rustc hangs and takes increasing amounts of memory when compiling json crate as dependency

[adrian17]

Reproduction:

• use nightly rustc (with wasm target installed)
• clone https://github.com/ruffle-rs/ruffle
• cd web
• cargo build --release --verbose -j1 --target=wasm32-unknown-unknown

Result: under a minute into the build, the compiler hangs when compiling the json dependency, taking increasingly more memory; after 40s it spiked to 1GB, then 2GB and more. The json crate is tiny with no dependencies of its own, normally it should take <5s to build.

Weirdly, I cannot reproduce it when building the json crate's repo on its own, but I may be missing some build configuration difference.

Reproduced on builds starting with nightly-2021-09-26 (rustc 1.57.0-nightly (addb4da68 2021-09-25)), it works fine on earlier build. Also someone else managed to reproduce this on latest nightly too.

[saethlin]

This might be the same bug as in #89524, I see the same sort of behavior. I need to be compiling toml as a dependency, not just a version of toml that produces the very long build time. I also see some memory growth initially, but it stops at ~0.5 GB. I think the biggest difference is that in #89524 memory growth eventually stops and LLVM does its best to optimize a tremendous amount of IR.

Also interesting that we're both seeing this related to parsing crates. I wonder if there's something about them that makes whatever pattern is present more likely.

[adrian17]

Do you think this set of issues could be considered a NewPM (I'm assuming it's NewPM-related) blocker?
I just verified that this reproduces on GH Actions and kills the build: https://github.com/adrian17/ruffle/runs/4253768752?check_suite_focus=true#step:8:291

[apiraino]

@adrian17 I'm not 100% sure but I'm thinking if a smaller reproducible would help correlating this to #89524 and - by extension - to the new LLVM 13 pass manager. Do some of the debugging done in #89524 also apply to this issue?

[adrian17]

Looks like this regression is now on stable 1.59.
This makes it impossible to build dependencies for our project locally and on CI, so we'll have to lock version to 1.58 for the time being.

@apiraino sorry, I missed the notification back then; but I didn't investigate this further. (I also don't know how to pass --debug-pass=Details)

[apiraino]

Summarizing here (for SEO purposes) from the linked issues the mitigating flags that make the issue disappear: setting the release build profile opt-level != 3 (mentioned here)

 [profile.release]
 panic = "abort"
+opt-level = 0       # or 2

or disabling the new LLVM pass manager altogether:

$ RUSTFLAGS="-Z new-llvm-pass-manager=no" \
    cargo +nightly build --release --verbose -j1 --target=wasm32-unknown-unknown

[nikic]

It looks like ruffle switched away from json in the meantime, so this also needs a git checkout fa459f7547e0d13c97394108ed0051956a92a1da now. It also looks like this reproduced with stable but not nightly -- probably the LLVM 14 upgrade perturbed things enough that it doesn't happen anymore. (The root issue still exists though.)

[nikic]

This is supposed to be fixed by #94764, but as mentioned above, I already couldn't reproduce this on nightly before that, so I can't verify whether this specific instance of the problem has been addressed.

If someone still sees hangs with the new pass manager, please let me know.