Different result when Release x64 configuration in VS2015

I am implementing a ray tracing program using VS2015 community. The program fills a screen size color buffer (width * height with element type of Eigen::Vector3f), then save the buffer content to a ppm file.

The basic loop (Note the output section at the end of the outer loop):

using Vec3f = Eigen::Vector3f;

Vec3f * buffer = new Vec3f[w * h];

// for each pixel
for (int y = 0; y < h; y++) for (int x = 0; x < w; x++) 
{
    int const i = (h - y - 1) * w + x;
    buffer[i] = Vec3f::Zero();

    // 2x2 subpixel 
    for (int sy = 0; sy < 2; sy++) for (int sx = 0; sx < 2; sx++)
    {
        Vec3f r = Vec3f::Zero();

        // sampling
        for (int s = 0; s < samps; s++) 
        {
            // do some computation and accumulation to r
            // r = ...
        }
        buffer[i] = buffer[i] + r;
    }
    buffer[i] = buffer[i] * 0.25f;

    // debug with an output section
    //if (x % 16 == 0 && y % 16 == 0)
    //  std::cout << buffer[i] << std::endl;

}

I can get proper result with configurations of:

• Debug, x86 or x64;
• Release, x86;
• Release (optimization = Od), x64.

However, the buffer is all zero vectors with Release (optimization = O1 or O2 or Ox), x64, and what I got is a black picture.

So with Release (optimization = O1 or O2 or Ox), x64, I uncommented the output section to check the values in the buffer. The strange thing is, each pixel that I checked has a correct value, those not checked remains zero vector. For example, if I check every 16 pixels like the upper code, I will get a picture like this (256*256 black tessellated every 16 pixel):

I googled and read some materials like Surviving the Release Version, but still have no idea. Could anyone provide some experiences dealing with these problems?

Update: The code above is not so detailed, full code is here, depends on Eigen 3.2.6.