Fwd: vectorization with negative offsets

[Nagaraju Mekala <gnuuser.raj@xxxxxxxxx>]

  I was working with X86 gcc-4.7.2 version. I found that gcc was not
able to vectorize small simple loops with negative offsets...


 assume ntimes & LEN to be some constant. In  my case they are 200000
int abc()

{
 for (int nl = 0; nl < ntimes*3; nl++) {
 for (int i = LEN - 1; i >= 0; i--) {
 a[i] = b[i] + (float) 1.;
}
}}

if we modify the above code as below Gcc has vectorized them.

int abc()
{
for (int nl = 0; nl < ntimes*3; nl++) {
for (int i = 0; i <= LEN - 1;  i++) {
a[i] = b[i] + (float) 1.;
}
}}

Can anyone explain why GCC is not able to vectorize is..

Thanks,
Nagaraju