Enable `outline-atomics` by default on more AArch64 platforms #144938

tgross35 · 2025-08-05T02:29:48Z

The baseline Armv8.0 ISA doesn't have atomics instructions, but in
practice most hardware is at least Armv8.1-A (2014), which includes
single-instruction atomics as part of the LSE feature. As a performance
optimization for these cases, GCC and LLVM have the -moutline-atomics flag
to turn atomic operations into calls to symbols like __aarch64_cas1_acq.
These can do runtime feature detection and use the LSE instructions if
available, falling back to more portable load-exclusive/store-exclusive
loops.

Since the recent 3b50253 ("compiler-builtins: plumb LSE support
for aarch64 on linux") our builtins support this LSE optimization, and
since 6936bb9 ("Dynamically enable LSE for aarch64 rust provided
intrinsics"), std will set the flag as part of its startup code. The first
commit in this PR configures this to work on all platforms built with
outline-atomics, not just Linux.

Thus, enable outline-atomics by default on Android, OpenBSD, Windows,
and Fuchsia platforms that don't have LSE in the baseline. The feature is
already enabled on Linux. Platform-specific details are included in each
commit message.

The current implementation can still be accessed by setting
-Ctarget-feature=-outline-atomics. Setting -Ctarget-feature=+lse or
a relevant CPU will use the single-instruction atomics without the call
overhead. https://rust.godbolt.org/z/dsdrzszoe

Link: https://learn.arm.com/learning-paths/servers-and-cloud-computing/lse/intro/
Original Clang outline-atomics benchmarks: https://reviews.llvm.org/D91157#2435844

try-job: aarch64-msvc-*
try-job: arm-android
try-job: dist-android
try-job: dist-aarch64-llvm-mingw
try-job: dist-aarch64-msvc
try-job: dist-various-*
try-job: test-various

rustbot · 2025-08-05T02:29:52Z

r? @ChrisDenton

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rustbot · 2025-08-05T02:29:55Z

These commits modify compiler targets.
(See the Target Tier Policy.)

tgross35 · 2025-08-05T02:30:02Z

Whoops

r? @ghost

[experiment] enable outline-atomics on more aarch64 platforms try-job: arm-android try-job: dist-android try-job: dist-x86_64-freebsd try-job: dist-aarch64-windows-gnullvm try-job: dist-aarch64-apple try-job: aarch64-msvc-1 try-job: aarch64-msvc-2 try-job: dist-aarch64-msvc

Enable outline-atomics by default on more aarch64 platforms try-job: aarch64-apple try-job: aarch64-msvc-1 try-job: aarch64-msvc-2 try-job: arm-android try-job: dist-android try-job: dist-aarch64-windows-gnullvm try-job: dist-aarch64-apple try-job: dist-aarch64-msvc try-job: dist-various try-job: dist-x86_64-freebsd

Windows has a similar flag `/forceInterlockedFunctions`, which uses names such as `_InterlockedAdd64_rel`.

Per LLVM commit c5e7e64 ("[AArch64][Clang][Linux] Enable out-of-line atomics by default.") [1], Clang enables these on Android. Thus, do the same in Rust. [1]: llvm/llvm-project@c5e7e649d537067de

Clang has done this by default since LLVM commit 1a963d3 ("[Driver] Make -moutline-atomics default for aarch64-fuchsia targets"), [1], so do the same here. [1]: llvm/llvm-project@1a963d3

Clang has recently started doing this, as of LLVM commit 5d774ec8d183 ("[Driver] Enable outline atomics for OpenBSD/aarch64") [1]. Thus, do the same here. [1]: llvm/llvm-project@5d774ec

tgross35 · 2025-12-04T08:44:11Z

@bors2 try

Enable `outline-atomics` by default on more AArch64 platforms try-job: aarch64-msvc-* try-job: arm-android try-job: dist-android try-job: dist-aarch64-windows-gnullvm try-job: dist-aarch64-msvc try-job: dist-various-* try-job: test-various

tgross35 · 2025-12-04T08:45:56Z

Finally got a chance to circle back here. I split FreeBSD to a separate PR #149633, and dropped the Apple with an open issue #149634. For everything else here, the target maintainers have said this should be okay either on this PR or on Zulip.

@rustbot ready

tgross35 · 2025-12-04T08:47:08Z

@bors2 try

Enable `outline-atomics` by default on more AArch64 platforms try-job: aarch64-msvc-* try-job: arm-android try-job: dist-android try-job: dist-aarch64-llvm-mingw try-job: dist-aarch64-msvc try-job: dist-various-* try-job: test-various

rust-bors · 2025-12-04T11:04:54Z

☀️ Try build successful (CI)
Build commit: 73cd8c5 (73cd8c5953271a06db9935692296d56895fd742b, parent: 556beb9ec72360512d0294eb0855c92fb2c20c88)

davidtwco · 2025-12-09T10:46:10Z

@bors r+ rollup

bors · 2025-12-09T10:46:14Z

📌 Commit 66c150c has been approved by davidtwco

It is now in the queue for this repository.

Rollup of 5 pull requests Successful merges: - #144938 (Enable `outline-atomics` by default on more AArch64 platforms) - #146579 (Handle macro invocation in attribute during parse) - #149400 (unstable proc_macro tracked::* rename/restructure) - #149664 (attempt to fix unreachable code regression ) - #149806 (Mirror `ubuntu:24.04` on ghcr) Failed merges: - #149789 (Cleanup in the attribute parsers) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #144938 - tgross35:more-outline-atomics, r=davidtwco Enable `outline-atomics` by default on more AArch64 platforms The baseline Armv8.0 ISA doesn't have atomics instructions, but in practice most hardware is at least Armv8.1-A (2014), which includes single-instruction atomics as part of the LSE feature. As a performance optimization for these cases, GCC and LLVM have the `-moutline-atomics` flag to turn atomic operations into calls to symbols like `__aarch64_cas1_acq`. These can do runtime feature detection and use the LSE instructions if available, falling back to more portable load-exclusive/store-exclusive loops. Since the recent 3b50253 ("compiler-builtins: plumb LSE support for aarch64 on linux") our builtins support this LSE optimization, and since 6936bb9 ("Dynamically enable LSE for aarch64 rust provided intrinsics"), std will set the flag as part of its startup code. The first commit in this PR configures this to work on all platforms built with `outline-atomics`, not just Linux. Thus, enable `outline-atomics` by default on Android, OpenBSD, Windows, and Fuchsia platforms that don't have LSE in the baseline. The feature is already enabled on Linux. Platform-specific details are included in each commit message. The current implementation can still be accessed by setting `-Ctarget-feature=-outline-atomics`. Setting `-Ctarget-feature=+lse` or a relevant CPU will use the single-instruction atomics without the call overhead. https://rust.godbolt.org/z/dsdrzszoe Link: https://learn.arm.com/learning-paths/servers-and-cloud-computing/lse/intro/ Original Clang outline-atomics benchmarks: https://reviews.llvm.org/D91157#2435844 try-job: aarch64-msvc-* try-job: arm-android try-job: dist-android try-job: dist-aarch64-llvm-mingw try-job: dist-aarch64-msvc try-job: dist-various-* try-job: test-various

Rollup of 5 pull requests Successful merges: - rust-lang/rust#144938 (Enable `outline-atomics` by default on more AArch64 platforms) - rust-lang/rust#146579 (Handle macro invocation in attribute during parse) - rust-lang/rust#149400 (unstable proc_macro tracked::* rename/restructure) - rust-lang/rust#149664 (attempt to fix unreachable code regression ) - rust-lang/rust#149806 (Mirror `ubuntu:24.04` on ghcr) Failed merges: - rust-lang/rust#149789 (Cleanup in the attribute parsers) r? `@ghost` `@rustbot` modify labels: rollup

rustbot assigned ChrisDenton Aug 5, 2025

rustbot unassigned ChrisDenton Aug 5, 2025

tgross35 marked this pull request as draft August 5, 2025 02:30

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 5, 2025

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tgross35 force-pushed the more-outline-atomics branch from a25e101 to e20add7 Compare September 5, 2025 19:39

tgross35 changed the title ~~[experiment] enable outline-atomics on more aarch64 platforms~~ Enable outline-atomics by default on more aarch64 platforms Sep 5, 2025

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tgross35 added 4 commits December 4, 2025 03:36

Enable outline-atomics by default on AArch64 Windows platforms

1ed1b6e

Windows has a similar flag `/forceInterlockedFunctions`, which uses names such as `_InterlockedAdd64_rel`.

Enable outline-atomics by default on AArch64 Android

c455903

Per LLVM commit c5e7e64 ("[AArch64][Clang][Linux] Enable out-of-line atomics by default.") [1], Clang enables these on Android. Thus, do the same in Rust. [1]: llvm/llvm-project@c5e7e649d537067de

Enable outline-atomics by default on AArch64 Fuchsia

21525f8

Clang has done this by default since LLVM commit 1a963d3 ("[Driver] Make -moutline-atomics default for aarch64-fuchsia targets"), [1], so do the same here. [1]: llvm/llvm-project@1a963d3

Enable outline-atomics by default on AArch64 OpenBSD

66c150c

Clang has recently started doing this, as of LLVM commit 5d774ec8d183 ("[Driver] Enable outline atomics for OpenBSD/aarch64") [1]. Thus, do the same here. [1]: llvm/llvm-project@5d774ec

tgross35 force-pushed the more-outline-atomics branch from 4401f9c to 66c150c Compare December 4, 2025 08:36

This was referenced Dec 4, 2025

Enable outline-atomics by default on AArch64 FreeBSD #149633

Open

Enable outline-atomics by default on aarch64 Apple targets #149634

Open

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davidtwco approved these changes Dec 9, 2025

View reviewed changes

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 Dec 9, 2025

matthiaskrgr mentioned this pull request Dec 9, 2025

Rollup of 5 pull requests #149818

Merged

bors merged commit 7637023 into rust-lang:main Dec 10, 2025
12 checks passed

rustbot added this to the 1.94.0 milestone Dec 10, 2025

tgross35 deleted the more-outline-atomics branch December 11, 2025 16:57

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Enable `outline-atomics` by default on more AArch64 platforms #144938

Enable `outline-atomics` by default on more AArch64 platforms #144938

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