rust-lang / rust-lang/rust

16k-aligned statics crash rustc on Windows

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A-align A-LLVM A-repr C-bug E-needs-investigation I-crash O-windows T-compiler
Dominant language
Rust
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Description

#[repr(align(16384))]
struct HighAlignment;

static EXAMPLE: HighAlignment = const { HighAlignment };

fn main() {}
$ cargo build --target x86_64-pc-windows-gnu
...
error: could not compile `cringe` (bin "cringe"); 2 warnings emitted

Caused by:
  process didn't exit successfully: `/home/purplesyringa/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --crate-name cringe --edition=2021 src/main.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=211 --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debuginfo=2 --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values())' -C metadata=2b980327f571f145 -C extra-filename=-97496ab9b6118996 --out-dir /home/purplesyringa/cringe/target/x86_64-pc-windows-gnu/debug/deps --target x86_64-pc-windows-gnu -C incremental=/home/purplesyringa/cringe/target/x86_64-pc-windows-gnu/debug/incremental -L dependency=/home/purplesyringa/cringe/target/x86_64-pc-windows-gnu/debug/deps -L dependency=/home/purplesyringa/cringe/target/debug/deps` (signal: 4, SIGILL: illegal instruction)

Also reproduces on other ABIs, also gives STATUS_ILLEGAL_INSTRUCTION when building on Windows. Also applies to thread locals. Something something IMAGE_SCN_ALIGN_ stops at 8k and LLVM fails an assertion?

Meta

rustc --version --verbose:

rustc 1.86.0-nightly (419b3e2d3 2025-01-15)
binary: rustc
commit-hash: 419b3e2d3e350822550eee0e82eeded4d324d584
commit-date: 2025-01-15
host: x86_64-unknown-linux-gnu
release: 1.86.0-nightly
LLVM version: 19.1.6

@rustbot label +I-crash +T-compiler +A-LLVM +E-needs-investigation +O-windows

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Research direction

No source files or tests are named. Start by reproducing the example with cargo build --target x86_64-pc-windows-gnu, then trace the Rust compiler's LLVM Windows handling for highly aligned statics and thread locals. Done means the reproducer no longer crashes or emits STATUS_ILLEGAL_INSTRUCTION, with coverage for the affected alignment case.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers, operating-systems
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
35/100

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