ENH: generalize MPI broadcast to basic and user-defined MPI types

- simplify and rationalize some of the broadcast methods for more code
  reuse.

  The bottom level UPstream::broadcast is now always to/from "root=0".
  This was previously passed as a default parameter, but never used
  anything other than '0' in the code. Fixing it as '0' makes it
  consistent with the 'top-down' logical for node-based broadcast.

applications/test/globalIndex3/Test-globalIndex3.cxx

@@ -345,8 +345,8 @@ static void reportOffsets(const globalIndex& gi)
 
         UPstream::broadcast
         (
-            allOffsets.data_bytes(),
-            allOffsets.size_bytes(),
+            allOffsets.data(),
+            allOffsets.size(),
             interNodeComm
         );
     }

src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H

@@ -109,6 +109,15 @@ public:
             const int communicator = UPstream::worldComm
         );
 
+        //- Broadcast fixed-list content (contiguous or non-contiguous) to all
+        //- communicator ranks. Does nothing in \b non-parallel.
+        template<class Type, unsigned N>
+        static void broadcast
+        (
+            FixedList<Type, N>& list,
+            const int communicator = UPstream::worldComm
+        );
+
         //- Broadcast multiple items to all communicator ranks.
         //- Does nothing in \b non-parallel.
         template<class Type, class... Args>

src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.txx

@@ -43,7 +43,6 @@ void Foam::Pstream::broadcast
     }
     else if constexpr (is_contiguous_v<Type>)
     {
-        // Note: contains parallel guard internally
         UPstream::broadcast
         (
             reinterpret_cast<char*>(&value),
@@ -65,6 +64,37 @@ void Foam::Pstream::broadcast
 }
 
 
+template<class Type, unsigned N>
+void Foam::Pstream::broadcast
+(
+    FixedList<Type, N>& list,
+    const int communicator
+)
+{
+    if (!UPstream::is_parallel(communicator))
+    {
+        return;
+    }
+    else if constexpr (is_contiguous_v<Type>)
+    {
+        // Size is known and identical on all ranks
+        UPstream::broadcast(list.data(), list.size(), communicator);
+    }
+    else
+    {
+        // Non-contiguous content - serialize it
+        if (UPstream::master(communicator))
+        {
+            OPBstream::send(list, communicator);
+        }
+        else
+        {
+            IPBstream::recv(list, communicator);
+        }
+    }
+}
+
+
 template<class Type, class... Args>
 void Foam::Pstream::broadcasts
 (
@@ -77,8 +107,15 @@ void Foam::Pstream::broadcasts
     {
         return;
     }
+    else if constexpr (!sizeof...(values) && is_contiguous_v<Type>)
+    {
+        // A single-value and contiguous
+        UPstream::broadcast(&value, 1, communicator);
+    }
     else
     {
+        // Non-contiguous data, or multiple data - needs serialization
+
         if (UPstream::master(communicator))
         {
             OPBstream::sends
@@ -129,19 +166,10 @@ void Foam::Pstream::broadcastList
             communicator
         );
 
-        if (UPstream::is_subrank(communicator))
-        {
-            list.resize_nocopy(len);
-        }
-
         if (len)
         {
-            UPstream::broadcast
-            (
-                list.data_bytes(),
-                list.size_bytes(),
-                communicator
-            );
+            // Only broadcast non-empty content
+            UPstream::broadcast(list.data(), list.size(), communicator);
         }
     }
     else
@@ -150,13 +178,28 @@ void Foam::Pstream::broadcastList
 
         if (UPstream::master(communicator))
         {
-            OPBstream os(communicator);
-            os << list;
+            if (list.empty())
+            {
+                // Do not serialize if empty.
+                // Just broadcast zero-size in a form that IPBstream can expect
+                OPBstream::send(Foam::zero{}, communicator);
+            }
+            else
+            {
+                OPBstream::send(list, communicator);
+            }
         }
         else
         {
             IPBstream is(communicator);
-            is >> list;
+            if (is.remaining() > 0)  // Received a non-empty buffer
+            {
+                is >> list;
+            }
+            else
+            {
+                list.clear();
+            }
         }
     }
 }

src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H

@@ -1720,16 +1720,16 @@ public:
 
     // Broadcast Functions
 
-        //- Broadcast buffer contents to all processes in given communicator.
-        //- The sizes must match on all processes.
+        //- Broadcast buffer contents (contiguous types), from rank=0
+        //- to all ranks. The sizes must match on all processes.
         //  For \b non-parallel : do nothing.
         //  \return True on success
-        static bool broadcast
+        template<class Type>
+        inline static bool broadcast
         (
-            char* buf,
-            const std::streamsize bufSize,
-            const label communicator,
-            const int rootProcNo = masterNo()
+            Type* buffer,
+            std::streamsize count,
+            const int communicator
         );
 
 

src/OpenFOAM/db/IOstreams/Pstreams/UPstream.txx

@@ -27,6 +27,44 @@ License
 
 // * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
 
+template<class Type>
+bool Foam::UPstream::broadcast
+(
+    Type* buffer,
+    std::streamsize count,
+    const int communicator
+)
+{
+    // Likely no reason to check for nullptr
+    if (!UPstream::is_parallel(communicator))
+    {
+        // Nothing to do - ignore
+        return true;
+    }
+    else if constexpr (!is_contiguous_v<Type>)
+    {
+        // Also report parameters to silence compiler warnings about unused
+        FatalErrorInFunction
+            << "Invalid for non-contiguous data types."
+            << " buffer:" << (buffer != nullptr)
+            << " count:" << count
+            << Foam::abort(FatalError);
+        return false;
+    }
+    else
+    {
+        // Use element or component type (or byte-wise) for data type
+        return UPstream::mpi_broadcast
+        (
+            buffer,     // The data or cmpt pointer
+            UPstream_dataType<Type>::size(count),
+            UPstream_dataType<Type>::datatype_id,
+            communicator
+        );
+    }
+}
+
+
 template<class T>
 Foam::List<T> Foam::UPstream::allGatherValues
 (

src/OpenFOAM/parallel/globalIndex/globalIndex.C

@@ -319,12 +319,7 @@ bool Foam::globalIndex::splitNodeOffsets
         allOffsets.resize_nocopy(numProc+1);
     }
 
-    UPstream::broadcast
-    (
-        allOffsets.data_bytes(),
-        allOffsets.size_bytes(),
-        interNodeComm
-    );
+    UPstream::broadcast(allOffsets.data(), allOffsets.size(), interNodeComm);
 
 
     if (FOAM_UNLIKELY(allOffsets.empty()))

src/Pstream/dummy/UPstreamBroadcast.C

@@ -42,19 +42,4 @@ bool Foam::UPstream::mpi_broadcast
 }
 
 
-// * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
-
-bool Foam::UPstream::broadcast
-(
-    char* buf,
-    const std::streamsize bufSize,
-    const label comm,
-    const int rootProcNo
-)
-{
-    // Nothing to do - ignore
-    return true;
-}
-
-
 // ************************************************************************* //

src/Pstream/mpi/UPstreamBroadcast.C

@@ -92,24 +92,4 @@ bool Foam::UPstream::mpi_broadcast
 }
 
 
-// * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
-
-bool Foam::UPstream::broadcast
-(
-    char* buf,
-    const std::streamsize count,
-    const label communicator,
-    const int rootProcNo
-)
-{
-    return UPstream::mpi_broadcast
-    (
-        buf,
-        count,
-        UPstream::dataTypes::type_byte,
-        communicator
-    );
-}
-
-
 // ************************************************************************* //