[hjl@gnu-2 vect]$ cat x.c
typedef char vec_t;

extern __attribute__((aligned(16))) vec_t x [64];
extern __attribute__((aligned(16))) vec_t y [64];
extern __attribute__((aligned(16))) vec_t m [64];

void
foo1 ()
{
  int i;

  for (i = 0; i < 64; i++)
    m [i] = x [i] == y [i] ? 1 : 2;
}

void
foo2 ()
{
  int i;

  for (i = 0; i < 64; i++)
    m [i] = x [i] == y [i];
}
[hjl@gnu-2 vect]$ /export/build/gnu/gcc-nni/build-x86_64-linux/gcc/xgcc -B./ -B/export/build/gnu/gcc-nni/build-x86_64-linux/gcc/ -O2 -msse4.2 -ftree-vectorize -ftree-vectorizer-verbose=3 -S x.c

x.c:12: note: LOOP VECTORIZED.
x.c:9: note: vectorized 1 loops in function.

x.c:21: note: not vectorized: stmt not supported: D.2111_8 = D.2109_5 == D.2110_7
x.c:18: note: vectorized 0 loops in function.
[hjl@gnu-2 vect]$ 

m [i] = x [i] == y [i] ? 1 : 2;

works. But

m [i] = x [i] == y [i];

doesn't work.