toml 0.4.9 to 0.5.3 compile for a long time on 2021-10-02 nightly

[saethlin]

[dependencies]
config = "=0.9.3"

[profile.release]
panic = "abort"
codegen-units = 1

It seems like both the panic setting and codegen-units setting are required.

I don't know what is going on, but LLVM seems to think it's doing something, but it's been doing the same something for the past 30 minutes. perf top says:

  41.22%  libLLVM-13-rust-1.57.0-nightly.so  [.] llvm::getUniqueExitBlocksHelper<llvm::BasicBlock, llvm::LoopBase<llvm::BasicBlock, llvm::Loop>, llvm::LoopBase<llvm::BasicBlock, llvm::Loop>::getUniq
  36.17%  libLLVM-13-rust-1.57.0-nightly.so  [.] llvm::ScalarEvolution::forgetMemoizedResults
   7.01%  libLLVM-13-rust-1.57.0-nightly.so  [.] llvm::Instruction::getSuccessor
   5.58%  libLLVM-13-rust-1.57.0-nightly.so  [.] llvm::ScalarEvolution::getLoopDisposition
   2.63%  libLLVM-13-rust-1.57.0-nightly.so  [.] llvm::ScalarEvolution::getSCEVAtScope

rustc --version --verbose:

rustc 1.57.0-nightly (f03eb6bef 2021-10-02)
binary: rustc
commit-hash: f03eb6bef8ced8a243858b819e013b9caf83d757
commit-date: 2021-10-02
host: x86_64-unknown-linux-gnu
release: 1.57.0-nightly
LLVM version: 13.0.0

[saethlin]

Update: Compilation finished after about 40 minutes.

[ehuss]

Bisected to #88243, cc @nikic

[apiraino]

@saethlin what's the command you use to reproduce this issue? Do you add some flags, such as -Znew-llvm-pass-manager?

[saethlin]

@apiraino No flags. Just cargo build --release with the dependencies and profile.release tables in Cargo.toml.

Are you having trouble reproducing?

[apiraino]

ah yes thank you @saethlin, I didn't read carefully your configuration. yes, I can now reproduce, I believe the issue is in one of the dependencies of config=0.9.3 (dated 2019, so all dependencies are equally old or even older). I've noticed that compiling config=0.11 doesn't reproduce anymore this issue.

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-medium +T-compiler

[saethlin]

I'm poking at this. It's out of my expertise, but maybe I'll turn up something useful.

I minimized config down to this function:

pub fn parse(text: &str) -> toml::Value {
    toml::from_str(text).unwrap()
}

So the regression is reproduced just by something in toml. It appears first in commit
f667495ae34395be9fc3e4bb8801cacf52c2253e and is removed by 7c9b0a39db0c4c235db372f88d725fae6e82889f

The older commits compile in release within a few seconds, while the more recent commits need just over a minute. I suggest investigating this based on the older toml commits in that range.