LLVM assertion with 16 codegen units and ThinLTO

[Luthaf]

Compiling the latest master of soa-derive with 16 codegen units and thinLTO activated makes the compiler hit an LLVM assertion:

$ RUSTFLAGS='-C codegen-units=16 -Z thinlto' cargo +nightly build --release --verbose
       Fresh unicode-xid v0.0.4
       Fresh quote v0.3.15
   Compiling syn v0.10.8
     Running `rustc --crate-name syn /home/guillaume/.cargo/registry/src/github.com-1ecc6299db9ec823/syn-0.10.8/src/lib.rs --crate-type lib --emit=dep-info,link -C opt-level=3 --cfg 'feature="default"' --cfg 'feature="parsing"' --cfg 'feature="printing"' --cfg 'feature="quote"' --cfg 'feature="unicode-xid"' -C metadata=119c35ede84429ac -C extra-filename=-119c35ede84429ac --out-dir /home/guillaume/perso/lumol/soa-derive/target/release/deps -L dependency=/home/guillaume/perso/lumol/soa-derive/target/release/deps --extern quote=/home/guillaume/perso/lumol/soa-derive/target/release/deps/libquote-fdddc1cdae0a2803.rlib --extern unicode_xid=/home/guillaume/perso/lumol/soa-derive/target/release/deps/libunicode_xid-e1922fe19127ad04.rlib --cap-lints allow -C codegen-units=16 -Z thinlto`
rustc: /checkout/src/llvm/lib/Transforms/Scalar/GVN.cpp:1933: bool llvm::GVN::replaceOperandsWithConsts(llvm::Instruction*) const: Assertion `!isa<Constant>(Operand) && "Replacing constants with constants is invalid"' failed.
error: Could not compile `syn`.

Caused by:
  process didn't exit successfully: `rustc --crate-name syn /home/guillaume/.cargo/registry/src/github.com-1ecc6299db9ec823/syn-0.10.8/src/lib.rs --crate-type lib --emit=dep-info,link -C opt-level=3 --cfg feature="default" --cfg feature="parsing" --cfg feature="printing" --cfg feature="quote" --cfg feature="unicode-xid" -C metadata=119c35ede84429ac -C extra-filename=-119c35ede84429ac --out-dir /home/guillaume/perso/lumol/soa-derive/target/release/deps -L dependency=/home/guillaume/perso/lumol/soa-derive/target/release/deps --extern quote=/home/guillaume/perso/lumol/soa-derive/target/release/deps/libquote-fdddc1cdae0a2803.rlib --extern unicode_xid=/home/guillaume/perso/lumol/soa-derive/target/release/deps/libunicode_xid-e1922fe19127ad04.rlib --cap-lints allow -C codegen-units=16 -Z thinlto` (signal: 6, SIGABRT: process abort signal)

The assertion error appears when compiling syn, but I could not reproduce with the syn crate by itself.

Meta

$ rustc +nightly -vV      
rustc 1.22.0-nightly (150b625a0 2017-10-08)
binary: rustc
commit-hash: 150b625a07f135d0a5e1693adf2b14315709779e
commit-date: 2017-10-08
host: x86_64-unknown-linux-gnu
release: 1.22.0-nightly
LLVM version: 4.0

[leoyvens]

ping @alexcrichton

[alexcrichton]

Thanks for the report! I'll try to investigate soon

[saethlin]

Same error comes out of the crate python3-sys.

     Running `rustc --crate-name build_script_build /home/ben/.cargo/registry/src/github.com-1ecc6299db9ec823/python3-sys-0.1.3/build.rs --crate-type bin --emit=dep-info,link -C opt-level=3 --cfg 'feature="default"' --cfg 'feature="extension-module"' --cfg 'feature="python-3"' -C metadata=60c44052e847b67b -C extra-filename=-60c44052e847b67b --out-dir /home/ben/rust-lather/target/release/build/python3-sys-60c44052e847b67b -L dependency=/home/ben/rust-lather/target/release/deps --extern regex=/home/ben/rust-lather/target/release/deps/libregex-6bf8361efa00f7a6.rlib --cap-lints allow -Ccodegen-units=4 -C target-cpu=native -C codegen-units=32 -Z thinlto`
rustc: /checkout/src/llvm/lib/Transforms/Scalar/GVN.cpp:1933: bool llvm::GVN::replaceOperandsWithConsts(llvm::Instruction*) const: Assertion `!isa<Constant>(Operand) && "Replacing constants with constants is invalid"' failed.
error: Could not compile `python3-sys`.

rustc 1.22.0-nightly (150b625a0 2017-10-08)
binary: rustc
commit-hash: 150b625a07f135d0a5e1693adf2b14315709779e
commit-date: 2017-10-08
host: x86_64-unknown-linux-gnu
release: 1.22.0-nightly
LLVM version: 4.0

[alexcrichton]

I've opened up an upstream bug for this as well as a fix, and I'll backport this to our branch once it lands.