Add Tier 3 Thumb-mode targets for Armv7-A, Armv7-R and Armv8-R #150556

thejpster · 2025-12-31T22:00:59Z

We currently have targets for bare-metal Armv7-R, Armv7-A and Armv8-R, but only in Arm mode. This PR adds five new targets enabling bare-metal support on these architectures in Thumb mode.

This has been tested using https://github.com/rust-embedded/aarch32/compare/main...thejpster:aarch32:support-thumb-mode-v7-v8?expand=1 and they all seem to work as expected.

However, I wasn't sure what to do with the maintainer lists as these are five new targets, but they share the docs page with the existing Arm versions. I can ask the Embedded Devices WG Arm Team about taking on these ones too, but whether Arm themselves want to take them on I guess is a bigger question.

rustbot · 2025-12-31T22:01:02Z

Some changes occurred in src/doc/rustc/src/platform-support

cc @Noratrieb

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

rustbot · 2025-12-31T22:01:05Z

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fee1-dead · 2026-01-01T02:51:26Z

@rustbot reroll

rustbot · 2026-01-01T17:16:31Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

lcnr · 2026-01-19T10:29:51Z

wooops, sry

@bors compiler

rust-bors · 2026-01-19T10:29:56Z

Unknown command "compiler". Run @bors help to see available commands.

lcnr · 2026-01-19T10:30:03Z

@rustbot reroll

petrochenkov · 2026-01-19T12:20:04Z

compiler/rustc_target/src/spec/targets/armv7a_none_eabi.rs

-        llvm_floatabi: Some(FloatAbi::Soft),
-        linker_flavor: LinkerFlavor::Gnu(Cc::No, Lld::Yes),
-        linker: Some("rust-lld".into()),
-        features: "+v7,+thumb2,+soft-float,-neon,+strict-align".into(),


+v7,+thumb2 was lost, is this intentional?

The feature flags were redundant as they are implied by the LLVM target name. The —print cfg output should be unchanged.

petrochenkov · 2026-01-19T12:20:52Z

compiler/rustc_target/src/spec/targets/armv7a_none_eabi.rs

-        linker: Some("rust-lld".into()),
-        features: "+v7,+thumb2,+soft-float,-neon,+strict-align".into(),
-        relocation_model: RelocModel::Static,
-        disable_redzone: true,


disable_redzone was changed to false, is this intentional?

I don’t think red zones apply to this architecture, so I made the setting the same as the other AArch32 targets.

petrochenkov · 2026-01-19T12:22:15Z

compiler/rustc_target/src/spec/targets/armv7a_none_eabihf.rs

-        linker: Some("rust-lld".into()),
-        features: "+v7,+vfp3d16,+thumb2,-neon,+strict-align".into(),
-        relocation_model: RelocModel::Static,
-        disable_redzone: true,


Again, features, disable_redzone and frame_pointer are different now.

I took out redundant flags and rationalised the options to match all the other targets.

petrochenkov · 2026-01-19T12:24:34Z

compiler/rustc_target/src/spec/targets/armv7a_none_eabi.rs

+            features: "+soft-float,-neon,+strict-align".into(),
+            max_atomic_width: Some(64),
+            has_thumb_interworking: true,
+            ..base::arm_none::opts()


Also, frame_pointer is now set to FramePointer::Always, is this intentional?

Yes. Other AArch32 targets have frame pointers always on.

petrochenkov · 2026-01-19T12:26:06Z

compiler/rustc_target/src/spec/targets/armv7r_none_eabi.rs

-            c_enum_min_bits: Some(8),
            has_thumb_interworking: true,
-            ..Default::default()
+            ..base::arm_none::opts()


frame_pointer changes here and in the other R targets.

Yes. Other AArch32 targets have frame pointers always on.

petrochenkov · 2026-01-19T12:28:25Z

compiler/rustc_target/src/spec/targets/thumbv7a_none_eabi.rs

+            llvm_floatabi: Some(FloatAbi::Soft),
+            features: "+soft-float,-neon,+strict-align".into(),
+            max_atomic_width: Some(64),
+            has_thumb_interworking: true,


Does has_thumb_interworking make sense on thumb targets?
Most of them don't have it set.

It enables the instruction_set attribute and AFAIK should be enabled on every target that supports both Arm mode and Thumb mode regardless of which is the default. The only AArch32 targets it shouldn’t apply to are the M-profile targets because they are Thumb-only.

rust/compiler/rustc_attr_parsing/src/attributes/instruction_set.rs

Line 48 in 058a763

if !cx.sess.target.has_thumb_interworking {

src/doc/rustc/src/platform-support/armv7a-none-eabi.md

thejpster · 2026-01-22T20:51:57Z

@petrochenkov - I've updated the docs as requested.

rustbot assigned fee1-dead Dec 31, 2025

rustbot assigned lcnr and unassigned fee1-dead Jan 1, 2026

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thejpster force-pushed the add-thumbv7a-thumbv7r-thumbv8r branch from a338091 to a556210 Compare January 1, 2026 17:16

rustbot assigned petrochenkov and unassigned lcnr Jan 19, 2026

petrochenkov reviewed Jan 19, 2026

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petrochenkov 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 Jan 19, 2026

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thejpster added 3 commits January 22, 2026 18:37

Rationalise the Armv7-A, Armv7-R and Armv8-R bare-metal target configs

5dd7c0b

Add Thumb-mode targets for Armv7-R, Armv7-A and Armv8-R.

96647dd

Update SUMMARY.md too

fb05cac

thejpster force-pushed the add-thumbv7a-thumbv7r-thumbv8r branch from 809a1a5 to fb05cac Compare January 22, 2026 18:37

thejpster added 3 commits January 22, 2026 20:48

Ensure armv7a-none-eabi.md mentions all four targets

0a2bb4b

Ensure armv7r-none-eabi.md mentions all four targets

19d1be5

Update armv8r-none-eabi.md - note both targets are hardfloat

fa526c4

thejpster force-pushed the add-thumbv7a-thumbv7r-thumbv8r branch from 75afffe to fa526c4 Compare January 22, 2026 20:51

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Add Tier 3 Thumb-mode targets for Armv7-A, Armv7-R and Armv8-R #150556

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thejpster commented Jan 22, 2026

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