Use cases for self-modifying code?

Question

On a Von Neumann architecture, program and data are both stored in memory, so a program can modify itself. Is this useful for a programmer? Could you give some examples?

Answer 1

Metamorphism

One (questionable) use case that comes to my mind is metamorphic computer viruses. These are malicious pieces of software that conceal themselves from signature based detection by rewriting their own machine code to an semantically equivalent representation that looks different.

Trampolining

Another (more complex, but also more common) use case is trampolining, a technique based on dynamic code generation to solve certain problems with nested function calls.

JIT compilation

The most common usage of dynamic code generation that I can think of is JIT (just-in-time) compilation. Modern languages like .NET or Java are not compiled into native machine code, but into some kind of intermediate language (called bytecode). This bytecode is then interpreted when the program is executed (by a virtual machine written for the target architecture). At the same time, a background process checks which parts of the code are executed very often. These parts then have a good chance of being dynamically compiled into native machine language for maximum performance. All this happens during the run time of the program!

Security implications

One thing to keep in mind is that the possibility to interpret data as code is useful for exploiting security holes in computer software, which is why the trend in modern hardware and operating systems is to enable and, if possible, even enforce the separation of code and data (also see NX bit and DEP).

Answer 2

I can best answer this by referring you to an answer to a similar (exceptionally well written and answered) question, also on StackOverflow - Homoiconic and "unrestricted" self modifying code + Is lisp really self modifying?. The answer focuses on Lisp, a family languages known for taking "code is data" to the next level, and explores the uses of that in AI.