https://github.com/bitcoin/bitcoin/blob/5f0c6a7b0e47e03f848dc992d37fe209dd9c6975/src/crypto/sha256.cpp#L183

This checks whether the CPU supports SSE4. It should also check if the OS supports SSE4. See this [StackOverflow answer](https://stackoverflow.com/a/22521619/6557621).
{
    if (TransformD64_8way) {
        while (blocks >= 8) {
            TransformD64_8way(out, in);
            out += 256;
            in += 512;
            blocks -= 8;
        }
    }
    if (TransformD64_4way) {
        while (blocks >= 4) {
            TransformD64_4way(out, in);
            out += 128;
            in += 256;
            blocks -= 4;
        }
    }
    while (blocks) {
        TransformD64(out, in);
        out += 32;
        in += 64;
        --blocks;
    }
}