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

From: Romain Francois <romain_at_r-enthusiasts.com>
Date: Wed, 21 Apr 2010 21:32:22 +0200

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
>
> ______________________________________________
> R-devel_at_r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>

-- 
Romain Francois
Professional R Enthusiast
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http://romainfrancois.blog.free.fr
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Received on Wed 21 Apr 2010 - 19:38:02 GMT

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