Difference in assembly

[Manmeet Singh Johar]

Hello,
I am building cross compilers for SPARC variant for Cygwin and MingW 
hosts. In the process I am faced with a very peculiar problem. For the 
code shown below, though I am getting same assembly instructions in same 
sequence, but the registers used in add are swapped. O0 has no problem, 
but the problem comes in as soon as I compile with O1.
Code:
#include <stdio.h>
extern unsigned int index;
extern unsigned char arr[100];
int main(int argc, char *argv[])
{
    if(arr[index] != 0) printf("\n FATAL....") ;
    return 0;
}
I also built the cross compiler for linux and the assembly generated 
matches with the one generated by compiler on Cygwin. Could someone 
please help me out.
Regards
Manmeet