GCC: vectorization difference between two similar loops

Question

When compiling with gcc -O3, why does the following loop not vectorize (automatically):

#define SIZE (65536)

int a[SIZE], b[SIZE], c[SIZE];

in         


        

Answer 1

The first loop is equivalent to

#define SIZE (65536)

int a[SIZE], b[SIZE], c[SIZE];

int foo () {
  int i, j;

  for (i=0; i c[SIZE - 1] ? b[i] : c[SIZE - 1];
  }
  return a[0];
}

The problem with the original expression is that it really doesn't make that much sense, so it's not too surprising gcc isn't able to vectorize it.