Can you think of any legitimate (smart) uses for runtime code modification (program modifying it's own code at runtime)?
Modern operating systems seem to frown upon programs that do this since this technique has been used by viruses to avoid detection.
All I can think of is some kind of runtime optimization that would remove or add some code by knowing something at runtime which cannot be known at compile time.
This has been done in computer graphics, specifically software renderers for optimization purposes. At runtime the state of many parameters is examined and an optimized version of the rasterizer code is generated (potentially eliminating a lot of conditionals) which allows one to render graphics primitives e.g. triangles much faster.
The Synthesis OS basically partially evaluated your program with respect to API calls, and replaced OS code with the results. The main benefit is that lots of error checking went away (because if your program isn't going to ask the OS to do something stupid, it doesn't need to check).
Yes, that's an example of runtime optimization.
You know the old chestnut that there is no logical difference between hardware and software...one can also say that there is no logical difference between code and data.
What is self-modifying code? Code that puts values in the execution stream so that it can be imterpreted not as data but as a command. Sure there is the theoretical viewpoint in functional languages that there really is no difference. I'm saying on e can do this in a straightforward manner in imperative languages and compiler/interpreters without the presumption of equal status.
What I'm referring to is in the practical sense that data can alter program execution paths (in some sense this is extremely obvious). I am thinking of something like a compiler-compiler that creates a table (an array of data) that one traverses through in parsing, moving from state to state (and also modifying other variables), just like how a program moves from command to command, modifying variables in the process.
So even in the usual instance of where a compiler creates code space and refers to a fully separate data space (the heap), one can still modify the data to explicitly change the execution path.
I have implemented a program using evolution to create the best algorithm. It used self-modifying code to modify the DNA blueprint.