[estebank]

> But, we write compilers and linkers as batch programs that redo all the compilation work from scratch every time.

I don't think that there are that many production level compilers that don't perform the kind of caching that you're advocating for. Part of them problem is what the language semantics are. https://www.pingcap.com/blog/rust-huge-compilation-units/ gives an example of this.

> Surely it’s possible to make a compiler that takes time proportional to how much of my code has changed, not how large the program is in total.

Language design also affects what can be done. For example Rust relies a lot on monomorphisation, which in turn makes much harder (not necessarily impossible) to do in-place patching, but a language like Java or Swift, where a lot of checks can be relegated to runtime, it becomesuch easier to do that kind of patching.

I think that there's a lot left to be done to get closer to what you want, but changing a compiler that has users in such an extensive way is a bit like changing the engine of a plane while it's flying.

[josephg]

Yes, rust has an incremental compilation mode that is definitely faster than compiling the whole program from scratch. But linking is still done from scratch every time, and that gets pretty slow with big programs.

I agree that it would be a lot of work to retrofit llvm like this. But personally I think that effort would be well worth it. Maybe the place to start is the linker.

[estebank]

You're in good company: multiple people want to experiment with taking ownership of the linking and codegen steps to enable this kind of behavior. I would be more than happy to see that happen. I feel that the problem is a project of that magnitude requires a benefactor for a small group of people to work completely dedicated to it for maybe 2 years. Those don't come along often. The alternative is that these projects don't happen, lose steam or take a really long time.

[pjmlp]

Given that VC++ does incremental linking, it seems to be the usual care of having someone caring enough to sort it out.