You can't branch at compile time. Well, you can, but that's less useful than branching at runtime. If you branch at compile time, then you need to produce binar

You can't branch at compile time. Well, you can, but that's less useful than branching at runtime. If you branch at compile time, then you need to produce binaries that have a minimum supported ISA extension. But if you branch at runtime, then you can produce maximally portable binaries that only use ISA extensions when they're available.