suggestion how to use memcpy in duplicate.c

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-bounces at r-project.org 
[mailto:r-devel-bounces at r-project.org] On Behalf Of Romain Francois
Sent: Wednesday, April 21, 2010 12:32 PM
To: Matthew Dowle
Cc: r-devel at stat.math.ethz.ch
Subject: Re: [Rd] suggestion how to use memcpy in duplicate.c

Le 21/04/10 17:54, Matthew Dowle a ?crit :

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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Romain Francois
Professional R Enthusiast
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http://romainfrancois.blog.free.fr
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