Build script fails when compiling with target-feature=+avx512bw

[tuzz]

Build script fails when compiling with target-feature=+avx512bw

[dtolnay]

RUSTFLAGS is applied to the compilation of both target code and build scripts when Cargo is not in cross-compile mode.

Your command makes build.rs crash on this line:

proc-macro2/build.rs

Line 95 in 49b26d2

let nightly = version.contains("nightly") || version.contains("dev");

If I add return None; above that line, it does not crash. If I add it on the next line, it crashes.

Based on RUSTFLAGS="-C target-feature=+avx512bw" cargo build --verbose, the build script itself is compiled with +avx512bw by Cargo, so a crash makes sense as soon as it hits an instruction that is not available in the host CPU.

Running `~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --crate-name build_scrip
  t_build --edition=2021 ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.7
  0/build.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diag
    ^^^^^^^^
  nostic-width=118 --crate-type bin --emit=dep-info,link -C embed-bitcode=no --cfg 'feature="defa
  ult"' --cfg 'feature="proc-macro"' -C metadata=5c68bf8a9ae873e5 -C extra-filename=-5c68bf8a9ae8
  73e5 --out-dir target/debug/build/proc-macro2-5c68bf8a9ae873e5 -C incremental=target/debug/incr
  emental -L dependency=target/debug/deps -C target-feature=+avx512bw`
                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^

You need to put Cargo in cross-compiling mode in order for it to pass different rustflags to host vs target builds, by passing --target. The correct syntax is:

$ cargo build --target x86_64-unknown-linux-gnu --config 'target.x86_64-unknown-linux-gnu.rustflags=["-C", "target_feature=+avx512bw"]'

or:

$ RUSTFLAGS='-C target-feature=+avx512bw' cargo build --target x86_64-unknown-linux-gnu

This is effective even if the host and target happen to be the same target triple.

[tuzz]

Ahh, thank you.

In my particular case, I had misunderstood how to do runtime detection of SIMD features. I needed to add #[target_feature(enable = "...")] to regions of code that used SIMD features. This guide was very helpful.

Closing now. Thanks again.