I have a array called A that contains 32 unsigned char values.
I want to unpack these values in 4 __m256 variables with this rule, assuming we have a index from 0 to 31 regarding all the values from A, the unpacked 4 variable would have these values:
B_0 = A[0], A[4], A[8], A[12], A[16], A[20], A[24], A[28]
B_1 = A[1], A[5], A[9], A[13], A[17], A[21], A[25], A[29]
B_2 = A[2], A[6], A[10], A[14], A[18], A[22], A[26], A[30]
B_3 = A[3], A[7], A[11], A[15], A[19], A[23], A[27], A[31]
To do that, I have this code:
const auto mask = _mm256_set1_epi32( 0x000000FF );
...
const auto A_values = _mm256_i32gather_epi32(reinterpret_cast<const int*>(A.data(), A_positions.values_, 4);
// This code bellow is equivalent to B_0 = static_cast<float>((A_value >> 24) & 0x000000FF)
const auto B_0 = _mm256_cvtepi32_ps(_mm256_and_si256(_mm256_srai_epi32(A_values, 24), mask));
const auto B_1 = _mm256_cvtepi32_ps(_mm256_and_si256(_mm256_srai_epi32(A_values, 16), mask));
const auto B_2 = _mm256_cvtepi32_ps(_mm256_and_si256(_mm256_srai_epi32(A_values, 8), mask));
const auto B_3 = _mm256_cvtepi32_ps(_mm256_and_si256(_mm256_srai_epi32(A_values, 0), mask));
This works great, but I wonder if there is some faster way to do that, specially regarding the shift right and and operator that I use to retrieve the values.
Also, just for clarification, I said that array A was of size 32, but that's not true, this array contains way more values, and I need to access it's elements from different positions (but always from blocks of 4 uint8_t) that's why I use _mm256_i32gather_epi23 to retrieve these values. I just restrain the array size in this example for simplicity.