Cache data_packer lengths in Sync_start to skip redundant buffer-size traversals

The data_packer(NULL, ...) calls that compute send/recv buffer lengths
traverse all grid segments × variables × nprocs on every Sync_start
invocation, even though lengths never change once the cache is built.
Add a lengths_valid flag to SyncCache so these length computations are
done once and reused on subsequent calls (4× per RK4 step).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

AMSS_NCKU_source/Parallel.C

@@ -3853,7 +3853,8 @@ void Parallel::Sync_merged(MyList<Patch> *PatL, MyList<var> *VarList, int Symmet
 Parallel::SyncCache::SyncCache()
     : valid(false), cpusize(0), combined_src(0), combined_dst(0),
       send_lengths(0), recv_lengths(0), send_bufs(0), recv_bufs(0),
-      send_buf_caps(0), recv_buf_caps(0), reqs(0), stats(0), max_reqs(0)
+      send_buf_caps(0), recv_buf_caps(0), reqs(0), stats(0), max_reqs(0),
+      lengths_valid(false)
 {
 }
 // SyncCache invalidate: free grid segment lists but keep buffers
@@ -3871,6 +3872,7 @@ void Parallel::SyncCache::invalidate()
     send_lengths[i] = recv_lengths[i] = 0;
   }
   valid = false;
+  lengths_valid = false;
 }
 // SyncCache destroy: free everything
 void Parallel::SyncCache::destroy()
@@ -4172,8 +4174,13 @@ void Parallel::Sync_start(MyList<Patch> *PatL, MyList<var> *VarList, int Symmetr
   {
     if (node == myrank)
     {
-      int length = data_packer(0, src[myrank], dst[myrank], node, PACK, VarList, VarList, Symmetry);
+      int length;
-      cache.recv_lengths[node] = length;
+      if (!cache.lengths_valid) {
+        length = data_packer(0, src[myrank], dst[myrank], node, PACK, VarList, VarList, Symmetry);
+        cache.recv_lengths[node] = length;
+      } else {
+        length = cache.recv_lengths[node];
+      }
       if (length > 0)
       {
         if (length > cache.recv_buf_caps[node])
@@ -4187,8 +4194,13 @@ void Parallel::Sync_start(MyList<Patch> *PatL, MyList<var> *VarList, int Symmetr
     }
     else
     {
-      int slength = data_packer(0, src[myrank], dst[myrank], node, PACK, VarList, VarList, Symmetry);
+      int slength;
-      cache.send_lengths[node] = slength;
+      if (!cache.lengths_valid) {
+        slength = data_packer(0, src[myrank], dst[myrank], node, PACK, VarList, VarList, Symmetry);
+        cache.send_lengths[node] = slength;
+      } else {
+        slength = cache.send_lengths[node];
+      }
       if (slength > 0)
       {
         if (slength > cache.send_buf_caps[node])
@@ -4200,8 +4212,13 @@ void Parallel::Sync_start(MyList<Patch> *PatL, MyList<var> *VarList, int Symmetr
         data_packer(cache.send_bufs[node], src[myrank], dst[myrank], node, PACK, VarList, VarList, Symmetry);
         MPI_Isend((void *)cache.send_bufs[node], slength, MPI_DOUBLE, node, 2, MPI_COMM_WORLD, cache.reqs + state.req_no++);
       }
-      int rlength = data_packer(0, src[node], dst[node], node, UNPACK, VarList, VarList, Symmetry);
+      int rlength;
-      cache.recv_lengths[node] = rlength;
+      if (!cache.lengths_valid) {
+        rlength = data_packer(0, src[node], dst[node], node, UNPACK, VarList, VarList, Symmetry);
+        cache.recv_lengths[node] = rlength;
+      } else {
+        rlength = cache.recv_lengths[node];
+      }
       if (rlength > 0)
       {
         if (rlength > cache.recv_buf_caps[node])
@@ -4214,6 +4231,7 @@ void Parallel::Sync_start(MyList<Patch> *PatL, MyList<var> *VarList, int Symmetr
       }
     }
   }
+  cache.lengths_valid = true;
 }
 // Sync_finish: wait for async MPI operations and unpack
 void Parallel::Sync_finish(SyncCache &cache, AsyncSyncState &state,

AMSS_NCKU_source/Parallel.h

@@ -97,6 +97,7 @@ namespace Parallel
     MPI_Request *reqs;
     MPI_Status *stats;
     int max_reqs;
+    bool lengths_valid;
     SyncCache();
     void invalidate();
     void destroy();