Use zero for initialized Once state #143881

orlp · 2025-07-13T10:28:09Z

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:

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:

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:

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.

rustbot · 2025-07-13T10:28:13Z

r? @tgross35

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tgross35 · 2025-07-13T10:31:14Z

One nit, could you add a note to these COMPLETE consts mentioning this so they don't accidentally get changed in the future? With that and passing CI, r=me

tgross35 · 2025-07-13T10:32:51Z

To test the queue implementation:

@bors2 try jobs=x86_64-msvc-*

rust-bors · 2025-07-13T10:32:55Z

⌛ Trying commit a2d4139 with merge a3c9ef7…

To cancel the try build, run the command @bors2 try cancel.

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. try-job: x86_64-msvc-*

orlp · 2025-07-13T10:38:30Z

One nit, could you add a note to these COMPLETE consts mentioning this so they don't accidentally get changed in the future? With that and passing CI, r=me

@tgross35 Done.

ibraheemdev · 2025-07-13T12:20:26Z

@bors r=tgross35 rollup

bors · 2025-07-13T12:20:28Z

📌 Commit f041962 has been approved by tgross35

It is now in the queue for this repository.

rust-bors · 2025-07-13T13:05:43Z

☀️ Try build successful (CI)
Build commit: a3c9ef7 (a3c9ef742934ddb66a7d2265ea705a16b21e524c, parent: d2baa49a106fad06fbf6202fb6ea8a0b3d2767cc)

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.

Rollup of 16 pull requests Successful merges: - #142885 (core: Add `BorrowedCursor::with_unfilled_buf`) - #143217 (Port #[link_ordinal] to the new attribute parsing infrastructure) - #143355 (wrapping shift: remove first bitmask and table) - #143448 (remote-test-client: Exit code `128 + <signal-number>` instead of `3`) - #143592 (UWP: link ntdll functions using raw-dylib) - #143681 (bootstrap/miri: avoid rebuilds for test builds) - #143710 (Updates to random number generation APIs) - #143724 (Tidy cleanup) - #143820 (Fixed a core crate compilation failure when enabling the `optimize_for_size` feature on some targets) - #143850 (Compiletest: Simplify {Html,Json}DocCk directive handling) - #143855 (Port `#[omit_gdb_pretty_printer_section]` to the new attribute parsing) - #143868 (warn on align on fields to avoid breaking changes) - #143875 (update issue number for `const_trait_impl`) - #143881 (Use zero for initialized Once state) - #143887 (Run bootstrap tests sooner in the `x test` pipeline) - #143893 (Don't require `eh_personality` lang item on targets that have a personality) Failed merges: - #143878 (Port `#[pointee]` to the new attribute parsing infrastructure) - #143891 (Port `#[coverage]` to the new attribute system) r? `@ghost` `@rustbot` modify labels: rollup

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, rust-lang#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.

Rollup of 17 pull requests Successful merges: - #142885 (core: Add `BorrowedCursor::with_unfilled_buf`) - #143217 (Port #[link_ordinal] to the new attribute parsing infrastructure) - #143355 (wrapping shift: remove first bitmask and table) - #143448 (remote-test-client: Exit code `128 + <signal-number>` instead of `3`) - #143681 (bootstrap/miri: avoid rebuilds for test builds) - #143710 (Updates to random number generation APIs) - #143724 (Tidy cleanup) - #143738 (Move several float tests to floats/mod.rs) - #143820 (Fixed a core crate compilation failure when enabling the `optimize_for_size` feature on some targets) - #143850 (Compiletest: Simplify {Html,Json}DocCk directive handling) - #143855 (Port `#[omit_gdb_pretty_printer_section]` to the new attribute parsing) - #143868 (warn on align on fields to avoid breaking changes) - #143875 (update issue number for `const_trait_impl`) - #143881 (Use zero for initialized Once state) - #143887 (Run bootstrap tests sooner in the `x test` pipeline) - #143893 (Don't require `eh_personality` lang item on targets that have a personality) - #143901 (Region constraint nits) Failed merges: - #143878 (Port `#[pointee]` to the new attribute parsing infrastructure) - #143891 (Port `#[coverage]` to the new attribute system) r? `@ghost` `@rustbot` modify labels: rollup

Use zero for initialized Once state

a2d4139

rustbot assigned tgross35 Jul 13, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jul 13, 2025

Add comment why we use zero for COMPLETE

f041962

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 13, 2025

fmease mentioned this pull request Jul 14, 2025

Rollup of 16 pull requests #143912

Open

jhpratt mentioned this pull request Jul 14, 2025

Rollup of 17 pull requests #143916

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Use zero for initialized Once state #143881

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