llvm assertion failure when trying to compile sha2 crate for skylake (with llvm+rustc debug assertions)

[matthiaskrgr]

I have a Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz, so when compiling with -Ctarget-cpu=native this is equivalent to -Ctarget-cpu=skylake.

I have a rustc+llvm that has debug assertions enabled and when I try to compile sha2 v0.10.1 this apparently hits a debug assertion inside llvm:

RUSTFLAGS="-Ctarget-cpu=skylake" RUSTC=/home/matthias/vcs/github/rust_debug_assertions/build/x86_64-unknown-linux-gnu/stage1/bin/rustc cargo build --release

   Compiling sha2 v0.10.1 (/tmp/sha)
rustc: /home/matthias/vcs/github/rust_debug_assertions/src/llvm-project/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:2556: llvm::slpvectorizer::BoUpSLP::TreeEntry *llvm::slpvectorizer::BoUpSLP::newTreeEntry(ArrayRef<llvm::Value *>, TreeEntry::EntryState, Optional<llvm::slpvectorizer::BoUpSLP::ScheduleData *>, const (anonymous namespace)::InstructionsState &, const llvm::slpvectorizer::BoUpSLP::EdgeInfo &, ArrayRef<int>, ArrayRef<unsigned int>): Assertion `!getTreeEntry(V) && "Scalar already in tree!"' failed.
sccache: Compiler killed by signal 6
error: could not compile `sha2`

Caused by:
  process didn't exit successfully: `/home/matthias/.cargo/bin/sccache /home/matthias/vcs/github/rust_debug_assertions/build/x86_64-unknown-linux-gnu/stage1/bin/rustc --crate-name sha2 --edition=2018 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C opt-level=3 -C embed-bitcode=no --cfg 'feature="default"' --cfg 'feature="std"' -C metadata=96ac895c949c4242 -C extra-filename=-96ac895c949c4242 --out-dir /tmp/sha/target/release/deps -L dependency=/tmp/sha/target/release/deps --extern cfg_if=/tmp/sha/target/release/deps/libcfg_if-19e7cc0b6f708960.rmeta --extern cpufeatures=/tmp/sha/target/release/deps/libcpufeatures-60eedbcf432061e3.rmeta --extern digest=/tmp/sha/target/release/deps/libdigest-d6ef0fdeb4fc592a.rmeta -Ctarget-cpu=skylake` (exit status: 254)

This rustc used is built with rustc debug assertions and llvm debug assertions and I noticed this crash during stage 2 of x.py build boostrap.

bootstrap was done with

changelog-seen = 2
[llvm]
download-ci-llvm = false
assertions = true
ninja = true
targets = "X86"
link-jobs = 2
[build]
extended = true
tools = ["cargo", "rls", "clippy", "rustfmt", "analysis", "src"] # + "rust-demangler" if `profiler`
low-priority = true
[install]
[rust]
debug = true
debug-assertions = true
debug-assertions-std = true
overflow-checks = true
overflow-checks-std = true
debuginfo-level = 1
backtrace-on-ice = true
verify-llvm-ir = true
[target.x86_64-unknown-linux-gnu]
[dist]

[matthiaskrgr]

I had a quick look into the crate and it seems to have some special avx2 related code, so there might be some unsafe that rustc cannot judge which later causes the assertion failure in llvm 🤔

[nikic]

Upstream issue: llvm/llvm-project#57447