compiler: Make +fix-cortex-a53-835769 a default feature for aarch64 fuchsia by PiJoules · Pull Request #161886 · rust-lang/rust

PiJoules · 2026-08-27T20:40:55Z

This matches the default target features for aarch64 fuchsia emitted by clang.

…uchsia This matches the default target features for aarch64 fuchsia emitted by clang.

rustbot · 2026-08-27T20:41:00Z

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

rustbot · 2026-08-27T20:41:02Z

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PiJoules · 2026-08-27T20:41:16Z

cc @ilovepi to make sure this is the right spelling of this

ilovepi · 2026-08-27T21:57:04Z

This should be the right way to spell this on Fuchsia. We should also consider just always setting this in LLVM if its targeting a Fuchsia triple.

We may also need to adjust the build Rules on the Fuchsia side when this lands, since its likely to change the ABI of the core snd std libraries. I have a vague memory that ordering may also have been important, so we should check that all the things that care (specifically zircon's build rules) use a compatible ordering.

jieyouxu

(This seems sensible, but I'll wait for a ~~target maintainer to double-check~~ EDIT: sorry, I think @ilovepi was meant to be one of the target maintainers?)

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jieyouxu · 2026-08-28T07:58:03Z

    let mut base = base::fuchsia::opts();
    base.cpu = "generic".into();
-    base.features = "+v8a,+crc,+aes,+sha2,+neon".into();
+    base.features = "+v8a,+crc,+aes,+sha2,+neon,+fix-cortex-a53-835769".into();


Is there more context for this? Would be helpful if the PR description could be slightly expanded to backlink to relevant bugs/context. I did a basic search and found:

ld.lld: error: unknown argument '--fix-cortex-a53-835769' when trying to compile rust on aarch64 llvm/llvm-project#155116

https://gcc.gnu.org/legacy-ml/gcc-patches/2014-10/msg00906.html

https://reviews.llvm.org/D5721

In particular for the gcc/llvm patch

Some early revisions of the Cortex-A53 have an erratum (835769) whereby it is possible for a 64-bit multiply-accumulate instruction in AArch64 state to generate an incorrect result. The details are quite complex and hard to determine statically, since branches in the code may exist in some circumstances, but all cases end with a memory (load, store, or prefetch) instruction followed immediately by the multiply-accumulate operation.

The safest work-around for this issue is to make the compiler avoid emitting multiply-accumulate instructions immediately after memory instructions and the simplest way to do this is to insert a NOP.

This patch implements such work-around. The work-around is only enabled when specifying the clang command line option -mfix-cortex-a53-835769 or the llvm backend option -aarch64-fix-cortex-a53-835769.

The work-around code generation is not enabled by default.

Presumably this PR here changes the default target features so that the workaround is enabled-by-default for the aarch64 fuchsia target?

cc @erickt @Nashenas88 (target maintainers, I don't know if Fuchsia has different target maintainers or sth)

Fushsia's C++ code already always adds this in clang (see https://github.com/llvm/llvm-project/blob/1a315dc409bd5eec1c4df57ae9225b518c1c265a/clang/lib/Driver/ToolChains/Fuchsia.cpp#L94). It's been an oversight that it wasn't happening in Rust too. This is unfortunate in that it likely has prevented cross language inlining in our LTO builds. We've been introducing Rust into our kernel and the build changes there are where we first noticed the incompatibility.

jieyouxu · 2026-08-28T08:01:22Z

    let mut base = base::fuchsia::opts();
    base.cpu = "generic".into();
-    base.features = "+v8a,+crc,+aes,+sha2,+neon".into();
+    base.features = "+v8a,+crc,+aes,+sha2,+neon,+fix-cortex-a53-835769".into();


This should be the right way to spell this on Fuchsia. We should also consider just always setting this in LLVM if its targeting a Fuchsia triple.

This sounds sensible to me. I think it's fine if we do it on the Rust side if you'd like to get this enabled-by-default sooner, but it sounds like the more sane default over the longer term is to change the LLVM side to enable this by default (and if there's some obscure reason to not want this, provide an opt-out).

jieyouxu · 2026-08-28T08:08:17Z

Oh hm, I think the fuchsia ping group / target maintainers never got updated, there's

Update fuchsia.toml to reflect team changes team#1614

https://doc.rust-lang.org/nightly/rustc/platform-support/fuchsia.html

@ilovepi do you happen to know what's the actual intended group of target maintainers for the Fuchsia targets?

View changes since the review

I think its roughly our Toolchain team. That includes @PiJoules @petrhosek @erickt @zeroomega @gulfemsavrun @Prabhuk @mysterymath and me.

@erickt can you confirm if there are more folks? We should probably also land the referenced PR w/ the full set of team members.

Updated in rust-lang/team#1614, thanks

Also FYI -- Fuchsia's current test job was disabled some ago due to hitting fuchsia server rate limits:

rust/src/ci/github-actions/jobs.yml

Lines 367 to 376 in 17fd5b8

# FIXME: temporarily disabled due to fuchsia server rate limits. See

# <https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/fuchsia.20failure/with/506637259>.

#

#- name: x86_64-fuchsia

# # Only run this job on the nightly channel. Fuchsia requires

# # nightly features to compile, and this job would fail if

# # executed on beta and stable.

# only_on_channel: nightly

# doc_url: https://rustc-dev-guide.rust-lang.org/tests/fuchsia.html

# <<: *job-linux-8c

I'm not sure if this is still intended (i.e. the server rate limits is annoying to solve), or if this fell off the radar. But just FYI that in terms of the integration test job, Fuchsia hasn't been running in CI for a while.

jieyouxu

Thanks
@bors r+ rollup

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rust-bors · 2026-08-29T07:09:12Z

📌 Commit 42f6596 has been approved by jieyouxu

It is now in the queue for this repository.

…uwer Rollup of 2 pull requests Successful merges: - #161692 (Allow Unpin impls for local extern type) - #161886 (compiler: Make +fix-cortex-a53-835769 a default feature for aarch64 fuchsia)

Rollup merge of #161886 - PiJoules:fix-cortex-a53-835769, r=jieyouxu compiler: Make +fix-cortex-a53-835769 a default feature for aarch64 fuchsia This matches the default target features for aarch64 fuchsia emitted by clang.

compiler: Make +fix-cortex-a53-835769 a default feature for aarch64 f…

42f6596

…uchsia This matches the default target features for aarch64 fuchsia emitted by clang.

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Aug 27, 2026

rustbot assigned jieyouxu Aug 27, 2026

jieyouxu reviewed Aug 28, 2026

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jieyouxu added the S-no-work-capacity-tracking Status: Exempted from triagebot work capacity tracking. label Aug 28, 2026

jieyouxu reviewed Aug 28, 2026

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jieyouxu added the relnotes Marks issues that should be documented in the release notes of the next release. label Aug 29, 2026

rustbot mentioned this pull request Aug 29, 2026

Tracking issue for release notes of #161886: compiler: Make +fix-cortex-a53-835769 a default feature for aarch64 fuchsia #161962

Open

jieyouxu added the O-fuchsia Operating system: Fuchsia label Aug 29, 2026

jieyouxu approved these changes Aug 29, 2026

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rust-bors Bot 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 Aug 29, 2026

JonathanBrouwer mentioned this pull request Aug 29, 2026

Rollup of 2 pull requests #161965

Merged

rust-bors Bot merged commit 711bd1a into rust-lang:main Aug 29, 2026
13 checks passed

rustbot added this to the 1.100.0 milestone Aug 29, 2026

	# FIXME: temporarily disabled due to fuchsia server rate limits. See
	# <https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/fuchsia.20failure/with/506637259>.
	#
	#- name: x86_64-fuchsia
	# # Only run this job on the nightly channel. Fuchsia requires
	# # nightly features to compile, and this job would fail if
	# # executed on beta and stable.
	# only_on_channel: nightly
	# doc_url: https://rustc-dev-guide.rust-lang.org/tests/fuchsia.html
	# <<: *job-linux-8c

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