Thread: Problem with gcc -O3 please help

Hi everyone,
I ran across a really strange problem with the -O3 optimization of gcc, and I would appreciate any help because I really have a hard time understanding the problem. I have boiled down my considerably longer code to a simple example that shows the behaviour. What it does is simply allocating an array, setting its 0-element to 0 and then setting it to one in a loop that is executed only once. The code

Code:

#include <stdlib.h>
#include <stdio.h>

int main() {

  int i;
  double R4 = 0;
  int *activedirections;
  int nactive=0;
  int holder;
  activedirections = malloc((2*3+1) *sizeof(int));
  activedirections[0] = 0;
  for (i=1; i<=1; i++) {
      activedirections[nactive] = i;
      nactive++;
  }
  holder = activedirections[0];
  printf("activedirections[0] = %i\n",activedirections[0]);
  printf("activedirections[0] = %i\n",activedirections[0]);
  printf("holder              = %i\n",holder);
  free(activedirections);
}

when compiled with gcc and no options produces:

Code:

activedirections[0] = 1
activedirections[0] = 1
holder              = 1

as expected. Compiling with -O3 however yields:

Code:

activedirections[0] = 0
activedirections[0] = 1
holder              = 0

I have no idea how a value can change in between two identical printf's

Any help would be great.

Thanks