$ cat a.c
void foo (float * restrict x, float * restrict y)
{
  int i;
  for (i = 0; i < 10000; i++)
    x[i] = y[i] * y[i];
}
$ gcc a.c -O1 -ffast-math -msse -mfpmath=sse -ftree-vectorize -ftree-vectorizer-verbose=5 -std=c99 -c

a.c:5: note: Alignment of access forced using peeling.
a.c:5: note: Vectorizing an unaligned access.
a.c:5: note: not vectorized: relevant stmt not supported: D.1353_14 =
__builtin_powf (D.1352_13, 2.0e+0)
a.c:5: note: vectorized 0 loops in function.


I find in fold-const.c:fold_binary, around line 9091, I found the following:
         /* Optimize x*x as pow(x,2.0), which is expanded as x*x.  */