Re: [Boost-users] Why use 'functional composition' with boost::bind

5 Aug 2010

      On 3 Aug 2010, at 15:35, Wolfram Brenig wrote:
...
Hi,
as a casual/newbie boost-user I'm trying to come to
grips with why/when nested functional composition with
boost::bind might be of help - following Karlsson's
book, in its fifth printing.
You are not comparing like with like in your example, the compiler is over optimising.

I tried comparing these functions:

void f1(vector<int>& v)
{
     for(int i=0;i<v.size();i++) {
	v[i] = (random()*M_PI)/RAND_MAX;
	double x = sin(v[i]); x = x*x;
	double y = cos(v[i]); y = y*y;
	v[i] = x + y;
     }
}

void f2(vector<int>& v)
{
     for(int i=0;i<v.size();i++) {
	v[i] = (random()*M_PI)/RAND_MAX;
	}
     for(int i=0;i<v.size();i++) {
	v[i] = sin(v[i])*sin(v[i])+cos(v[i])*cos(v[i]);
     }
}

void f3(vector<int>& v)
{
     generate(v.begin(),v.end(),
	       bind(multiplies<double>(),
		    bind(&random),
		    M_PI/RAND_MAX));

     transform(v.begin(),v.end(),v.begin(),
		bind(plus<double>(),
		     bind(multiplies<double>(),bind(sin,_1),bind(sin,_1)),
		     bind(multiplies<double>(),bind(cos,_1),bind(cos,_1))
		     )
		);
}

In a seperate file, I compiled:

#include <vector>
using namespace std;

void f1(vector<int>& v);
void f2(vector<int>& v);
void f3(vector<int>& v);

int main(void)
{
	std::vector<int> v(10000);
	for(int i = 0; i < 10000; ++i)
		FUN(v);
}

Replacing FUN with f1,f2 and f3.

Using g++ 4.4 and -O3 there was no difference in performance between them. In eariler versions of g++, the first one was faster, as the compiler didn't figure out it could fuse the loops together.

One very useful g++ flag is "-fdump-tree-all". This will dump out a number of files which show you how g++ is optimising the code. The format isn't documented, but it is fairly readable. Using this flag, you will see that f1,f2 and f3 are all turned into the same code in a recent g++.

Chris

Re: [Boost-users] Why use 'functional composition' with boost::bind

Christopher Jefferson