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

[Romain Francois]

Le 21/04/10 17:54, Matthew Dowle a écrit :
>
>> 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;

or at least with something like this:

int* p_s = INTEGER(s) ;
int* p_t = INTEGER(t) ;
for( i=0 ; i < ns ; i++){
	p_s[i] = p_t[i % nt];
}

since expanding the INTEGER macro over and over has a price.

> 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
>
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>
>


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Romain Francois
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http://romainfrancois.blog.free.fr
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