If you're discussing Microsoft usage, no header is required by ICL (under Windows). For non-Microsoft-compatible compilers, you should change to data types defined in . If you're trying to sort out which intrinsics from immintrin.h involve 64-bit integers, that's a sore point, it's not organized to make it easy. The organization is somewhat different between open and closed source versions. I don't know that you can do better than the like of grep __m64 *intrin.h in the include directory tied to your compiler. immintrin.h includes all its predecessors. Other proprietary compilers, such as CL, have their own intrinsics.