Like others have said it's not a "hard thing" per se. It's that few implementations actually work correctly in most of the edge cases. Manual dispatch is, of co

Like others have said it's not a "hard thing" per se. It's that few implementations actually work correctly in most of the edge cases. Manual dispatch is, of course, doable. Agner Fog's optimization guide has a pretty decent walkthrough of writing your own dispatcher and even he has to go through a bunch of caveats.