assembly - 将 float 向量转换为 uint32 向量的最有效方法？

Question

这是 this one 的后续问题。现在我想以相反的方向转换 float --> unsigned int。以下标量运算的最优且准确的向量序列是什么？

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

Answer 1

这是基于旧的但有用的 Apple AltiVec-SSE 迁移文档中的示例，不幸的是，该文档现在不再可用 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;
}