mitigate MSVC alignment issue on x86-32 #139261

RalfJung · 2025-04-02T15:27:22Z

This implements mitigation for #112480 by stopping to emit align attributes on loads and function arguments when building for a win32 MSVC target. MSVC is known to not properly align u64 and similar types, and claiming to LLVM that everything is properly aligned increases the chance that this will cause problems.

Of course, the misalignment is still a bug, but we can't fix that bug, only MSVC can.

Also add an errata note to the platform support page warning users about this known problem.

try-job: i686-msvc*

rustbot · 2025-04-02T15:27:26Z

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rustbot · 2025-04-02T15:27:28Z

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

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

RalfJung · 2025-04-02T15:28:09Z

compiler/rustc_target/src/spec/mod.rs

+    pub fn has_unreliable_alignment(&self) -> bool {
+        // FIXME(#112480) MSVC on x86-32 is unsound and fails to properly align many types with
+        // more-than-4-byte-alignment on the stack.
+        if self.is_like_msvc && self.arch == "x86" {


This function is called a lot during codegen. Should I be worried about the (short) string comparison here?

We can also add a new bool flag to the target spec that directly controls this.

Hm, thinking about this some more it might make sense to make it a part of the target spec so that custom targets can be supported.

Currently we auto-detect this for custom targets, if we make it part of the spec custom target specs will have to remember to do this.

Right but I mean the trade-off is that a potential non-msvc target that's not in-tree will have no way to set this. I don't know how likely that is but you mentioned the possibility earlier. And maybe it's fine in any case if such a targets are likely to have other custom requirements that will necessitate forking.

RalfJung · 2025-04-02T15:30:23Z

This will probably be hellish for the codegen test suite since all the align things are missing now...
@bors try

mitigate MSVC alignment issue on x86-32 This implements mitigation for rust-lang#112480 by stopping to emit `align` attributes on loads and function arguments when building for a win32 MSVC target. MSVC is known to not properly align `u64` and similar types, and claiming to LLVM that everything is properly aligned increases the chance that this will cause problems. Of course, the misalignment is still a bug, but we can't fix that bug, only MSVC can. Also add an errata note to the platform support page warning users about this known problem. try-job: `i686-msvc*`

bors · 2025-04-02T15:31:36Z

⌛ Trying commit ce44d17 with merge 21a86ec...

compiler/rustc_target/src/spec/mod.rs

compiler/rustc_codegen_llvm/src/builder.rs

beetrees · 2025-04-02T17:04:54Z

Looking at #112480 (comment), I think the only alignment this PR needs to change as it that a type with an alignment of 8 bytes might only have an alignment of 4 bytes. MSVC will always align primitive types with an alignment < 8 bytes correctly, and the only types with an alignment > 8 bytes are SIMD types which MSVC aligns correctly anyway.

RalfJung · 2025-04-02T19:42:54Z

Looking at #112480 (comment), I think the only alignment this PR needs to change as it that a type with an alignment of 8 bytes might only have an alignment of 4 bytes. MSVC will always align primitive types with an alignment < 8 bytes correctly, and the only types with an alignment > 8 bytes are SIMD types which MSVC aligns correctly anyway.

I'd rather not rely on the "> 8" part. But yeah I think changing the logic to "clamp to 4" rather than "skip applying alignment" will probably be better, also to affect fewer tests.

RalfJung · 2025-04-03T15:48:43Z

@bors try

mitigate MSVC alignment issue on x86-32 This implements mitigation for rust-lang#112480 by stopping to emit `align` attributes on loads and function arguments when building for a win32 MSVC target. MSVC is known to not properly align `u64` and similar types, and claiming to LLVM that everything is properly aligned increases the chance that this will cause problems. Of course, the misalignment is still a bug, but we can't fix that bug, only MSVC can. Also add an errata note to the platform support page warning users about this known problem. try-job: `i686-msvc*`

bors · 2025-04-03T15:49:56Z

⌛ Trying commit ca29777 with merge 0b5c0be...

bors · 2025-04-03T17:39:20Z

💔 Test failed - checks-actions

RalfJung · 2025-04-04T13:23:36Z

@saethlin I adjusted the alignment check so that it is still active on MSVC but only checks an alignment of up to 4. I hope I did this right. :D What would be the best way to test this? An only-msvc MIR test?

compiler/rustc_mir_transform/src/check_alignment.rs

tests/codegen/align-struct.rs

RalfJung · 2025-04-04T16:08:04Z

@bors try

mitigate MSVC alignment issue on x86-32 This implements mitigation for rust-lang#112480 by stopping to emit `align` attributes on loads and function arguments when building for a win32 MSVC target. MSVC is known to not properly align `u64` and similar types, and claiming to LLVM that everything is properly aligned increases the chance that this will cause problems. Of course, the misalignment is still a bug, but we can't fix that bug, only MSVC can. Also add an errata note to the platform support page warning users about this known problem. try-job: `i686-msvc*`

bors · 2025-04-04T16:09:17Z

⌛ Trying commit df69e5d with merge ae95269...

bors · 2025-04-07T19:17:27Z

💔 Test failed - checks-actions

…32-msvc targets also mention the MSVC alignment issue in platform-support.md

RalfJung · 2025-04-07T21:31:16Z

@bors try

mitigate MSVC alignment issue on x86-32 This implements mitigation for rust-lang#112480 by stopping to emit `align` attributes on loads and function arguments when building for a win32 MSVC target. MSVC is known to not properly align `u64` and similar types, and claiming to LLVM that everything is properly aligned increases the chance that this will cause problems. Of course, the misalignment is still a bug, but we can't fix that bug, only MSVC can. Also add an errata note to the platform support page warning users about this known problem. try-job: `i686-msvc*`

bors · 2025-04-07T21:32:28Z

⌛ Trying commit e702f96 with merge 27ef8fe...

bors · 2025-04-08T00:21:10Z

☀️ Try build successful - checks-actions
Build commit: 27ef8fe (27ef8fe238fd8ed3348e55d4d58f946056e636d8)

RalfJung · 2025-04-08T06:21:53Z

All right this should be @rustbot ready now.

RalfJung · 2025-04-23T13:55:26Z

@oli-obk friendly reviewer ping :)

oli-obk · 2025-04-23T15:16:15Z

@bors r+

bors · 2025-04-23T15:16:19Z

📌 Commit e702f96 has been approved by oli-obk

It is now in the queue for this repository.

…oli-obk mitigate MSVC alignment issue on x86-32 This implements mitigation for rust-lang#112480 by stopping to emit `align` attributes on loads and function arguments when building for a win32 MSVC target. MSVC is known to not properly align `u64` and similar types, and claiming to LLVM that everything is properly aligned increases the chance that this will cause problems. Of course, the misalignment is still a bug, but we can't fix that bug, only MSVC can. Also add an errata note to the platform support page warning users about this known problem. try-job: `i686-msvc*`

Rollup of 23 pull requests Successful merges: - rust-lang#139261 (mitigate MSVC alignment issue on x86-32) - rust-lang#139307 (std: Add performance warnings to HashMap::get_disjoint_mut) - rust-lang#139700 (Autodiff flags) - rust-lang#139752 (set subsections_via_symbols for ld64 helper sections) - rust-lang#139809 (Don't warn about `v128` in wasm ABI transition) - rust-lang#139852 (StableMIR: Implement `CompilerInterface`) - rust-lang#139945 (Extend HIR to track the source and syntax of a lifetime) - rust-lang#140028 (`deref_patterns`: support string and byte string literals in explicit `deref!("...")` patterns) - rust-lang#140139 (rustc_target: Adjust RISC-V feature implication) - rust-lang#140143 (Move `sys::pal::os::Env` into `sys::env`) - rust-lang#140148 (CI: use aws codebuild for job dist-arm-linux) - rust-lang#140150 (fix MAX_EXP and MIN_EXP docs) - rust-lang#140172 (Make algebraic functions into `const fn` items.) - rust-lang#140177 ([compiletest] Parallelize test discovery) - rust-lang#140181 (Remove `synstructure::Structure::underscore_const` calls.) - rust-lang#140184 (Update doc of cygwin target) - rust-lang#140186 (Rename `compute_x` methods) - rust-lang#140187 ([AIX] Handle AIX dynamic library extensions within c-link-to-rust-dylib run-make test) - rust-lang#140191 (Remove git repository from git config) - rust-lang#140194 (minicore: Have `//@ add-core-stubs` also imply `-Cforce-unwind-tables=yes`) - rust-lang#140195 (triagebot: label minicore changes w/ `A-test-infra-minicore` and ping jieyouxu on changes) - rust-lang#140202 (Make #![feature(let_chains)] bootstrap conditional in compiler/) - rust-lang#140214 (Remove comment about handling non-global where bounds with corresponding projection) r? `@ghost` `@rustbot` modify labels: rollup

…oli-obk mitigate MSVC alignment issue on x86-32 This implements mitigation for rust-lang#112480 by stopping to emit `align` attributes on loads and function arguments when building for a win32 MSVC target. MSVC is known to not properly align `u64` and similar types, and claiming to LLVM that everything is properly aligned increases the chance that this will cause problems. Of course, the misalignment is still a bug, but we can't fix that bug, only MSVC can. Also add an errata note to the platform support page warning users about this known problem. try-job: `i686-msvc*`

…iaskrgr Rollup of 8 pull requests Successful merges: - rust-lang#139261 (mitigate MSVC alignment issue on x86-32) - rust-lang#140075 (Mention average in midpoint documentations) - rust-lang#140184 (Update doc of cygwin target) - rust-lang#140186 (Rename `compute_x` methods) - rust-lang#140194 (minicore: Have `//@ add-core-stubs` also imply `-Cforce-unwind-tables=yes`) - rust-lang#140195 (triagebot: label minicore changes w/ `A-test-infra-minicore` and ping jieyouxu on changes) - rust-lang#140214 (Remove comment about handling non-global where bounds with corresponding projection) - rust-lang#140228 (Revert overzealous parse recovery for single colons in paths) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of rust-lang#139261 - RalfJung:msvc-align-mitigation, r=oli-obk mitigate MSVC alignment issue on x86-32 This implements mitigation for rust-lang#112480 by stopping to emit `align` attributes on loads and function arguments when building for a win32 MSVC target. MSVC is known to not properly align `u64` and similar types, and claiming to LLVM that everything is properly aligned increases the chance that this will cause problems. Of course, the misalignment is still a bug, but we can't fix that bug, only MSVC can. Also add an errata note to the platform support page warning users about this known problem. try-job: `i686-msvc*`

rustbot assigned oli-obk Apr 2, 2025

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RalfJung added 2 commits April 7, 2025 23:30

mitigate MSVC unsoundness by not emitting alignment attributes on win…

2678d04

…32-msvc targets also mention the MSVC alignment issue in platform-support.md

check_align: we can still check low alignments on MSVC

e702f96

RalfJung force-pushed the msvc-align-mitigation branch from d9b828f to e702f96 Compare April 7, 2025 21:31

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 Apr 8, 2025

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 Apr 23, 2025

Zalathar mentioned this pull request Apr 24, 2025

Rollup of 23 pull requests #140231

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joshlf mentioned this pull request Apr 24, 2025

Mitigate MSVC alignment? google/zerocopy#2508

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This was referenced Apr 24, 2025

Rollup of 9 pull requests #140242

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Rollup of 8 pull requests #140245

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bors merged commit a8ebfb2 into rust-lang:master Apr 24, 2025
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Amanieu mentioned this pull request Apr 25, 2025

fix - aarch64_be tests rust-lang/stdarch#1786

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mitigate MSVC alignment issue on x86-32 #139261

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