d514263ce3
rustc: Default 32 codegen units at O0 This commit changes the default of rustc to use 32 codegen units when compiling in debug mode, typically an opt-level=0 compilation. Since their inception codegen units have matured quite a bit, gaining features such as: * Parallel translation and codegen enabling codegen units to get worked on even more quickly. * Deterministic and reliable partitioning through the same infrastructure as incremental compilation. * Global rate limiting through the `jobserver` crate to avoid overloading the system. The largest benefit of codegen units has forever been faster compilation through parallel processing of modules on the LLVM side of things, using all the cores available on build machines that typically have many available. Some downsides have been fixed through the features above, but the major downside remaining is that using codegen units reduces opportunities for inlining and optimization. This, however, doesn't matter much during debug builds! In this commit the default number of codegen units for debug builds has been raised from 1 to 32. This should enable most `cargo build` compiles that are bottlenecked on translation and/or code generation to immediately see speedups through parallelization on available cores. Work is being done to *always* enable multiple codegen units (and therefore parallel codegen) but it requires #44841 at least to be landed and stabilized, but stay tuned if you're interested in that aspect!
NB: This crate is part of the Rust compiler. For an overview of the
compiler as a whole, see
the README.md file found in librustc.
The trans crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.