For setjmp to have a measurable performance impact, you have to be calling it an awful lot. It is just a handful of mov instructions without dependencies.
C++ and other languages tend to make the fast path of no exception extremely fast and store additional data about functions that allow for stack unwinding when an exception is thrown.
It is not. The dominant C++ exception system is based on "zero-cost exception handling", which refers to the fact that you do not call any extra code (like setjmp) until an exception is to be thrown. All of that logic is instead encapsulated in tables of exception-handling data and sophisticated unwind routines that look up those tables to figure out where to transfer control-flow to.
> For setjmp to have a measurable performance impact, you have to be calling it an awful lot.
If you built the C++ exception system on top of setjmp/longjmp, you would have to call setjmp on every instance that begins a try block. This includes implicit try blocks generated for every object that has a nontrivial destructor (which must be called if you unwind through the block). So yeah, you would be calling it an awful lot...
It also doesn’t really compose well, so if library A is using it for exceptions and library B is doing something else clever then it’s hard to coordinate those two uses.