Why doesn't rustc eliminate this empty loop?

1

https://rust.godbolt.org/z/rY95oGMYd

pub fn test(){
    for _ in 0..=1024 {}
}
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2

Interestingly, it does eliminate this loop:

pub fn test(){
    for _ in 0..1024 {}
}
3

That's so weird. It doesn't even loop 1024 times. Does the compiler think we are gonna use the value of eax?

4

There are quite a few known performance problems with inclusive ranges...
https://github.com/rust-lang/rust/issues/45222

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5

Interestingly this loop was optimized out in 1.42.0, regressed to horrible code generation in 1.43.0 and recovered a bit to the current non-optimal state of affairs in 1.47.0 - Godbolt.

1 Like
6

This is just the extreme case of "you should use internal iteration for cheap loop bodies". If you

(0..=1024).for_each(|_| {})

then it optimizes out just fine, even at only opt-level 1: https://rust.godbolt.org/z/oP8TEhadr

More in previous threads like

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7

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