What Is JIT? How Apple's Rules Are Holding Back iPhone Game Emulators

howtogeek.com:

While Apple seems to have opened the floodgates when it comes to emulator apps on iPhone, iPad, and Apple TV, you may have noticed that certain systems are conspicuously missing from the menu. There's a good chance that's because of a specific feature known as "JIT" compilation being restricted by App Store rules.

What Is JIT?

JIT compilation is short for "Just In Time" compilation. Compilation is the process of turning programming code into machine code. You can write something in, for example, C code and then compile it for any system, whether it's an x86 CPU as found on most Windows computers or an ARM CPU as found on mobile phones and tablets.

JIT compilation, as opposed to ahead-of-time compilation, has numerous advantages, not least of which is adaptability to what's currently happening on the computer and what the user is asking the program to do.

Why Do Emulators Need JIT?

The developers of emulators have found that the JIT approach lets them achieve levels of performance and efficiency that would otherwise be impossible. Using the JIT technique, the emulated CPU code from the original system can be recompiled to native code for the target host system on the fly. Once that code has been recompiled, it can be cached and used the next time the same call is made without compiling again.

Using the JIT method, it's possible to create self-modifying code, which has become a common approach in video game emulation. This is necessary in some systems because many older systems, particularly cartridge based systems, would load code from the cartridge dynamically and seemingly arbitrarily based on the exact game state. JIT allows for essentially real-time handling of this approach. JIT also lets the emulator optimize the recompiled code to take advantage of the host system as much as possible.

I'm not a programmer, but even from my point of view, JIT seems like an extremely powerful tool when applied to the tough problem of emulation, based on what I've