How to use mold without clang

[sebastian-de]

gcc 12.1 is included in freedesktop-SDK 22.08, so it should be possible to use mold without clang.
I'm not able to figure this out however. I have append-path: /usr/lib/sdk/rust-stable/bin in my manifest, so mold is in $PATH.
If I set RUSTFLAGS: -C link-arg=-fuse-ld=mold the build fails:

  = note: collect2: fatal error: cannot find 'ld'
          compilation terminated.
          

error: could not compile `serde` due to previous error

Caused by:
  process didn't exit successfully: `rustc --crate-name build_script_build /run/build/lapce/cargo/vendor/serde-1.0.143/build.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debug-assertions=off --cfg 'feature="default"' --cfg 'feature="derive"' --cfg 'feature="serde_derive"' --cfg 'feature="std"' -C metadata=defc46304c21cac7 -C extra-filename=-defc46304c21cac7 --out-dir /run/build/lapce/target/release/build/serde-defc46304c21cac7 -L dependency=/run/build/lapce/target/release/deps --cap-lints allow -C link-arg=-fuse-ld=mold` (exit status: 1)

[sebastian-de]

I also tried to use mold -run cargo --offline build --release, but that resulted in mold: fatal: mold-wrapper.so is missing.
Does any of the maintainers know how to achieve this? @Hofer-Julian maybe?

[Hofer-Julian]

No sorry, I never tried that and just followed the instructions on mold's README.

[teohhanhui]

It seems like fuse-ld=mold only works if mold is in one of gcc's search dirs:

[📦 org.freedesktop.Sdk ~]$ gcc --print-search-dirs | grep ^programs:
programs: =/usr/libexec/gcc/x86_64-unknown-linux-gnu/12.1.0/:/usr/libexec/gcc/x86_64-unknown-linux-gnu/12.1.0/:/usr/libexec/gcc/x86_64-unknown-linux-gnu/:/usr/lib/gcc/x86_64-unknown-linux-gnu/12.1.0/:/usr/lib/gcc/x86_64-unknown-linux-gnu/:/usr/lib/gcc/x86_64-unknown-linux-gnu/12.1.0/../../../../x86_64-unknown-linux-gnu/bin/x86_64-unknown-linux-gnu/12.1.0/:/usr/lib/gcc/x86_64-unknown-linux-gnu/12.1.0/../../../../x86_64-unknown-linux-gnu/bin/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-unknown-linux-gnu/12.1.0/../../../../x86_64-unknown-linux-gnu/bin/

Perhaps this needs to be fixed in org.freedesktop.Sdk?

[sonnyp]

@teohhanhui is correct

To expand a bit on that, gcc does not support path for -fuse-ld so what works for llvm doesn't for gcc : -fuse-ld=/usr/lib/sdk/rust-stable/bin/mold

-fuse-ld=mold would be the way to go but mold needs to be in the search dirs of gcc.

There is an option -B to add a path to search dirs of gcc but I couldn't make it work

-C link-arg=-fuse-ld=mold -C link-arg=-B=/usr/lib/sdk/rust-stable/ results in note: collect2: fatal error: cannot find 'ld'

There is a whole thread on the gcc mailing list https://gcc.gnu.org/pipermail/gcc-patches/2021-June/thread.html#573768

Regardless I agree that's something that should be fixed in the sdk extension but I don't know if it's possible to add a search dir to gcc from there.

[teohhanhui]

There's also the option of just replacing /usr/bin/ld with lld like what Fedora offers:

https://fedoraproject.org/wiki/Changes/Add_LLD_As_Update_Alternatives_Option_For_LD

(/usr/bin/ld is a symlimk managed by update-alternatives in Fedora)

I don't know how feasible this option is for Flatpak though. Could there be an SDK extension that overrides files from another SDK extension?