Compiling with -mxxx defines the corresponding macro of the intrinsics. For example, -mssse3 defines __SSSE3__ macro to 1. In GCC versions < 4.9, it is not possible to use and call x86 intrinsics only at runtime without compiling the entire file with the -mxxx option. For example, if we want to call SSSE3 intrinsics without compiling with -mssse3, the macro __SSSE3__ is not defined. Therefore, when including <tmmintrin.h>, this results in "error "SSSE3 instruction set not enabled"" because of : #ifndef __SSSE3__ # error "SSSE3 instruction set not enabled" Since GCC 4.9, this has been fixed and it is possible to call x86 intrinsics from select functions in a file that are tagged with the corresponding target attribute without having to compile the entire file with the -mxxx option. This can be seen in <tmmintrin.h> which in recent versions (>= 4.9) contains : #ifndef __SSSE3__ #pragma GCC push_options #pragma GCC target("ssse3") #define __DISABLE_SSSE3__ Since SSSE3 is only used under Windows for ChaCha256, this can be fixed by preceding '#include <tmmintrin.h>' with #if defined (_MSC_VER) && !defined (TC_WINDOWS_BOOT). See https://gcc.gnu.org/gcc-4.9/changes.html