If statements with comparison SSE in C

c, sse

Solution

You're close:

const __m128 P2f = _mm_set1_ps(2.0f);
const __m128 M2f = _mm_set1_ps(-2.0f);
for (int i = 0; i < n; i += 4)
{
    __m128 xv = _mm_load_ps(x + i);
    __m128 av = _mm_load_ps(a + i);

    __m128 c1v = _mm_cmpgt_ps(xv, P2f);
    __m128 c2v = _mm_cmplt_ps(xv, M2f);

    __m128 cv = _mm_or_ps(c1v, c2v);

    xv = _mm_and_ps(xv, cv);

    av = _mm_add_ps(av, xv);

    _mm_store_ps(a + i, av);
}

The trick is to `OR` the two comparison results and then use this combined result as a mask to zero out the X values which do not pass the test using a bitwise `AND` operation. You then add the masked X vector, which will add 0 or the original X value to each element of A according to the mask.

For the alternate version as mentioned in your comment below you'd do this:

const __m128 P2f = _mm_set1_ps(2.0f);
const __m128 M2f = _mm_set1_ps(-2.0f);
for (int i = 0; i < n; i += 4)
{
    __m128 xv = _mm_load_ps(x + i);
    __m128 av = _mm_load_ps(a + i);

    __m128 c1v = _mm_cmpgt_ps(xv, P2f);
    __m128 c2v = _mm_cmplt_ps(xv, M2f);

    __m128 cv = _mm_or_ps(c1v, c2v);

    xv = _mm_and_ps(P2f, cv); // <<< change this line to get a[i] += 2.0f
                              //     instead of a[i] += x[i]

    av = _mm_add_ps(av, xv);

    _mm_store_ps(a + i, av);
}

For the third version you mention in later comments below (`a[i] *= 2.0`) it's slightly trickier, but you can do it by thinking of the expression as `a[i] += a[i]`:

const __m128 P2f = _mm_set1_ps(2.0f);
const __m128 M2f = _mm_set1_ps(-2.0f);
for (int i = 0; i < n; i += 4)
{
    __m128 xv = _mm_load_ps(x + i);
    __m128 av = _mm_load_ps(a + i);

    __m128 c1v = _mm_cmpgt_ps(xv, P2f);
    __m128 c2v = _mm_cmplt_ps(xv, M2f);

    __m128 cv = _mm_or_ps(c1v, c2v);

    xv = _mm_and_ps(av, cv)); // <<< change this line to get a[i] *= 2.0f (a[i] += a[i])
                              //     instead of a[i] += x[i]

    av = _mm_add_ps(av, xv);

    _mm_store_ps(a + i, av);
}

Problem

Original source