I thought I could make a difference by trying some loop unrolling
voodoo, with this macro:
#define LOOP_UNROLL(EXPR) \
int __trip_count = n >> 2 ; \
int i = 0 ; \
for ( ; __trip_count > 0 ; --__trip_count) { \
EXPR ; i++ ; \
EXPR ; i++ ; \
EXPR ; i++ ; \
EXPR ; i++ ; \
} \
switch (n - i){ \
case 3: \
EXPR ; i++ ; \
case 2: \
EXPR ; i++ ; \
case 1: \
EXPR ; i++ ; \
case 0: \
default: \
{} \
}
and these versions:
count_bin_unroll <- cxxfunction(signature(x = "numeric", binwidth =
"numeric", origin = "numeric", nbins = "integer"), '
int nbins_ = as<int>(nbins);
double binwidth_ = as<double>(binwidth);
double origin_ = as<double>(origin);
Rcpp::NumericVector counts(nbins_);
Rcpp::NumericVector x_(x);
int n = x_.size();
LOOP_UNROLL(( counts[(int) ((x_[i] - origin_) / binwidth_)]++ ))
return counts;
', plugin = "Rcpp", includes = readLines( "loopunroll.h" ) )
count_bini_unroll <- cxxfunction(signature(x = "numeric", binwidth =
"numeric", origin = "numeric", nbins = "integer"), '
int nbins_ = as<int>(nbins);
double binwidth_ = as<double>(binwidth);
double origin_ = as<double>(origin);
Rcpp::NumericVector counts(nbins_);
Rcpp::NumericVector x_(x);
int n = x_.size();
Rcpp::NumericVector::iterator x_i = x_.begin();
Rcpp::NumericVector::iterator counts_i = counts.begin();
LOOP_UNROLL(( counts_i[(int) ( (x_i[i] - origin_) / binwidth_)]++ ))
return counts;
', plugin = "Rcpp", includes = readLines( "loopunroll.h" ) )
But it turns out it does not help much. I get these:
Unit: milliseconds
expr min lq median uq max
1 iterator 31.26655 31.52351 31.64775 31.81314 89.9450
2 iterator_unroll 32.12131 32.34650 32.48870 32.69931 109.7043
3 operator 33.35224 33.59498 33.73557 33.91297 126.1293
4 operator_unroll 33.62351 33.77934 33.93898 34.28303 118.9024
But I wanted to share it anyway.
Romain
Le 04/01/12 15:16, Hadley Wickham a écrit :
Hi all,
Slightly less dense question (hopefully). In the code below I have
two versions of the same function - one uses operator[] and the other
uses iterators. Following the Rcpp introduction, I had expected the
iterator version to be substantially faster, but I'm only seeing a
minor improvement (~10%). Why doesn't using iterators help me much
here? Possible explanations:
* I'm using iterators incorrectly in my code
* Iterators help most when the vector access is sequential, and here
the counts index is bouncing all over the place, so I shouldn't expect
much improvement.
Any ideas would be much appreciated. Thanks!
Hadley
library(inline)
count_bin<- cxxfunction(signature(x = "numeric", binwidth =
"numeric", origin = "numeric", nbins = "integer"), '
int nbins_ = as<int>(nbins);
double binwidth_ = as<double>(binwidth);
double origin_ = as<double>(origin);
Rcpp::NumericVector counts(nbins_);
Rcpp::NumericVector x_(x);
int n = x_.size();
for(int i = 0; i< n; i++) {
counts[(int) ((x_[i] - origin_) / binwidth_)]++;
}
return counts;
', plugin = "Rcpp")
count_bini<- cxxfunction(signature(x = "numeric", binwidth =
"numeric", origin = "numeric", nbins = "integer"), '
int nbins_ = as<int>(nbins);
double binwidth_ = as<double>(binwidth);
double origin_ = as<double>(origin);
Rcpp::NumericVector counts(nbins_);
Rcpp::NumericVector x_(x);
int n = x_.size();
Rcpp::NumericVector::iterator x_i = x_.begin();
Rcpp::NumericVector::iterator counts_i = counts.begin();
for(int i = 0; i< n; i++) {
counts_i[(int) ((x_i[i] - origin_) / binwidth_)]++;
}
return counts;
', plugin = "Rcpp")
x<- rnorm(1e7, sd = 3)
origin<- min(x)
binwidth<- 1
n<- ceiling((max(x) - origin) / binwidth)
system.time(y1<- count_bin(x, binwidth, origin, nbins = n))
system.time(y2<- count_bini(x, binwidth, origin, nbins = n))
all.equal(y1, y2)
library(microbenchmark)
microbenchmark(
operator = count_bin(x, binwidth, origin, nbins = n),
iterator = count_bini(x, binwidth, origin, nbins = n))
)
# The real reason I'm exploring this is as a more efficient version
# of tabulate for doing equal bin counts. The Rcpp version is about 10x
# faster, mainly (I think) because it avoids creating a modified copy of the
# vector
system.time(y3<- tabulate((x - origin) / binwidth + 1, nbins = n))
all.equal(y1, y3)
--
Romain Francois
Professional R Enthusiast
http://romainfrancois.blog.free.fr
_______________________________________________
Rcpp-devel mailing list
Rcpp-devel@lists.r-forge.r-project.org
https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
