Isn't this crippling a compile-time thing? Is there something in binary that executes best-performing instructions (as opposed to execute just the instructions compiled in) when it's being executed on a specific CPU? If so, how exactly does it work?
It's actually a runtime switch. A compiled x86 binary that uses extra-wide number-crunching instructions (SSE etc) must also work on older processors that don't have those instructions, so it will have two or more code paths. The code paths all perform equivalent computations, but using different instructions. For example, if you are adding 4 pairs of 64-bit numbers, and there's a special add-4-pairs-of-64-bit-number…
Thanks for your answer, it did not occur to me initially, but it makes a lot of sense!