Comment by IainIreland

Cranelift does not use copy-and-patch. Consider, for example, this file, which implements part of the instruction generation logic for x64: https://github.com/bytecodealliance/wasmtime/blob/main/crane...

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-...