If I were worried about the time this loop takes,
I would avoid using i%nt. For the attached C code
compile with gcc 4.3.3 with -O2 I get
> # INTEGER() in loop
> system.time( r1 <- .Call("my_rep1", 1:3, 1e7) )
user system elapsed
0.060 0.012 0.071
> # INTEGER() before loop
> system.time( r2 <- .Call("my_rep2", 1:3, 1e7) )
user system elapsed
0.076 0.008 0.086
> # replace i%src_length in loop with j=0 before loop and
> # if(++j==src_length) j=0 ;
> # in the loop.
> system.time( r3 <- .Call("my_rep3", 1:3, 1e7) )
user system elapsed
0.024 0.028 0.050
> identical(r1,r2) && identical(r2,r3)
[1] TRUE
The C code is:
#define USE_RINTERNALS /* pretend we are in the R kernel */
#include <R.h>
#include <Rinternals.h>
SEXP my_rep1(SEXP s_src, SEXP s_dest_length)
{
int src_length = length(s_src) ;
int dest_length = asInteger(s_dest_length) ;
int i,j ;
SEXP s_dest ;
PROTECT(s_dest = allocVector(INTSXP, dest_length)) ;
if(TYPEOF(s_src) != INTSXP) error("src must be integer data") ;
for(i=0;i<dest_length;i++) {
INTEGER(s_dest)[i] = INTEGER(s_src)[i % src_length] ;
}
UNPROTECT(1) ;
return s_dest ;
}
SEXP my_rep2(SEXP s_src, SEXP s_dest_length)
{
int src_length = length(s_src) ;
int dest_length = asInteger(s_dest_length) ;
int *psrc = INTEGER(s_src) ;
int *pdest ;
int i ;
SEXP s_dest ;
PROTECT(s_dest = allocVector(INTSXP, dest_length)) ;
pdest = INTEGER(s_dest) ;
if(TYPEOF(s_src) != INTSXP) error("src must be integer data") ;
/* end of boilerplate */
for(i=0;i<dest_length;i++) {
pdest[i] = psrc[i % src_length] ;
}
UNPROTECT(1) ;
return s_dest ;
}
SEXP my_rep3(SEXP s_src, SEXP s_dest_length)
{
int src_length = length(s_src) ;
int dest_length = asInteger(s_dest_length) ;
int *psrc = INTEGER(s_src) ;
int *pdest ;
int i,j ;
SEXP s_dest ;
PROTECT(s_dest = allocVector(INTSXP, dest_length)) ;
pdest = INTEGER(s_dest) ;
if(TYPEOF(s_src) != INTSXP) error("src must be integer data") ;
/* end of boilerplate */
for(j=0,i=0;i<dest_length;i++) {
*pdest++ = psrc[j++] ;
if (j==src_length) {
j = 0 ;
}
}
UNPROTECT(1) ;
return s_dest ;
}
Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com
> -----Original Message-----
> From: r-devel-boun...@r-project.org
> [mailto:r-devel-boun...@r-project.org] On Behalf Of Romain Francois
> Sent: Wednesday, April 21, 2010 12:32 PM
> To: Matthew Dowle
> Cc: r-de...@stat.math.ethz.ch
> Subject: Re: [Rd] suggestion how to use memcpy in duplicate.c
>
> 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@r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
> >
> >
>
>
> --
> Romain Francois
> Professional R Enthusiast
> +33(0) 6 28 91 30 30
> http://romainfrancois.blog.free.fr
> |- http://bit.ly/9aKDM9 : embed images in Rd documents
> |- http://tr.im/OIXN : raster images and RImageJ
> |- http://tr.im/OcQe : Rcpp 0.7.7
>
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>
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