Re: Array optimizing problem in C++?

On Mar 25, 12:20 pm, Lionel B <m...@privacy.net> wrote:


Try the program I gave earlier on this topic.
Compile it with :
g++ -O3

Then uncomment the line :
//#define NO_ALIASING_OPTIMIZATION

and compile it again with g++ -O3.

There should be a difference.
Let me know if you don't find any.

Here is the program :

#include <iostream>
#include <ctime>

//#define NO_ALIASING_OPTIMIZATION

const int len = 50000;

__attribute__((noinline))
#ifndef NO_ALIASING_OPTIMIZATION
void smooth (int* dest, int * src )
#else
void smooth (int* __restrict dest, int * __restrict src )
#endif
{
    for ( int i = 0 ; i < 17 ; ++i )
       dest[ i ] = src[ i ] + src[ i + 1 ] + src[ i + 2 ];
}

void fill (int* src)
{
    for (int i = 0 ; i < len ; ++ i )
        src[i] = i;
}

int main()
{

int src_array [len] = {0} ;
int dest_array [len] = {0};

fill(src_array);

smooth (dest_array, dest_array); // dummy call

clock_t start=clock();

for (int i = 0; i < 100000000; i++)
smooth (dest_array, src_array);

clock_t endt=clock();

        std::cout <<"Time smooth(): " <<
          double(endt-start)/CLOCKS_PER_SEC * 1000 << " ms\n";

// doesn't work without the following cout on vc++
std::cout << dest_array [0] ;

return 0;
}

Alexandre Courpron.