Mis-compilation of infinite recursions

[ksqsf]

Consider this code

use std::time::Instant;

fn collatz(i: usize) -> bool {
    if i == 1 {
        true
    } else if i % 2 == 0 {
        collatz(i / 2)
    } else {
        collatz(3 * i + 1)
    }
}

fn main() {
    static TIMES: usize = 500000;
    let start = Instant::now();
    for i in 0..TIMES {
        if ! collatz(i) {
            println!("{}", i);
            return;
        }
    }
    println!("Done, {} millis elapsed!", start.elapsed().as_millis());
}

It contains infinite recursion (collatz(0)), but the whole loop gets optimized away with Nightly + Release.

Related: #28728 (Perhaps also related to this commit).

Not sure if this is an LLVM bug?

[moxian]

This requires --release to elide the function, but is reproducible on both nightly and stable (and as far back as at least 1.15.0) (so technically not a regression)

[oxalica]

In addition:

1. According to Unexpected code elimination of using .. instead of ..=  #85750, using 0..=(TIMES - 1) instead of 0..TIMES produces the correct result (infinite loop).
2. Decrease TIMES to less than 100, both .. and ..= get mis-compiled to an immediate return.
   Maybe related to loop-unroll or inline?

[nikic]

Upstream issue: https://bugs.llvm.org/show_bug.cgi?id=50511

[apiraino]

thanks @nikic and @oxalica for the analysis.

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

@rustbot label -I-prioritize +P-critical

[nikic]

Upstream fix: llvm/llvm-project@1b84acb

[nikic]

This issue has been fixed a while ago by #87018, which picks up the referenced fix.