encode.avx512vl.cpp

// This file is a copy of encode.avx512bw.cpp with necessary changes to
// use VPMULTISHIFTQB instruction

namespace base64 {

    namespace avx512vl {

        void encode(const uint8_t* input, size_t bytes, uint8_t* output) {

            uint8_t* out = output;

            static const char* lookup_tbl = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

            // 32-bit input: [ 0  0  0  0  0  0  0  0|c1 c0 d5 d4 d3 d2 d1 d0|b3 b2 b1 b0 c5 c4 c3 c2|a5 a4 a3 a2 a1 a0 b5 b4]
            //                                                   2                         1                    0
            // output order  [1, 2, 0, 1], i.e.:
            //               [b3 b2 b1 b0 c5 c4 c3 c2|c1 c0 d5 d4 d3 d2 d1 d0|a5 a4 a3 a2 a1 a0 b5 b4|b3 b2 b1 b0 c3 c2 c1 c0]
            //                            ^^^^^^^^^^^ ^^^^^ ----------------- ^^^^^^^^^^^^^^^^^ -----------------
            // constants generated by script/permutexvar_parameters.py
            const __m512i shuffle_input = _mm512_setr_epi32(
                        0x01020001, 0x04050304, 0x07080607, 0x0a0b090a,
                        0x0d0e0c0d, 0x10110f10, 0x13141213, 0x16171516,
                        0x191a1819, 0x1c1d1b1c, 0x1f201e1f, 0x22232122,
                        0x25262425, 0x28292728, 0x2b2c2a2b, 0x2e2f2d2e);
            const __m512i lookup = _mm512_loadu_si512(reinterpret_cast<const __m512i*>(lookup_tbl));

            for (size_t i = 0; i < bytes; i += 4 * 12) {
                const __m512i v = _mm512_loadu_si512(reinterpret_cast<const __m512i*>(input + i));

                // reorder bytes
                // [b3 b2 b1 b0 c5 c4 c3 c2|c1 c0 d5 d4 d3 d2 d1 d0|a5 a4 a3 a2 a1 a0 b5 b4|b3 b2 b1 b0 c3 c2 c1 c0]
                const __m512i in = _mm512_permutexvar_epi8(shuffle_input, v);

                // after multishift a single 32-bit lane has following layout:
                // [c1 c0 d5 d4 d3 d2 d1 d0|b1 b0 c5 c4 c3 c2 c1 c0|a1 a0 b5 b4 b3 b2 b1 b0|d1 d0 a5 a4 a3 a2 a1 a0]
                //        ^^^^^^^^^^^^^^^^^       ^^^^^^^^^^^^^^^^^       ^^^^^^^^^^^^^^^^^       ^^^^^^^^^^^^^^^^^
                // i.e.: (a = [10:17], b = [4:11], c = [22:27], d = [16:21])

                const __m512i shifts  = _mm512_set1_epi64(0x3036242a1016040alu); // 48, 54, 36, 42, 16, 22, 4, 10
                const __m512i indices = _mm512_multishift_epi64_epi8(shifts, in);

                // Note: the two higher bits of each indices' byte have garbage,
                //       but the following permutexvar instruction masks them out.

                // translation 6-bit values -> ASCII
                const __m512i result = _mm512_permutexvar_epi8(indices, lookup);

                _mm512_storeu_si512(reinterpret_cast<__m512i*>(out), result);

                out += 64;
            }
        }

    } // namespace avx512

} // namespace base64