Single file takes 30 minutes to compile at opt-level=3, regression from <1 sec

[dabek]

Put this in your main.rs file: https://gist.github.com/dabek/3474504ab3dae4d264b78504a4d2ee21
Run:

cargo run --release

See it take half an hour (at least it did for me the only time I had the patience to wait for it).

[dtolnay]

Labeling as a regression because 🤣 something is going on:

rustc	time cargo build --release
1.0 - 1.18	<1 sec
1.19	7.5 sec
1.20	30 sec
1.21 - 1.23	4 sec
1.24	2 sec
1.25 - 1.41-nightly	30 minutes

[alex]

I got bored after 40 seconds (I am not a patient man) -- this reproduces at opt-level=3, but not opt-level=2. Hopefully that helps narrow it down.

[vivekvpandya]

On rustc 1.39.0 I see it hangs when running following command :
rustc -C opt-level=3 --emit link main.rs

however
rustc -C opt-level=3 --emit obj main.rs generates .a file quickly , so it seems that linker is not happy with something generate by optimization.

[jsgf]

-Ztime-passes is showing:

  time: 0.226; rss: 134MB       codegen
    time: 0.002; rss: 134MB     llvm function passes [slow.7rcbfp3g-cgu.4]
    time: 0.001; rss: 134MB     llvm function passes [slow.7rcbfp3g-cgu.2]
    time: 0.004; rss: 134MB     llvm module passes [slow.7rcbfp3g-cgu.2]
    time: 0.014; rss: 136MB     llvm module passes [slow.7rcbfp3g-cgu.4]
    time: 0.039; rss: 136MB     llvm module passes [slow.7rcbfp3g-cgu.3]
    time: 0.050; rss: 136MB     llvm function passes [slow.7rcbfp3g-cgu.0]
    time: 24.948; rss: 267MB    llvm module passes [slow.7rcbfp3g-cgu.0]
>>  time: 4923.626; rss: 297MB  llvm module passes [slow.7rcbfp3g-cgu.1]
    time: 0.004; rss: 267MB     LTO passes
    time: 0.004; rss: 267MB     LTO passes
    time: 0.010; rss: 268MB     codegen passes [slow.7rcbfp3g-cgu.2]
    time: 0.010; rss: 268MB     codegen passes [slow.7rcbfp3g-cgu.4]
    time: 0.056; rss: 269MB     LTO passes
    time: 0.082; rss: 270MB     codegen passes [slow.7rcbfp3g-cgu.3]
    time: 25.502; rss: 270MB    LTO passes
    time: 3.664; rss: 279MB     codegen passes [slow.7rcbfp3g-cgu.0]

It hasn't completed yet. I'm also running it under perf. So far the top hot functions seem to be llvm::isSafeToSpeculativelyExecute and llvm::SmallPtrSetImplBase::FindBucketFor.

[andjo403]

at nightly (rustc 1.43.0-nightly (8aa9d20 2020-02-21)) around 2sec
the big regression was due to before one huge function was generated for core::ptr::drop_in_place but now multiple smaller functions is generated and that makes llvm mush happier :)
I do not know when this was changed only noticed it now.

[jonas-schievink]

Marking needstest to add a test to https://github.com/rust-lang-nursery/rustc-perf/