96fe3c31c2
Remove current code for embedding command-line args in PDB The compiler currently has code that will obtain a list of quoted command-line arguments, and pass it through to TargetMachine creation, so that the command-line args can be embedded in PDB output. This PR removes that code, due to subtle concerns that might not have been apparent when it was originally added. --- Those concerns include: - The entire command-line quoting process is repeated every time a target-machine-factory is created. In incremental builds this typically occurs 500+ times, instead of happening only once. The repeated quoting constitutes a large chunk of instructions executed in the `large-workspace` benchmark. - See https://github.com/rust-lang/rust/pull/146804#issuecomment-3317322958 for an example of the perf consequences of skipping all that work. - This overhead occurs even when building for targets or configurations that don't emit PDB output. - Command-line arguments are obtained in a way that completely bypasses the query system, which is a problem for the integrity of incremental compilation. - Fixing this alone is likely to inhibit incremental rebuilds for most or all CGUs, even in builds that don't emit PDB output. - Command-line arguments and the executable path are obtained in a way that completely bypasses the compiler's path-remapping system, which is a reproducibility hazard. - https://github.com/rust-lang/rust/issues/128842 --- Relevant PRs: - https://github.com/rust-lang/rust/pull/113492 - https://github.com/rust-lang/rust/pull/130446 - https://github.com/rust-lang/rust/pull/131805 - https://github.com/rust-lang/rust/pull/146700 - https://github.com/rust-lang/rust/pull/146973 Zulip thread: - https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Some.20PDB.20info.20bypasses.20the.20query.20system.20and.20path.20remapping/with/541432211 --- According to rust-lang/rust#96475, one of the big motivations for embedding the command-line arguments was to enable tools like Live++. [It appears that Live++ doesn't actually support Rust yet](https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/embeded.20compiler.20args.20and.20--remap-path-prefix/near/523800010), so it's possible that there aren't any existing workflows for this removal to break. In the future, there could be a case for reintroducing some or all of this functionality, guarded behind an opt-in flag so that it doesn't cause problems for other users. But as it stands, the current implementation puts a disproportionate burden on other users and on compiler maintainers.
The codegen 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.
For more information about how codegen works, see the rustc dev guide.