allocate the cell_buff arrays aligned, as gcc assumes they will be and emits...

allocate the cell_buff arrays aligned, as gcc assumes they will be and emits aligned read/writes in memswap.

src/space.c

@@ -1481,8 +1481,8 @@ void space_split_recursive(struct space *s, struct cell *c,
   const int allocate_buffer = (buff == NULL && gbuff == NULL);
   if (allocate_buffer) {
     if (count > 0) {
-      if ((buff = (struct cell_buff *)malloc(sizeof(struct cell_buff) *
-                                             count)) == NULL)
+      if (posix_memalign((void *)&buff, SWIFT_STRUCT_ALIGNMENT,
+                       sizeof(struct cell_buff) * count) != 0)
         error("Failed to allocate temporary indices.");
       for (int k = 0; k < count; k++) {
         buff[k].x[0] = parts[k].x[0];
@@ -1491,8 +1491,8 @@ void space_split_recursive(struct space *s, struct cell *c,
       }
     }
     if (gcount > 0) {
-      if ((gbuff = (struct cell_buff *)malloc(sizeof(struct cell_buff) *
-                                              gcount)) == NULL)
+      if (posix_memalign((void *)&gbuff, SWIFT_STRUCT_ALIGNMENT,
+                       sizeof(struct cell_buff) * gcount) != 0)
         error("Failed to allocate temporary indices.");
       for (int k = 0; k < gcount; k++) {
         gbuff[k].x[0] = gparts[k].x[0];