Found a bug in the Intel compiler.

iccbug.c

// Code sample illustrating a bug in the Intel compiler 
// icpc (ICC) 13.0.1 20121010


// compile with:
// icc  -restrict -std=c99  -O2 iccbug.c -o iccbug
// then run iccbug
// Tested in Linux Ubuntu 12.10 (Intel Core i7)

#include <stdint.h>
#include <stdio.h>

// expect: out[0] = in[0] + in[1] + in[2] + in[3];
//         out[1] = in[4] + in[5] + in[6] + in[7];
__attribute__((noinline))
void broken_with_O2(int * restrict in,
                    int *  out) {
     for(int outer = 0; outer < 2; outer++) {
        *out = *in++;
         for (int inner = 1; inner < 4; inner++) {
            *out += *in++;
         }
        ++out;
     }
}

int main() {
  int in[8] = {1,1,1,1,1,1,1,1};
  int out[2];

  broken_with_O2(&in[0],&out[0]);

  printf(" got = %d %d\n",out[0], out[1]);
  printf(" expected = %d %d\n", 4,4);
}