vexos: clear .bss from assembly by tropicaaal · Pull Request #163960 · rust-lang/rust

tropicaaal · 2026-10-08T01:14:12Z

On target_os = "vexos", libstd is responsible for clearing the .bss section during runtime initialization. This PR moves that step from Rust (previously done via core::ptr::write_bytes) to some assembly that runs even earlier into execution.

The previous approach was sketchy at best when considering soundness, because:

Pointer provenance (probably) makes this UB since .bss lives inside the bounds of the abstract machine and we'd be writing to memory that will eventually hold statics. I have no idea how this plays out in practice, especially because it's a linkerscript symbol which isn't an actual value.
core::ptr::write_bytes gets lowered to a memclr intrinsic, which may access .bss depending on how its implemented in compiler-rt.
On debug profiles, UB precondition checks might cause problems depending on what's being asserted before the runtime environment is sane.

Anyways, for peace-of-mind it's much better to do this from assembly. The compiler changes here are trivial, just a change to the linkerscript to ensure that __bss_start gets aligned to a 4-byte boundary.

tropicaaal · 2026-10-08T01:14:29Z

marked as a draft until I get a chance to test this

this loop was originally written for thumb-2, but since we start in ARM mode we want regular mov

tropicaaal · 2026-10-08T06:18:52Z

tested in QEMU, seems to work fine

rustbot · 2026-10-08T06:18:57Z

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

rustbot · 2026-10-08T06:19:01Z

r? @aapoalas

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Owners of files modified in this PR: @ChrisDenton, libs
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hanna-kruppe · 2026-10-08T18:20:52Z

I don't know anything about vexos, but this looks good to me:

This only affects the thumbv7a-vex-v5 target (tier 3) and the PR comes from a target maintainers
I agree that doing this initialization from Rust (or C) is extremely sketchy for all the reasons described.
The new code look clear and plausible to me
This is very target-specific so I don't expect there's any good way to have more target-independent / shared code for this

r? me @bors r+ rollup

rust-bors · 2026-10-08T18:20:56Z

📌 Commit 810627f has been approved by hanna-kruppe

It is now in the queue for this repository.

…uwer Rollup of 19 pull requests Successful merges: - #156112 (Distinguish `repr(C)` ZSTs from others in ABI computation) - #157941 (Deny partial `-Z stack-protector` by default in all editions) - #163462 (Fix COFF renaming of decorated (stdcall/fastcall/vectorcall) exports) - #163855 (Include enclosing item's context on const errors) - #163861 (Run LLDB debuginfo tests on `x86_64-mingw`) - #163913 ( Silence redundant failed obligations on the same statement) - #161201 (Speed up tidy again) - #163380 (Tweak "name not found" resolution error when it happens from within a derive expansion) - #163538 (Add `rustc::missing_generic_type_visitable` lint) - #163617 (mips: make `Complex<T>` ABI match GCC) - #163772 (comptime fn error: suggest wrapping in const block) - #163813 (use pre-borrowck typing env for early MIR validation) - #163924 (replace instances of NonNull::new(&mut x).expect("...") with NonNull::from_mut) - #163958 (only make `RustaceansAreAwesome` satisfy trait clauses) - #163960 (vexos: clear .bss from assembly) - #163974 (Condense AdtDef lang item checks into one match) - #163979 (Add `bf16` to arm features) - #163989 (Revert "compiletest: stream output of executor process when --no-capture is set") - #163995 (m68k-unknown-none-elf: Remove code model) Failed merges: - #163832 (reject non-async coroutine closures as async callables) - #163972 (const-eval: ICE when we hit a non-const fn)

Rollup merge of #163960 - vexide:tropicaaal/asm-bss-clear, r=hanna-kruppe vexos: clear .bss from assembly On `target_os = "vexos"`, libstd is responsible for clearing the `.bss` section during runtime initialization. This PR moves that step from Rust (previously done via `core::ptr::write_bytes`) to some assembly that runs even earlier into execution. The previous approach was sketchy at best when considering soundness, because: - Pointer provenance (probably) makes this UB since `.bss` lives inside the bounds of the abstract machine and we'd be writing to memory that will eventually hold statics. I have no idea how this plays out in practice, especially because it's a linkerscript symbol which isn't an actual value. - `core::ptr::write_bytes` gets lowered to a memclr intrinsic, which may access `.bss` depending on how its [implemented](https://github.com/rust-lang/compiler-builtins/blob/7f5edc1fc0f7b736d9beb6f817650feac7247114/compiler-builtins/src/arm.rs#L248) in `compiler-rt`. - On debug profiles, UB precondition checks might cause problems depending on what's being asserted before the runtime environment is sane. Anyways, for peace-of-mind it's much better to do this from assembly. The compiler changes here are trivial, just a change to the linkerscript to ensure that `__bss_start` gets aligned to a 4-byte boundary.

…uwer Rollup of 19 pull requests Successful merges: - rust-lang/rust#156112 (Distinguish `repr(C)` ZSTs from others in ABI computation) - rust-lang/rust#157941 (Deny partial `-Z stack-protector` by default in all editions) - rust-lang/rust#163462 (Fix COFF renaming of decorated (stdcall/fastcall/vectorcall) exports) - rust-lang/rust#163855 (Include enclosing item's context on const errors) - rust-lang/rust#163861 (Run LLDB debuginfo tests on `x86_64-mingw`) - rust-lang/rust#163913 ( Silence redundant failed obligations on the same statement) - rust-lang/rust#161201 (Speed up tidy again) - rust-lang/rust#163380 (Tweak "name not found" resolution error when it happens from within a derive expansion) - rust-lang/rust#163538 (Add `rustc::missing_generic_type_visitable` lint) - rust-lang/rust#163617 (mips: make `Complex<T>` ABI match GCC) - rust-lang/rust#163772 (comptime fn error: suggest wrapping in const block) - rust-lang/rust#163813 (use pre-borrowck typing env for early MIR validation) - rust-lang/rust#163924 (replace instances of NonNull::new(&mut x).expect("...") with NonNull::from_mut) - rust-lang/rust#163958 (only make `RustaceansAreAwesome` satisfy trait clauses) - rust-lang/rust#163960 (vexos: clear .bss from assembly) - rust-lang/rust#163974 (Condense AdtDef lang item checks into one match) - rust-lang/rust#163979 (Add `bf16` to arm features) - rust-lang/rust#163989 (Revert "compiletest: stream output of executor process when --no-capture is set") - rust-lang/rust#163995 (m68k-unknown-none-elf: Remove code model) Failed merges: - rust-lang/rust#163832 (reject non-async coroutine closures as async callables) - rust-lang/rust#163972 (const-eval: ICE when we hit a non-const fn)

vexos: clear .bss from assembly

307ecc0

tropicaaal added 2 commits October 7, 2026 20:28

std: vexos: use mov rather than movs to clear bss

85bdd9d

this loop was originally written for thumb-2, but since we start in ARM mode we want regular mov

vexos: use post-indexed str over stm for clarity

810627f

tropicaaal marked this pull request as ready for review October 8, 2026 06:18

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 8, 2026

rustbot assigned aapoalas Oct 8, 2026

rustbot assigned hanna-kruppe Oct 8, 2026

rust-bors Bot added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Oct 8, 2026

rustbot unassigned aapoalas Oct 8, 2026

rust-bors Bot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Oct 8, 2026

JonathanBrouwer mentioned this pull request Oct 8, 2026

Rollup of 19 pull requests #164005

Merged

rust-bors Bot merged commit b0c172e into rust-lang:main Oct 8, 2026
14 checks passed

rustbot added this to the 1.101.0 milestone Oct 8, 2026

github-actions Bot mentioned this pull request Oct 9, 2026

Toolchain upgrade to nightly-2026-10-09 flux-rs/flux#1811

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vexos: clear .bss from assembly - #163960

vexos: clear .bss from assembly#163960
rust-bors[bot] merged 3 commits into
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vexide:tropicaaal/asm-bss-clear

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