MMX was the first and oldest SIMD extension and initially filled a need for wider (64-bit) integer and additional registers. This is back when processors were 32-bit and 8 x 32-bit registers was starting to cramp our programming style. Now 64-bit processors, larger register sets, and 128-bit (or larger) vector SIMD extensions are common. There is simply no good reason to write new code using the (now) very limited MMX capabilities. We recommend that existing MMX codes be rewritten to use the newer SSE and VMX/VSX intrinsics or using the more portable GCC  builtin vector support or in the case of si64 operations use C scalar code. The MMX si64 scalars are just (64-bit) operations on long long int types and any modern C compiler can handle this type. The char / short in SIMD operations should all be promoted to 128-bit SIMD operations on GCC builtin vectors. Both will improve cross platform portability and performance.