Xray is currently being implemented in LLVM: it will allow exactly this. The code is instrumented at compile-time by leaving at the beginning of the function ju

Xray is currently being implemented in LLVM: it will allow exactly this. The code is instrumented at compile-time by leaving at the beginning of the function just the minimum amount of extra space (nop) so that at runtime a selected list of function can be instrumented arbitrarily. It means that even for a long-lived process, you can turn on/off various instrumentations while still having ~0 cost when instrumentation is off.