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sseavx2

How to make an operation similar to _mm_extract_epi8 with non-immediate input?

What I want is extracting a value from vector using a variable scalar index.

Like _mm_extract_epi8 / _mm256_extract_epi8 but with non-immediate input.

(There are some results in the vector, the one with the given index is found out to be the true result, the rest are discarded)

Solution

  • Especially, if index is in a GPR, the easiest way is probably to store val to memory and then movzx it into another GPR. Sample implementation using C:

    uint8_t extract_epu8var(__m256i val, int index) {
    union {
        __m256i m256;
        uint8_t array[32];
    } tmp;
    tmp.m256 = val;
    return tmp.array[index];
}

    Godbolt translation (note that a lot of overhead happens for stack alignment -- if you don't have an aligned temporary storage area, you could just vmovdqu instead of vmovdqa): https://godbolt.org/z/Gj6Eadq9r