> But linking new binaries with old binaries, and at the same time being concerned with performance? I know of at least one case where a developer linked an an

> But linking new binaries with old binaries, and at the same time being concerned with performance? I know of at least one case where a developer linked an ancient Fortran object with a fresh C++ object file in order to fuzz test both against each other. The C++ code had to not only match the specification, it had to be bug for bug compatible at the same time, even down to the possible bugs in the 1990s Fortran compiler used to build the machine code. Or, if a bug was fixed it had to be fully documented as to the behavior change. I don't think the Fortran code used MMX but it could have in theory. So pretty rare, but can obviously happen.