The problem with LLVM has always been that it takes a long time to produce code. The post in the link promises a new backend that produces a slower artifact, but does so 10-20x quicker. This is great for debug builds.
This doesn’t mean the compilation as a whole gets quicker. There are 3 steps in compilation
- Front end: transforms source code into an LLVM intermediation representation (IR)
- Backend: this is where LLVM comes in. It accepts LLVM IR and transforms it into machine code
- Linking: a separate program links the artifacts produced by LLVM.
How long does each step take? Really depends on the program we’re trying to compile. This blog post contains timings for one example program (https://blog.rust-lang.org/2023/11/09/parallel-rustc/) to give you an idea. It also depends on whether LLVM is asked to produce a debug build (not performant, but quicker to produce) or a release build (fully optimised, takes longer).
The 10-20x improvement described here doesn’t work yet for clang or rustc, and when it does it will only speed up the backend portion. Nevertheless, this is still an incredible win for compile times because the other two steps can be optimised independently. Great work by everyone involved.