How can I convert a vector of float to short int using avx instructions?

Question

Basically how can I write the equivalent of this with AVX2 intrinsics? We assume here that result_in_float is of type __m256, while result is of type short int* or short int[8].

for(i = 0; i < 8; i++)
    result[i] = (short int)result_in_float[i];

I know that floats can be converted to 32 bit integers using the __m256i _mm256_cvtps_epi32(__m256 m1) intrinsic, but have no idea how to convert these 32 bit integers further to 16 bit integers. And I don't want just that but also to store those values (in the form of 16 bit integers) to the memory, and I want to do that all using vector instructions.

Searching around the internet, I found an intrinsic by the name of_mm256_mask_storeu_epi16, but I'm not really sure if that would do the trick, as I couldn't find an example of its usage.

Answer 1

_mm256_cvtps_epi32 is a good first step, the conversion to a packed vector of shorts is a bit annoying, requiring a cross-slice shuffle (so it's good that it's not in a dependency chain here).

Since the values can be assumed to be in the right range (as per the comment), we can use _mm256_packs_epi32 instead of _mm256_shuffle_epi8 to do the conversion, either way it's a 1-cycle instruction on port 5 but using _mm256_packs_epi32 avoids having to get a shuffle mask from somewhere.

So to put it together (not tested)

__m256i tmp = _mm256_cvtps_epi32(result_in_float);
tmp = _mm256_packs_epi32(tmp, _mm256_setzero_si256());
tmp = _mm256_permute4x64_epi64(tmp, 0xD8);
__m128i res = _mm256_castsi256_si128(tmp);
// _mm_store_si128 that

The last step (cast) is free, it just changes the type.

If you had two vectors of floats to convert, you could re-use most of the instructions, eg: (not tested either)

__m256i tmp1 = _mm256_cvtps_epi32(result_in_float1);
__m256i tmp2 = _mm256_cvtps_epi32(result_in_float2);
tmp1 = _mm256_packs_epi32(tmp1, tmp2);
tmp1 = _mm256_permute4x64_epi64(tmp1, 0xD8);
// _mm256_store_si256 this