Internal representation of double variables - 3.4.6 vs 4.1.0

[max]

Hi gcc developers and users,

I have discovered that my code gives different results if compiled
with different gcc versions, namely 3.4.6 and 4.1.0.
Since I wanted to understand why, I compile my code again w/o any
optimization (-O0) and with debug symbols (-g).
I have found that differences (very small, 10e-12 on a 32-bit machine)
started to appear in the return value of a routine which performs
vector-vector multiplication, i.d.

double vecdot(double *v)
{
  double sum = 0;
  for(i = 0; i < n; i++)
    sum += v[i] * v[i];
  return sum;
}

even if elements of v[] are the same. Do these versions use different
"internal" representation of doubles?
I agree that the sum above is ill-conditioned, but why do different
gccs give (w/o optimization) different results?

Thanks for your help,
Max