Copy-and-Patch: a Copy-and-Patch Tutorial

Closest thing in (relatively) recent news that uses copy-and-patch I can think of is CPython's new JIT.

[0]: https://github.com/bytecodealliance/rfcs/pull/27

Copy-and-patch is a technique for reducing the amount of effort it takes to write a JIT by leaning on an existing AOT compiler's code generator. Instead of generating machine code yourself, you can get LLVM (or another compiler) to generate a small snippet of code for each operation in your internal IR. Then codegen is simply a matter of copying the precompiled snippet and patching up the references.

The more resources are poured into a JIT, the less it is likely to use copy-and-patch. You get more control/flexibility doing codegen yourself.

But see also Deegen for a pretty cool example of trying to push this approach as far as possible: https://aha.stanford.edu/deegen-meta-compiler-approach-high-...

This is, but only for someone who wants to do JIT work without writing assembly code, but can read assembly code back into C (or can automate that part).

Instead of doing all manual register allocations in the JIT, you get to fill in the blanks with the actual inputs after a more (maybe) diligent compiler has allocated the registers, pushed them and all that.

There's a similar set of implementation techniques in Apache Impala, where the JIT only invokes the library functions when generating JIT code, instead of writing inline JIT operations, so that they can rely on shorter compile times for the JIT and deeper optimization passes for the called functions.