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If you make a small change to your application, the Rust compiler does a significant amount of rework. That is, it recompiles a lot of code that it has already

If you make a small change to your application, the Rust compiler does a significant amount of rework. That is, it recompiles a lot of code that it has already compiled before. There are valid technical reasons for this because of how LLVM works or that the linker needs to rewrite all addresses. Yes, incremental compilation is a thing but it’s too coarse IMO. To me it seems that taking an extremely fine grained approach to compilation would improve the ergonomics of the iterative hack-and-run method of writing software. Some sort of local database of diffs or some such.

makapuf 3y ago
Depends on your target; if you have tiny compilation units you won't be able to optimize /inline on a broad target, that's why single unit compilation is an option (that may or may not improve the resut)
- davidhyde 3y ago
  This is true but what is really happening is that the frontend and backend cannot communicate intent effectively because of they way they are separated. The frontend doesn’t know what is important for optimisation because that’s not its job and the backend only sees the code the front gives it (never a wholistic view). So the easiest (and slowest) approach is to do everything over and over again. Increasing Codegen units (multi unit compilation) is just the user taking a risk that splitting things up will not affect performance optimisations. Nothing smart about it. If you had tiny compilation units and the frontend understood their significance to the backend then it would be able to build a graph of dirty code to be recompiled when a small piece of it changes.
- yxhuvud 3y ago
  One problem they really need to address though is that as soon as you put everything in a single compilation unit, then compilation is single threaded and dog slow. Compilation units maybe make sense for C and C++, but for other languages they are just a way to structure the compilation. It should be automatic, and just better all around.
hun3 3y ago
As a workaround, split into multiple crates. Disclaimer: I concur that this is merely a workaround, not the real fix

makapuf 3y ago

Depends on your target; if you have tiny compilation units you won't be able to optimize /inline on a broad target, that's why single unit compilation is an option (that may or may not improve the resut)

davidhyde 3y ago
This is true but what is really happening is that the frontend and backend cannot communicate intent effectively because of they way they are separated. The frontend doesn’t know what is important for optimisation because that’s not its job and the backend only sees the code the front gives it (never a wholistic view). So the easiest (and slowest) approach is to do everything over and over again. Increasing Codegen units (multi unit compilation) is just the user taking a risk that splitting things up will not affect performance optimisations. Nothing smart about it. If you had tiny compilation units and the frontend understood their significance to the backend then it would be able to build a graph of dirty code to be recompiled when a small piece of it changes.
yxhuvud 3y ago
One problem they really need to address though is that as soon as you put everything in a single compilation unit, then compilation is single threaded and dog slow. Compilation units maybe make sense for C and C++, but for other languages they are just a way to structure the compilation. It should be automatic, and just better all around.

hun3 3y ago

As a workaround, split into multiple crates. Disclaimer: I concur that this is merely a workaround, not the real fix