commit bc5c05c9fe74b17f2d74f4cfb3acff274b9b8f6f
author Jakub Beránek <berykubik@gmail.com> Mon Jul 14 11:04:55 2025 +0200
committer GitHub <noreply@github.com> Mon Jul 14 11:04:55 2025 +0200
tree dd82ad7ceaed6976da7e6fbc4108292a2750fe21
parent 4da2bbaac8dd8be137bcb8419076d37a5fe922d5
parent 14d7e8819bf97a31f7fb38789b17696a529dcd2e

Rollup merge of #143881 - orlp:once-state-repr, r=tgross35

Use zero for initialized Once state

By re-labeling which integer represents which internal state for `Once` we can ensure that the initialized state is the all-zero state. This is beneficial because some CPU architectures (such as Arm) have specialized instructions to specifically branch on non-zero, and checking for the initialized state is by far the most important operation.

As an example, take this:

```rust
use std::sync::atomic::{AtomicU32, Ordering};

const INIT: u32 = 3;

#[inline(never)]
#[cold]
pub fn slow(state: &AtomicU32) {
    state.store(INIT, Ordering::Release);
}

pub fn ensure_init(state: &AtomicU32) {
    if state.load(Ordering::Acquire) != INIT {
        slow(state)
    }
}
```

If `INIT` is 3 (as is currently the state for `Once`), we see the following assembly on `aarch64-apple-darwin`:

```asm
example::ensure_init::h332061368366e313:
        ldapr   w8, [x0]
        cmp     w8, #3
        b.ne    LBB1_2
        ret
LBB1_2:
        b       example::slow::ha042bd6a4f33724e
```

By changing the `INIT` state to zero we get the following:

```asm
example::ensure_init::h332061368366e313:
        ldapr   w8, [x0]
        cbnz    w8, LBB1_2
        ret
LBB1_2:
        b       example::slow::ha042bd6a4f33724e
```

So this PR saves 1 instruction every time a `LazyLock` gets accessed on platforms such as these.