restricted pointers

["Carlos Alvarez" <carl dot alv at gmail dot com>]

Good morning,

I am currently trying to make use of the "restrict" ("__restrict__")
keyord with the gcc-4.2 (-O3 -std=c99) and g++-4.2 (-O3) compilers to
allow restricted pointers. For this I am making a test with this code:


#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <sys/time.h>

void vecmult(int n, int * restrict a, int * restrict b, int * restrict c)
{
  int i;
  for (i=0; i<n; ++i) {
    a[i] = b[i] * c[i];
  }
}

int main(){
  int Nsteps = 100000;
  int n = 1000;
  int* a=NULL;
  int* b=NULL;
  int* c=NULL;

  //allocate memory
  a = malloc(n*sizeof(int));
  b = malloc(n*sizeof(int));
  c = malloc(n*sizeof(int));

  //initialize arrays
  for(int i = 0; i < n; ++i){
    a[i] = i;
    b[i] = 1;
    c[i] = 0;
  }

  //initialize time
  struct timeval tim;
  gettimeofday(&tim, NULL);
  long tcpu = clock();

  for(int i = 0; i < Nsteps; ++i){
    vecmult(n, a, b, c);
  }

  //time difference evaluation
  double t1 = tim.tv_sec + tim.tv_usec / 1000000.0;
  double start = (double)(tcpu);
  gettimeofday(&tim, NULL);
  double t2 = tim.tv_sec + tim.tv_usec / 1000000.0;
  tcpu = clock();
  double stop = (double)(tcpu);
  double t_elap = (t2 - t1);
  double t_cpu = (stop - start) / 1000000.0;
  //print
  printf("%f %f\n",t_elap, t_cpu);

  //deallocate memory
  free(a);
  free(b);
  free(c);

  return 0;
}


but the times with or without restrict are the same. Why isn't it
improving performance, am I doing something wrong?

Thank you in advance for your help.

Sincerely,

Carlos Alvarez