Compiler doesn't optimize out no-op loop.

[theemathas]

I tried this code:

#[unsafe(no_mangle)]
pub fn foo() -> u64 {
    let mut a = 0u64;
    for _ in 0..1024 {
        a ^= 1;
    }
    a
}

I compiled it with -Copt-level=3 on godbolt, and got this assembly:

foo:
        mov     eax, 1024
.LBB0_1:
        add     eax, -32
        jne     .LBB0_1
        xor     eax, eax
        ret

For some reason, the generated assembly counts from 1024 to 0 in a loop that does nothing. This seems wasteful.

Other operations that flip-flop between two states also have a similar effect. For example, a = -a;, or a = !a;.

Meta

Compiler version on godbolt:

rustc 1.90.0 (1159e78c4 2025-09-14)
binary: rustc
commit-hash: 1159e78c4747b02ef996e55082b704c09b970588
commit-date: 2025-09-14
host: x86_64-unknown-linux-gnu
release: 1.90.0
LLVM version: 20.1.8
Internal compiler ID: r1900

[nikic]

There is currently no dedicated optimization to compute the exit value of the IV for such cases. It looks like in this case it's only folded to zero as a result of vectorization.

Did this come up in practice somewhere?

[theemathas]

Didn't really come up "in practice". It came up while experimenting with how pure works in asm!(). (It involved XOR-ing values generated by rdseed but incorrectly marked as pure.)