GCC Auto-Vectorization with Strided Writes

[SDS <sds dot biz dot main at gmail dot com>]

When compiled on GCC 5.1 using -std=c99, -O3, and -mavx2, the
following code snippet auto-vectorizes:

#include <stdint.h>

void test(uint32_t *restrict a,
          uint32_t *restrict b) {
  uint32_t *a_aligned = __builtin_assume_aligned(a, 32);
  uint32_t *b_aligned = __builtin_assume_aligned(b, 32);

  for (int i = 0; i < (1L << 10); i += 2) {
    a_aligned[i] = 2 * b_aligned[i];
    a_aligned[i+1] = 3 * a_aligned[i+1];
  }
}

But the following code snippet does not auto-vectorize:

#include <stdint.h>

void test(uint32_t *restrict a,
          uint32_t *restrict b) {
  uint32_t *a_aligned = __builtin_assume_aligned(a, 32);
  uint32_t *b_aligned = __builtin_assume_aligned(b, 32);

  for (int i = 0; i < (1L << 10); i += 2) {
    a_aligned[i] = 2 * b_aligned[i];
    a_aligned[i+1] = a_aligned[i+1];
  }
}

This was also the case for GCC 4.8 and 4.9. Adding volatile to
a_aligned's declaration inhibits auto-vectorization completely.

Is there a way to make the latter code snippet auto-vectorize?


Cheers,

S.D.S.