[Rd] suggestion how to use memcpy in duplicate.c

[Matthew Dowle]

>From copyVector in duplicate.c :

void copyVector(SEXP s, SEXP t)
{
    int i, ns, nt;
    nt = LENGTH(t);
    ns = LENGTH(s);
    switch (TYPEOF(s)) {
...
    case INTSXP:
    for (i = 0; i < ns; i++)
        INTEGER(s)[i] = INTEGER(t)[i % nt];
    break;
...

could that be replaced with :

    case INTSXP:
    for (i=0; i<ns/nt; i++)
        memcpy((char *)DATAPTR(s)+i*nt*sizeof(int), (char *)DATAPTR(t), 
nt*sizeof(int));
    break;

and similar for the other types in copyVector.  This won't help regular 
vector copies, since those seem to be done by the DUPLICATE_ATOMIC_VECTOR 
macro, see next suggestion below, but it should help copyMatrix which calls 
copyVector, scan.c which calls copyVector on three lines, dcf.c (once) and 
dounzip.c (once).

For the DUPLICATE_ATOMIC_VECTOR macro there is already a comment next to it 
:

    <FIXME>: surely memcpy would be faster here?

which seems to refer to the for loop  :

    else { \
    int __i__; \
    type *__fp__ = fun(from), *__tp__ = fun(to); \
    for (__i__ = 0; __i__ < __n__; __i__++) \
      __tp__[__i__] = __fp__[__i__]; \
  } \

Could that loop be replaced by the following ?

   else { \
   memcpy((char *)DATAPTR(to), (char *)DATAPTR(from), __n__*sizeof(type)); \
   }\

In the data.table package, dogroups.c uses this technique, so the principle 
is tested and works well so far.

Are there any road blocks preventing this change, or is anyone already 
working on it ?  If not then I'll try and test it (on Ubuntu 32bit) and 
submit patch with timings, as before.  Comments/pointers much appreciated.

Matthew