Are you sure? AFAIU, __m128i isn't for 128-bit arithmetic, it's simply a convenience type for packing 8-bit, 32-bit, or 64-bit values for SIMD operations. GCC

Are you sure? AFAIU, __m128i isn't for 128-bit arithmetic, it's simply a convenience type for packing 8-bit, 32-bit, or 64-bit values for SIMD operations. GCC and clang do have a 128-bit integral type, __int128, but arithmetic operations are synthesized inline, and judging by the generated assembly don't even make use of SIMD at all. Note that an unintended consequence of C99's uintmax_t made the introduction of a standard 128-bit integral type impossible on platforms that cared about ABI compatibility, an effect somewhat contradictory to the original intention of improving portability and code integration. A future C standard will loosen some language and introduce some new facilities to help promote the introduction of larger integral types. At that point Unix platforms may begin typedef'ing __int128 to int128_t.