizeof(T),  // The send/recv size per rank
+                comm
+            );
+        }
+        else
+        {
+            if (UPstream::master(comm))
+            {
+                // Non-trivial to manage non-blocking gather without a
+                // PEX/NBX approach (eg, PstreamBuffers) but leave with
+                // with simple exchange for now
 
-            if (is_contiguous<T>::value)
-            {
-                UOPstream::write
-                (
-                    UPstream::commsTypes::scheduled,
-                    belowID,
-                    reinterpret_cast<const char*>(&value),
-                    sizeof(T),
-                    tag,
-                    comm
-                );
+                allValues[0] = localValue;
+
+                for (int proci = 1; proci < numProc; ++proci)
+                {
+                    IPstream fromProc
+                    (
+                        UPstream::commsTypes::scheduled,
+                        proci,
+                        0,  // bufsize
+                        tag,
+                        comm
+                    );
+                    fromProc >> allValues[proci];
+                }
             }
-            else
+            else if (UPstream::is_rank(comm))
             {
-                OPstream toBelow
+                OPstream toProc
                 (
                     UPstream::commsTypes::scheduled,
-                    belowID,
-                    0,
+                    UPstream::masterNo(),
+                    0,  // bufsize
                     tag,
                     comm
                 );
-                toBelow << value;
+                toProc << localValue;
             }
         }
     }
-    #endif
-}
+    else
+    {
+        // non-parallel: return own value
+        // TBD: only when UPstream::is_rank(comm) as well?
+        allValues.resize(1);
+        allValues[0] = localValue;
+    }
 
-
-template<class T, class BinaryOp>
-void Foam::Pstream::gather
-(
-    T& value,
-    const BinaryOp& bop,
-    const int tag,
-    const label comm
-)
-{
-    Pstream::gather(UPstream::whichCommunication(comm), value, bop, tag, comm);
+    return allValues;
 }
 
 
 template<class T>
-void Foam::Pstream::scatter(T& value, const int tag, const label comm)
+T Foam::Pstream::listScatterValues
+(
+    const UList<T>& allValues,
+    const label comm,
+    const int tag
+)
 {
-    #ifndef Foam_Pstream_scatter_nobroadcast
-    Pstream::broadcast(value, comm);
-    #else
-    Pstream::scatter(UPstream::whichCommunication(comm), value, tag, comm);
-    #endif
+    // OR
+    // if (is_contiguous<T>::value)
+    // {
+    //     return UPstream::listScatterValues(allValues, comm);
+    // }
+
+    T localValue{};
+
+    if (UPstream::is_parallel(comm))
+    {
+        const label numProc = UPstream::nProcs(comm);
+
+        if (UPstream::master(comm) && allValues.size() < numProc)
+        {
+            FatalErrorInFunction
+                << "Attempting to send " << allValues.size()
+                << " values to " << numProc << " processors" << endl
+                << Foam::abort(FatalError);
+        }
+
+        if (is_contiguous<T>::value)
+        {
+            UPstream::mpiScatter
+            (
+                allValues.cdata_bytes(),
+                reinterpret_cast<char*>(&localValue),
+                sizeof(T),  // The send/recv size per rank
+                comm
+            );
+        }
+        else
+        {
+            if (UPstream::master(comm))
+            {
+                const label startOfRequests = UPstream::nRequests();
+
+                List<DynamicList<char>> sendBuffers(numProc);
+
+                for (int proci = 1; proci < numProc; ++proci)
+                {
+                    UOPstream toProc
+                    (
+                        UPstream::commsTypes::nonBlocking,
+                        proci,
+                        sendBuffers[proci],
+                        tag,
+                        comm
+                    );
+                    toProc << allValues[proci];
+                }
+
+                // Wait for outstanding requests
+                UPstream::waitRequests(startOfRequests);
+
+                return allValues[0];
+            }
+            else if (UPstream::is_rank(comm))
+            {
+                IPstream fromProc
+                (
+                    UPstream::commsTypes::scheduled,
+                    UPstream::masterNo(),
+                    0,  // bufsize
+                    tag,
+                    comm
+                );
+                fromProc >> localValue;
+            }
+        }
+    }
+    else
+    {
+        // non-parallel: return first value
+        // TBD: only when UPstream::is_rank(comm) as well?
+
+        if (!allValues.empty())
+        {
+            return allValues[0];
+        }
+     }
+
+     return localValue;
 }
 
 
+
 // ************************************************************************* //
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGatherList.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGatherList.C
index e4dd9f8440..d009e43eb8 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGatherList.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGatherList.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2015-2023 OpenCFD Ltd.
+    Copyright (C) 2015-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -51,7 +51,7 @@ void Foam::Pstream::gatherList
     const label comm
 )
 {
-    if (UPstream::is_parallel(comm))
+    if (!comms.empty() && UPstream::is_parallel(comm))
     {
         if (values.size() < UPstream::nProcs(comm))
         {
@@ -62,7 +62,7 @@ void Foam::Pstream::gatherList
         }
 
         // My communication order
-        const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
+        const auto& myComm = comms[UPstream::myProcNo(comm)];
 
         // Receive from my downstairs neighbours
         for (const label belowID : myComm.below())
@@ -199,7 +199,7 @@ void Foam::Pstream::scatterList
     // between scatterList() and using broadcast(List<T>&) or a regular
     // scatter(List<T>&) is that processor-local data is skipped.
 
-    if (UPstream::is_parallel(comm))
+    if (!comms.empty() && UPstream::is_parallel(comm))
     {
         if (values.size() < UPstream::nProcs(comm))
         {
@@ -210,7 +210,7 @@ void Foam::Pstream::scatterList
         }
 
         // My communication order
-        const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
+        const auto& myComm = comms[UPstream::myProcNo(comm)];
 
         // Receive from up
         if (myComm.above() != -1)
@@ -323,7 +323,13 @@ void Foam::Pstream::gatherList
     const label comm
 )
 {
-    Pstream::gatherList(UPstream::whichCommunication(comm), values, tag, comm);
+    Pstream::gatherList
+    (
+        UPstream::whichCommunication(comm),
+        values,
+        tag,
+        comm
+    );
 }
 
 
@@ -336,7 +342,13 @@ void Foam::Pstream::scatterList
     const label comm
 )
 {
-    Pstream::scatterList(UPstream::whichCommunication(comm), values, tag, comm);
+    Pstream::scatterList
+    (
+        UPstream::whichCommunication(comm),
+        values,
+        tag,
+        comm
+    );
 }
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamReduceOps.H b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamReduceOps.H
index 7c114a54e8..aff75a17c0 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamReduceOps.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamReduceOps.H
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2016-2023 OpenCFD Ltd.
+    Copyright (C) 2016-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -46,28 +46,6 @@ namespace Foam
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
-//- Reduce inplace (cf. MPI Allreduce)
-//- using specified communication schedule.
-template<class T, class BinaryOp>
-void reduce
-(
-    const List<UPstream::commsStruct>& comms,
-    T& value,
-    const BinaryOp& bop,
-    const int tag,
-    const label comm
-)
-{
-    if (UPstream::warnComm >= 0 && comm != UPstream::warnComm)
-    {
-        Pout<< "** reducing:" << value << " with comm:" << comm << endl;
-        error::printStack(Pout);
-    }
-    Pstream::gather(comms, value, bop, tag, comm);
-    Pstream::broadcast(value, comm);
-}
-
-
 //- Reduce inplace (cf. MPI Allreduce)
 //- using linear/tree communication schedule
 template<class T, class BinaryOp>
@@ -81,7 +59,13 @@ void reduce
 {
     if (UPstream::is_parallel(comm))
     {
-        Foam::reduce(UPstream::whichCommunication(comm), value, bop, tag, comm);
+        if (UPstream::warnComm >= 0 && comm != UPstream::warnComm)
+        {
+            Pout<< "** reducing:" << value << " with comm:" << comm << endl;
+            error::printStack(Pout);
+        }
+        Pstream::gather(value, bop, tag, comm);
+        Pstream::broadcast(value, comm);
     }
 }
 
@@ -436,8 +420,7 @@ Pstream_SumReduce(double);
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
-// Convenience wrappers for some reduction operations
-// - defined after all specialisations are known
+// Convenience wrappers - defined after all specialisations are known
 
 //- Perform reduction on a copy, using specified binary operation
 //  \return the resulting value
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
index 6e9fa98636..5c96b9a380 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
@@ -52,6 +52,7 @@ Foam::UPstream::commsTypeNames
 ({
     { commsTypes::blocking, "blocking" },
     { commsTypes::scheduled, "scheduled" },
+    // { commsTypes::nonBlocking, "non-blocking" },
     { commsTypes::nonBlocking, "nonBlocking" },
 });
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
index 83b24683fe..83760dfc3e 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2015-2023 OpenCFD Ltd.
+    Copyright (C) 2015-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -895,21 +895,32 @@ public:
         );
 
         //- Communication schedule for tree all-to-master (proc 0)
-        static const List<commsStruct>& treeCommunication
+        static const List<commsStruct>&
+        treeCommunication
         (
             const label communicator = worldComm
         );
 
-        //- Communication schedule for linear/tree all-to-master (proc 0).
-        //- Chooses based on the value of UPstream::nProcsSimpleSum
+        //- Communication schedule for all-to-master (proc 0) as
+        //- linear/tree/none with switching based on UPstream::nProcsSimpleSum
+        //- and the is_parallel() state
         static const List<commsStruct>& whichCommunication
         (
             const label communicator = worldComm
         )
         {
+            const label np
+            (
+                parRun_ && is_rank(communicator)  // cf. is_parallel()
+              ? nProcs(communicator)
+              : 0
+            );
+
             return
             (
-                nProcs(communicator) < nProcsSimpleSum
+                np <= 1
+              ? List<commsStruct>::null()
+              : np < nProcsSimpleSum
               ? linearCommunication(communicator)
               : treeCommunication(communicator)
             );
@@ -1138,7 +1149,7 @@ public:
         //  On master input list length == nProcs, ignored on other procs.
         //  \n
         //  For \b non-parallel :
-        //  returns the first list element (or zero).
+        //  returns the first list element (or default initialized).
         template<class T>
         static T listScatterValues
         (
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamTemplates.C b/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamTemplates.C
index 10065bff3f..6f71ec548f 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamTemplates.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamTemplates.C
@@ -34,14 +34,6 @@ Foam::List<T> Foam::UPstream::allGatherValues
     const label comm
 )
 {
-    if (!is_contiguous<T>::value)
-    {
-        FatalErrorInFunction
-            << "Cannot all-gather values for non-contiguous types" << endl
-            << Foam::abort(FatalError);
-    }
-
-
     List<T> allValues;
 
     if (UPstream::is_parallel(comm))
@@ -49,7 +41,17 @@ Foam::List<T> Foam::UPstream::allGatherValues
         allValues.resize(UPstream::nProcs(comm));
         allValues[UPstream::myProcNo(comm)] = localValue;
 
-        UPstream::mpiAllGather(allValues.data_bytes(), sizeof(T), comm);
+        if (is_contiguous<T>::value)
+        {
+            UPstream::mpiAllGather(allValues.data_bytes(), sizeof(T), comm);
+        }
+        else
+        {
+            FatalErrorInFunction
+                << "Cannot all-gather values for non-contiguous types"
+                   " - consider Pstream variant instead" << endl
+                << Foam::abort(FatalError);
+        }
     }
     else
     {
@@ -70,14 +72,6 @@ Foam::List<T> Foam::UPstream::listGatherValues
     const label comm
 )
 {
-    if (!is_contiguous<T>::value)
-    {
-        FatalErrorInFunction
-            << "Cannot gather values for non-contiguous types" << endl
-            << Foam::abort(FatalError);
-    }
-
-
     List<T> allValues;
 
     if (UPstream::is_parallel(comm))
@@ -87,13 +81,23 @@ Foam::List<T> Foam::UPstream::listGatherValues
             allValues.resize(UPstream::nProcs(comm));
         }
 
-        UPstream::mpiGather
-        (
-            reinterpret_cast<const char*>(&localValue),
-            allValues.data_bytes(),
-            sizeof(T),  // The send/recv size per rank
-            comm
-        );
+        if (is_contiguous<T>::value)
+        {
+            UPstream::mpiGather
+            (
+                reinterpret_cast<const char*>(&localValue),
+                allValues.data_bytes(),
+                sizeof(T),  // The send/recv size per rank
+                comm
+            );
+        }
+        else
+        {
+            FatalErrorInFunction
+                << "Cannot gather values for non-contiguous types"
+                   " - consider Pstream variant instead" << endl
+                << Foam::abort(FatalError);
+        }
     }
     else
     {
@@ -114,47 +118,46 @@ T Foam::UPstream::listScatterValues
     const label comm
 )
 {
-    if (!is_contiguous<T>::value)
-    {
-        FatalErrorInFunction
-            << "Cannot scatter values for non-contiguous types" << endl
-            << Foam::abort(FatalError);
-    }
-
-
-    T localValue;
+    T localValue{};
 
     if (UPstream::is_parallel(comm))
     {
-        const label nproc = UPstream::nProcs(comm);
+        const label numProc = UPstream::nProcs(comm);
 
-        if (UPstream::master(comm) && allValues.size() < nproc)
+        if (UPstream::master(comm) && allValues.size() < numProc)
         {
             FatalErrorInFunction
                 << "Attempting to send " << allValues.size()
-                << " values to " << nproc << " processors" << endl
+                << " values to " << numProc << " processors" << endl
                 << Foam::abort(FatalError);
         }
 
-        UPstream::mpiScatter
-        (
-            allValues.cdata_bytes(),
-            reinterpret_cast<char*>(&localValue),
-            sizeof(T),  // The send/recv size per rank
-            comm
-        );
-    }
-    else
-    {
-        // non-parallel: return local value
-
-        if (UPstream::is_rank(comm) && !allValues.empty())
+        if (is_contiguous<T>::value)
         {
-            localValue = allValues[0];
+            UPstream::mpiScatter
+            (
+                allValues.cdata_bytes(),
+                reinterpret_cast<char*>(&localValue),
+                sizeof(T),  // The send/recv size per rank
+                comm
+            );
         }
         else
         {
-            localValue = Zero;
+            FatalErrorInFunction
+                << "Cannot scatter values for non-contiguous types"
+                   " - consider Pstream variant instead" << endl
+                << Foam::abort(FatalError);
+        }
+    }
+    else
+    {
+        // non-parallel: return first value
+        // TBD: only when UPstream::is_rank(comm) as well?
+
+        if (!allValues.empty())
+        {
+            return allValues[0];
         }
      }