Kirill Lykov Kirill Lykov - 4 months ago 21
C Question

Difference in converting unsigned/signed integer into float

In SSE there is a function

_mm_cvtepi32_ps(__m128i input)
which takes input vector of signed integers and converts them into float. Now, I want to interpret input integers as not signed. But there is no function
_mm_cvtepu32_ps
and I could not find an implementation of one. Do you know where I can find such a function or at least give a hint on the implementation?
To illustrate the the difference in results:

unsigned int a = 2480160505; // 10010011 11010100 00111110 11111001
float a1 = a; // 01001111 00010011 11010100 00111111;
float a2 = (signed int)a; // 11001110 11011000 01010111 10000010

Answer

This functionality exists in AVX-512, but if you can't wait until then the only thing I can suggest is to convert the unsigned int input values into pairs of smaller values, convert these, and then add them together again, e.g.

inline __m128 _mm_cvtepu32_ps(const __m128i v)
{
    __m128i v2 = _mm_srli_epi32(v, 1);     // v2 = v / 2
    __m128i v1 = _mm_sub_epi32(v, v2);     // v1 = v - (v / 2)
    __m128 v2f = _mm_cvtepi32_ps(v2);
    __m128 v1f = _mm_cvtepi32_ps(v1);
    return _mm_add_ps(v2f, v1f); 
}

UPDATE

As noted by @wim in his answer, the above solution fails for an input value of UINT_MAX. Here is a more robust, but slightly less efficient solution, which should work for the full uint32_t input range:

inline __m128 _mm_cvtepu32_ps(const __m128i v)
{
    __m128i v2 = _mm_srli_epi32(v, 1);                 // v2 = v / 2
    __m128i v1 = _mm_and_si128(v, _mm_set1_epi32(1));  // v1 = v & 1
    __m128 v2f = _mm_cvtepi32_ps(v2);
    __m128 v1f = _mm_cvtepi32_ps(v1);
    return _mm_add_ps(_mm_add_ps(v2f, v2f), v1f);      // return 2 * v2 + v1
}