Question

This a follow-up question to this one. Now I would like to convert in the opposite direction float –> unsigned int.
What is the optimal and accurate vector sequence of the following scalar operation?

float x = ...
unsigned int res = (unsigned int)x;

Answer 1

This is based on an example from the old but useful Apple AltiVec-SSE migration documentation which unfortunately is now no longer available at http://developer.apple.com:

inline __m128i _mm_ctu_ps(const __m128 f)
{
    const __m128 two31 = _mm_set1_ps(0x1.0p31f);
    const __m128 two32 = _mm_add_ps(two31, two31);
    const __m128 zero = _mm_xor_ps(f,f);

    // check for overflow before conversion to int
    const __m128 overflow = _mm_cmpge_ps(f, two31);
    const __m128 overflow2 = _mm_cmpge_ps(f, two32);
    const __m128 subval = _mm_and_ps(overflow, two31);
    const __m128i addval = _mm_slli_epi32((__m128i)overflow, 31);
    __m128i result;

    // bias the value to signed space if it is >= 2**31
    f = _mm_sub_ps(f, subval);

    // clip at zero
    f = _mm_max_ps(f, zero);

    // convert to int with saturation
    result = _mm_cvtps_epi32(f); // rounding mode should be round to nearest

    // unbias
    result = _mm_add_epi32(result, addval);

    // patch up the overflow case
    result = _mm_or_si128(result, (__m128i)overflow2);

    return result;
}