git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@11028 f3b2605a-c512-4ea7-a41b-209d697bcdaa

src/atom_vec.h

@@ -115,6 +115,17 @@ class AtomVec : protected Pointers {
   int deform_groupbit;
   double *h_rate;
 
+  // union data struct for packing 32-bit and 64-bit ints into double bufs
+  // this avoids aliasing issues by having 2 pointers (double,int)
+  //   to same buf memory
+  // constructor for 32-bit int prevents compiler
+  //   from possibly calling the double constructor when passed an int
+  // copy to a double *buf:
+  //   buf[m++] = ubuf(foo).d, where foo is a 32-bit or 64-bit int
+  // copy from a double *buf:
+  //   foo = (int) ubuf(buf[m++]).i;, where (int) or (tagint) match foo
+  //   the cast prevents compiler warnings about possible truncation
+
   union ubuf {
     double   d;
     int64_t  i;