rustc 4x compilation time slowdown in rust 1.55

[bazald]

Moving from rustc 1.54 to 1.55, I'm observing a 4x compilation time slowdown for the examples for the hash-trie crate I'm working on. permalink to current revision

I'm not sure if this is a bug, or some new compilation feature, but compilation times increasing from 45-60s to 180-240s is not something I was expecting.

cargo run --release --example set
cargo run --release --example set_xor

Perhaps I'm doing something pathological in my crate, but I'd be very curious to know what change is causing this.

[jyn514]

@bazald does this only happen with --release or does cargo build also reproduce the issue? Can you run cargo rustc --release --example set -Z time-passes and post the output? (-Z self-profile would also be helpful, it will generate a profdata file you can upload.)

[bazald]

@jyn514 I can say that non-release build times seem marginally slower, but nothing I'd have noticed without the --release issue. Definitely appears that some optimization is going haywire on this code in 1.55... or is very expensive.

Regarding your other requests, I'm afraid I get the following message:

error: the `-Z` flag is only accepted on the nightly channel of Cargo, but this is the `stable` channel
See https://doc.rust-lang.org/book/appendix-07-nightly-rust.html for more information about Rust release channels.

This sent me down a rabbit hole. Which nightlies are roughly equivalent? Can I even install old nightlies? If I can do that, can I basically run a bisect? Turns out I can.

nightly-2021-07-05-x86_64-pc-windows-msvc gives the old compilation time. (59.93s including dependencies)
nightly-2021-07-06-x86_64-pc-windows-msvc gives the new, drastically slowed compilation time. (3m 03s including dependencies)

I'm not sure I have the energy to run more commands tonight. Can you clarify whether you want -Z time-passes and -Z self-profile separately, and whether you want them for both 2021-07-05 and 2021-07-06 or just 06?

[hkratz]

Some findings on a Mac M1:

• cargo clean; cargo +1.54.0 build --release --example set # 17.24s
• cargo clean; cargo +1.55.0 build --example set # 4.75s
• cargo clean; cargo +1.55.0 build --release --example set # 57.07s
• cargo clean; cargo +beta build --release --example set # (1.56.0-beta.2) - 16.44s
• cargo clean; cargo +nightly build --release --example set # (2021-09-15) - 16.61s

So only with 1.55.0 and --release, fixed in 1.56 beta and nightly.

[hkratz]

 Regression bisection led to #84560 (!), fix bisection lead to the update to LLVM 13 (#87570).

[jyn514]

cc @nikic

[jyn514]

Oh wait, I misread - this is only an issue with two releases of the rust compiler? I'm going to close this then since it's fixed on nightly.