Fix halo exchange in `fmpi_task_run_sync()`

src/fmpi_task.c

@@ -138,34 +138,34 @@ int fmpi_task_run_sync(
         memcpy(dest, src, size);
 
         const size_t idx_1 = type_size;
-        const size_t idx_n = task->domains[0].halo.cnt * type_size;
-        const size_t idx_np1 = idx_n + idx_1;
+        const size_t idx_nm2 = (task->domains[0].halo.cnt-2) * type_size;
+        const size_t idx_nm1 = idx_nm2 + idx_1;
 
               void * const buf_0   = task->domains[0].halo.raw;
         const void * const buf_1   = (char *)buf_0 + idx_1;
-        const void * const buf_n   = (char *)buf_0 + idx_n;
-              void * const buf_np1 = (char *)buf_0 + idx_np1;
+        const void * const buf_nm2 = (char *)buf_0 + idx_nm2;
+              void * const buf_nm1 = (char *)buf_0 + idx_nm1;
 
         MPI_Datatype type = fmpi_mpi_type(task->domains[0].halo.type.base);
         if((rank % 2) == 0) {
             int left = (rank != 0) ? (rank - 1) : MPI_PROC_NULL;
             int right = rank + 1;
-            MPI_Send(buf_n  , 1, type, right, 0, ctx->mpi->world);
+            MPI_Send(buf_nm2, 1, type, right, 0, ctx->mpi->world);
             MPI_Recv(buf_0  , 1, type, left,  1, ctx->mpi->world, MPI_STATUS_IGNORE);
-            MPI_Send(buf_1  , 1, type, left,  0, ctx->mpi->world);
-            MPI_Recv(buf_np1, 1, type, right, 1, ctx->mpi->world, MPI_STATUS_IGNORE);
+            MPI_Send(buf_1  , 1, type, left,  2, ctx->mpi->world);
+            MPI_Recv(buf_nm1, 1, type, right, 3, ctx->mpi->world, MPI_STATUS_IGNORE);
 
         } else {
             int left = rank - 1;
             int right = (rank != (ctx->mpi->size-1)) ? (rank + 1) : MPI_PROC_NULL;
             MPI_Recv(buf_0  , 1, type, left,  0, ctx->mpi->world, MPI_STATUS_IGNORE);
-            MPI_Send(buf_n  , 1, type, right, 1, ctx->mpi->world);
-            MPI_Recv(buf_np1, 1, type, right, 0, ctx->mpi->world, MPI_STATUS_IGNORE);
-            MPI_Send(buf_1  , 1, type, left,  1, ctx->mpi->world);
+            MPI_Send(buf_nm2, 1, type, right, 1, ctx->mpi->world);
+            MPI_Recv(buf_nm1, 1, type, right, 2, ctx->mpi->world, MPI_STATUS_IGNORE);
+            MPI_Send(buf_1  , 1, type, left,  3, ctx->mpi->world);
         }
         const int err = fmpi_futhark_free_data_sync(
             ctx->fut, task->args.in[0].gpu, task->domains[0].halo.type.base,
-            task->domains[0].halo.cnt
+            task->domains[0].halo.dim_cnt
         );
         if(err != FMPI_SUCCESS) {
             FMPI_RAISE_ERROR(ctx->err_handler, "FMPI",