aarch64-pc-windows-msvc: -Cprofile-generate binary segfaults during PGO training

Summary

On aarch64-pc-windows-msvc, a Rust binary built with -Cprofile-generate segfaults during ordinary execution. The same project built as a normal LTO release binary (cargo build --release --locked) runs and self-checks successfully.

This was observed while building release artifacts for https://github.com/vjovanov/grund. The failure looks specific to the PGO-instrumented binary or profiling runtime: the instrumented executable builds successfully, then crashes during the PGO training run. llvm-profdata then reports malformed .profraw files.

I am filing this against rust-lang/rust first because the user-visible trigger is rustc -Cprofile-generate; this may ultimately belong in LLVM/compiler-rt.

Not transient

This reproduced in three separate GitHub Actions runs on windows-11-arm / windows-11-arm64:

1. Failed release run: https://github.com/vjovanov/grund/actions/runs/26000032777/job/76421409507
2. Recovery release run, with fallback added after PGO failure: https://github.com/vjovanov/grund/actions/runs/26000896601/job/76423823987
3. Fresh non-publishing dry-run release to re-check transience: https://github.com/vjovanov/grund/actions/runs/26001277253/job/76424832805

All three runs used:

Image: windows-11-arm64
Image Version: 20260510.38.1
Runner version: 2.334.0
rustc 1.95.0 (59807616e 2026-04-14)
host: aarch64-pc-windows-msvc
LLVM version: 22.1.2

The same PGO pipeline passed on Linux x86_64/aarch64, macOS x86_64/aarch64, and Windows x86_64 in the same workflow runs.

Reproducer

On Windows ARM64 with Git Bash available:

git clone https://github.com/vjovanov/grund.git
cd grund
git checkout v0.2.0

rustup toolchain install 1.95.0-aarch64-pc-windows-msvc
rustup override set 1.95.0
rustup component add llvm-tools-preview

bash scripts/pgo-build.sh

The script does the following:

1. Builds the binary with RUSTFLAGS=-Cprofile-generate=<target/pgo-data> cargo build --release --locked.
2. Runs the freshly built target/release/grund.exe against this repository as its PGO training workload.
3. Merges the generated .profraw files with llvm-profdata.
4. Rebuilds with -Cprofile-use.

The failure happens in step 2. A more direct manual reduction from the script is:

repo="$PWD"
pgo_dir="$repo/target/pgo-data"
rm -rf "$pgo_dir"
mkdir -p "$pgo_dir"

# On Git Bash, rustc wants a Windows-style profile path.
pgo_dir_rustc="$(cygpath -m "$pgo_dir")"
RUSTFLAGS="-Cprofile-generate=$pgo_dir_rustc" cargo build --release --locked

./target/release/grund.exe list "$repo"

Observed: the instrumented grund.exe segfaults.

Expected: the instrumented binary should run normally and emit valid .profraw profile data, as it does on the other release targets.

CI log excerpts

Fresh dry-run release, run 26001277253:

PGO binary (windows-11-arm) Set up job
Current runner version: '2.334.0'
Runner Image Provisioner Version: 20260415.520
Image: windows-11-arm64
Version: 20260510.38.1

rustc 1.95.0 (59807616e 2026-04-14)
host: aarch64-pc-windows-msvc
LLVM version: 22.1.2

==> 1/3  build instrumented binary (-Cprofile-generate)
Finished `release` profile [optimized] target(s) in 52.76s
==> 2/3  training run — the §AR-benchmarks workload
scripts/pgo-build.sh: line 66:   324 Segmentation fault         "$grund" list "$repo" > /dev/null 2>&1
scripts/pgo-build.sh: line 66:   325 Segmentation fault         "$grund" show FS-check --brief "$repo" > /dev/null 2>&1
scripts/pgo-build.sh: line 66:   326 Segmentation fault         "$grund" show FS-check "$repo" > /dev/null 2>&1
scripts/pgo-build.sh: line 66:   327 Segmentation fault         "$grund" show FS-check --full "$repo" > /dev/null 2>&1
scripts/pgo-build.sh: line 66:   328 Segmentation fault         "$grund" refs GOAL-fast-feedback "$repo" > /dev/null 2>&1
scripts/pgo-build.sh: line 66:   329 Segmentation fault         "$grund" cover "$repo" > /dev/null 2>&1
scripts/pgo-build.sh: line 66:   330 Segmentation fault         "$grund" fmt --check "$repo" > /dev/null 2>&1
scripts/pgo-build.sh: line 66:   331 Segmentation fault         "$grund" check "$repo" > /dev/null 2>&1
...
warning: C:/a/grund/grund/target/pgo-data/default_2211884963031461884_0.profraw: malformed instrumentation profile data: symbol name is empty
error: no profile can be merged

Original failed release, run 26000032777:

Image: windows-11-arm64
Version: 20260510.38.1
rustc 1.95.0 (59807616e 2026-04-14)
host: aarch64-pc-windows-msvc
LLVM version: 22.1.2

==> 2/3  training run — the §AR-benchmarks workload
scripts/pgo-build.sh: line 66:  1286 Segmentation fault         "$grund" list "$repo" > /dev/null 2>&1
scripts/pgo-build.sh: line 66:  1287 Segmentation fault         "$grund" show FS-check --brief "$repo" > /dev/null 2>&1
scripts/pgo-build.sh: line 66:  1288 Segmentation fault         "$grund" show FS-check "$repo" > /dev/null 2>&1
...
warning: C:/a/grund/grund/target/pgo-data/default_15662148217428528836_0.profraw: malformed instrumentation profile data: symbol name is empty
warning: C:/a/grund/grund/target/pgo-data/default_6987841190259789436_0.profraw: malformed instrumentation profile data: symbol name is empty
warning: C:/a/grund/grund/target/pgo-data/default_2211884963031461884_0.profraw: malformed instrumentation profile data: symbol name is empty
error: no profile can be merged

Recovery release, run 26000896601, reproduced the same PGO failure and then succeeded with the workflow's non-PGO fallback:

Image: windows-11-arm64
Version: 20260510.38.1
rustc 1.95.0 (59807616e 2026-04-14)
host: aarch64-pc-windows-msvc
LLVM version: 22.1.2

==> 2/3  training run — the §AR-benchmarks workload
scripts/pgo-build.sh: line 66:  1371 Segmentation fault         "$grund" list "$repo" > /dev/null 2>&1
scripts/pgo-build.sh: line 66:  1372 Segmentation fault         "$grund" show FS-check --brief "$repo" > /dev/null 2>&1
...
warning: C:/a/grund/grund/target/pgo-data/default_15662148217428528836_0.profraw: malformed instrumentation profile data: symbol name is empty
warning: C:/a/grund/grund/target/pgo-data/default_6987841190259789436_0.profraw: malformed instrumentation profile data: symbol name is empty
warning: C:/a/grund/grund/target/pgo-data/default_2211884963031461884_0.profraw: malformed instrumentation profile data: symbol name is empty
error: no profile can be merged

Build LTO release binary fallback:
cargo build --release --locked
Finished `release` profile [optimized] target(s) in 44.33s

Additional notes

• The repo has no Windows-active unsafe in this path. The only unsafe found is a Unix-only SIGPIPE restore guarded by #[cfg(unix)].
• The non-PGO fallback binary built immediately after the PGO failure passed the repo self-check in CI.
• I searched for existing reports in rust-lang/rust and llvm/llvm-project with terms around aarch64-pc-windows-msvc, profile-generate, segfault, malformed instrumentation profile data, and symbol name is empty, and did not find an obvious duplicate.

Start by reproducing the failure on aarch64-pc-windows-msvc using scripts/pgo-build.sh and the direct -Cprofile-generate command in the issue. Compare the instrumented training run with the successful cargo build --release --locked fallback, then inspect the llvm-profdata output. Done means the PGO binary completes training without segfaults and produces valid .profraw data that can be merged.