Using retain() on a vector a lot slower in version 1.38.0 than 1.36.0

[Miguel-c-s]

I posted this on stackoverflow.

What happens is that when using retain over a vector, it seems to be a lot slower in version 1.38.0.
host is: x86_64-unknown-linux-gpu (according to rustc--version --verbose) and LLVM version is 8.0.

I'm using ubuntu 19.04.

As I wrote on SO, I tried:

use std::time::Instant;
fn main() {
    let start = Instant::now();
    let mut my_vec:Vec<u32>;
    for _ in 0..100_000{
        my_vec = (0..10_000).collect();
        my_vec.retain(|&x| x < 9000);
        my_vec.retain(|&x| x < 8000);
        my_vec.retain(|&x| x < 7000);
        my_vec.retain(|&x| x < 6000);
        my_vec.retain(|&x| x < 5000);
        my_vec.retain(|&x| (x < 5) & (x > 2));
    }
    let duration = start.elapsed();
    println!("Program took: {:?}", duration);
}

And changed between versions with rustup default 1.36.0 and rustup default stable, having updated rust before with rustup update.
p.s.: From my complete program I also noticed that version 1.38.0 was still a little bit slower without using retain(). But the difference was minimal (~5%) and hard to come up with another culprit.

[peterjoel]

Performance of this code in 1.37 is similar to that in 1.36, so this would appear to be a regression in 1.38.

• Rust 1.37.0: 2.8 seconds
• Rust 1.38.0: 5.7 seconds

[mati865]

Minimal example: https://godbolt.org/z/f81qSU

[tesuji]

Could you check against nightly compiler?

[Miguel-c-s]

In the nightly version it takes about the same time as in stable.

[tesuji]

stable is Rust 1.38.0?

[Miguel-c-s]

yes, stable is 1.38.0.

[Miguel-c-s]

Will this ever be fixed or looked at? I understand that the devs may have too much work and too little help, I wish I had the knowledge to help, just sincerely asking.

[mati865]

I bet it's fallout from upgrade to LLVM 9 but bisecting it would greatly help (either manually or with https://github.com/rust-lang/cargo-bisect-rustc/blob/master/TUTORIAL.md).