SIMD Accelerated bytes to hex string

bin2hex.cpp

#include <immintrin.h> // SSE
#include <emmintrin.h> // SSE2
#include <tmmintrin.h> // SSSE3

alignas(16) static constexpr const char hex_chars[16] = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F' };

void bin2hex(const byte* src, char* dst, size_t num) noexcept
{
    if (num >= 8)
    {
        const __m128i mask = _mm_set1_epi8(0xF);

#if 1 // SSSE3
        const __m128i chars = _mm_load_si128(reinterpret_cast<const __m128i*>(hex_chars));
#else // SSE2
        const __m128i mask1 = _mm_set1_epi8(0x9);
        const __m128i mask2 = _mm_set1_epi8('A' - '0' - 0xA);
        const __m128i mask3 = _mm_set1_epi8('0');
#endif

        do
        {
            __m128i value = _mm_loadu_si64(src);

            num -= 8;
            src += 8;

            value = _mm_and_si128(_mm_unpacklo_epi8(_mm_srli_epi64(value, 4), value), mask);

#if 1 // SSSE3
            value = _mm_shuffle_epi8(chars, value);
#else // SSE2
            value = _mm_add_epi8(_mm_add_epi8(value, mask3), _mm_and_si128(_mm_cmpgt_epi8(value, mask1), mask2));
#endif

            _mm_storeu_si128(reinterpret_cast<__m128i*>(dst), value);

            dst += 16;
        } while (num >= 8);
    }

    while (num != 0)
    {
        --num;

        byte value = *src++;

        *dst++ = hex_chars[size_t(value >> 4)];
        *dst++ = hex_chars[size_t(value & 0xF)];
    }
}