Re: [OE-core] [PATCH V5 2/3] rust: Use llvm instead of rust-llvm

Khem Raj
2025-10-17T06:01:38Z

I am seeing a build regression when using clang as default compiler

https://errors.yoctoproject.org/Errors/Details/885430/

PREFERRED_TOOLCHAIN_TARGET ?= "clang"
PREFERRED_TOOLCHAIN_SDK ?= "clang"
TOOLCHAIN_NATIVE:pn-rust ?= "clang"
TOOLCHAIN_NATIVE:pn-rust-native ?= "clang"
TOOLCHAIN_NATIVE:pn-nativesdk-rust ?= "clang"

On Wed, Oct 15, 2025 at 2:59 AM Varatharajan, Deepesh via
lists.openembedded.org
<deepesh.varatharajan=windriver.com@lists.openembedded.org> wrote:
>
> From: Deepesh Varatharajan <Deepesh.Varatharajan@windriver.com>
>
> Updated the Rust build to depend on llvm instead.
>
> *Summary of discussion with the rust upstream about using latest LLVM instead of Rust maintained LLVM fork.
> https://internals.rust-lang.org/t/can-we-use-proper-clang-instead-of-llvm-fork-what-rust-uses/23489
>
> *Upstream LLVM is generally compatible:
> - Rust does support building with upstream (vanilla) LLVM, especially the latest
> major release and the one or two preceding ones.
> https://rustc-dev-guide.rust-lang.org/backend/updating-llvm.html#updating-llvm
>
> *Impact on Yocto Rust upgrades:
> - Rust upgrades shall always check for updates on rust forked llvm and backport
> the relevant patches to llvm.
>
> *Regarding the rust forked llvm local patches:
> - There are no local patches on rust forked llvm other than the backported fixes
> from llvm master.
>
> *We are copying the natively built `llvm-config` binary into the target sysroot and running
> it. However, this `llvm-config` has compile time dependencies on various other arch's LLVM
> libraries because native-llvm is built for all oe-core supported targets.
>
> Attempting to work around this by symlinking the missing libraries from the native sysroot
> into the target sysroot leads to mixed architectures in the final `.rlib`. Specifically,
> the object files extracted from those symlinked libraries within `librustc_llvm-<hash>.rlib`
> are built for the host, while others are correctly built for the target This results in linker
> failures due to file format not recognized.
>
> To resolve this, we now build llvm-target also for all oe-core supported architectures in
> addition to the native-llvm build. This ensures that `llvm-config` and all associated
> libraries are built for the correct target, eliminating cross-architecture contamination
> and linker issues.
>
> *We are enabling -DLLVM_INSTALL_UTILS=ON to ensure essential LLVM utilities like FileCheck
> are available, as they are required by the Rust build.
>
> Without this, the build fails with an error as below:
> | thread 'main' panicked at src/bootstrap/src/core/sanity.rs:315:21:
> | FileCheck executable "poky/build/tmp/work/x86_64-linux/rust-native/1.90.0/recipe-sysroot
> -native/usr/bin/FileCheck" does not exist
>
> *We now add these flags "-Clink-arg=-lz -Clink-arg=-lzstd" because of this following
> diff otherwise we will get errors during link time.
>
> Setup in rust-llvm
> -DLLVM_ENABLE_ZLIB=OFF \
> -DLLVM_ENABLE_ZSTD=OFF \
> -DLLVM_ENABLE_FFI=OFF \
>
> Setup in llvm
> -DLLVM_ENABLE_FFI=ON \
>
> *When multilibs enabled:
>
> llvm-config expects static libraries to be located in the lib directory rather than
> lib64. However, since we are copying the natively built llvm-config to target sysroot
> and running it and llvm-config doesn't know anything about lib64 existence. To accommodate
> this without breaking multilib behavior, we are creating a symlink from 'lib' to 'lib64'
> directory.
>
> Previously, when we depended on rust-llvm, this worked because we specified:
> -DCMAKE_INSTALL_PREFIX:PATH=${libdir}/llvm-rust
>
> With this setup, llvm-config was installed inside ${libdir}/llvm-rust, which included
> its own bin and lib directories. Thus, llvm-config located in bin would correctly find
> the libraries in the adjacent lib directory.
>
> Even when multilib was enabled or not, llvm-config would still look for libraries under
> lib in this structure, so everything functioned as expected.
>
> Signed-off-by: Deepesh Varatharajan <Deepesh.Varatharajan@windriver.com>
> ---
> meta/recipes-devtools/clang/llvm_git.bb | 2 +-
> meta/recipes-devtools/rust/rust_1.90.0.bb | 39 ++++++++++++++++++++---
> 2 files changed, 36 insertions(+), 5 deletions(-)
>
> diff --git a/meta/recipes-devtools/clang/llvm_git.bb b/meta/recipes-devtools/clang/llvm_git.bb
> index d2b060ff88..2f47af8d7a 100644
> --- a/meta/recipes-devtools/clang/llvm_git.bb
> +++ b/meta/recipes-devtools/clang/llvm_git.bb
> @@ -27,7 +27,6 @@ OECMAKE_SOURCEPATH = "${S}/llvm"
> # https://github.com/llvm/llvm-project/blob/main/llvm/CMakeLists.txt
> LLVM_TARGETS_GPU ?= "${@bb.utils.contains_any('DISTRO_FEATURES', 'opencl opengl vulkan', 'AMDGPU;NVPTX;SPIRV', '', d)}"
> LLVM_TARGETS_TO_BUILD ?= "AArch64;ARM;BPF;Mips;PowerPC;RISCV;X86;LoongArch;${LLVM_TARGETS_GPU}"
> -LLVM_TARGETS_TO_BUILD:class-target ?= "${@get_clang_host_arch(bb, d)};BPF;${LLVM_TARGETS_GPU}"
>
> LLVM_EXPERIMENTAL_TARGETS_TO_BUILD ?= ""
>
> @@ -37,6 +36,7 @@ HF[vardepvalue] = "${HF}"
>
> EXTRA_OECMAKE += "-DCMAKE_BUILD_TYPE=MinSizeRel \
> -DLLVM_ENABLE_BINDINGS=OFF \
> + -DLLVM_INSTALL_UTILS=ON \
> -DLLVM_ENABLE_FFI=ON \
> -DLLVM_ENABLE_RTTI=ON \
> -DLLVM_TARGETS_TO_BUILD='${LLVM_TARGETS_TO_BUILD}' \
> diff --git a/meta/recipes-devtools/rust/rust_1.90.0.bb b/meta/recipes-devtools/rust/rust_1.90.0.bb
> index 5d804c7398..0319d73b93 100644
> --- a/meta/recipes-devtools/rust/rust_1.90.0.bb
> +++ b/meta/recipes-devtools/rust/rust_1.90.0.bb
> @@ -7,7 +7,7 @@ LIC_FILES_CHKSUM = "file://COPYRIGHT;md5=11a3899825f4376896e438c8c753f8dc"
> inherit rust
> inherit cargo_common
>
> -DEPENDS += "rust-llvm"
> +DEPENDS += "llvm"
> # native rust uses cargo/rustc from binary snapshots to bootstrap
> # but everything else should use our native builds
> DEPENDS:append:class-target = " cargo-native rust-native"
> @@ -28,8 +28,8 @@ PV .= "${@bb.utils.contains('RUST_CHANNEL', 'stable', '', '-${RUST_CHANNEL}', d)
>
> export FORCE_CRATE_HASH = "${BB_TASKHASH}"
>
> -RUST_ALTERNATE_EXE_PATH ?= "${STAGING_LIBDIR}/llvm-rust/bin/llvm-config"
> -RUST_ALTERNATE_EXE_PATH_NATIVE = "${STAGING_LIBDIR_NATIVE}/llvm-rust/bin/llvm-config"
> +RUST_ALTERNATE_EXE_PATH ?= "${STAGING_BINDIR}/llvm-config"
> +RUST_ALTERNATE_EXE_PATH_NATIVE = "${STAGING_BINDIR_NATIVE}/llvm-config"
>
> # We don't want to use bitbakes vendoring because the rust sources do their
> # own vendoring.
> @@ -188,6 +188,37 @@ python do_configure() {
> bb.build.exec_func("setup_cargo_environment", d)
> }
>
> +# llvm-config expects static libraries to be in the 'lib' directory rather than 'lib64' when
> +# multilibs enabled. Since we are copying the natively built llvm-config into the target sysroot
> +# and executing it there, it will default to searching in 'lib', as it is unaware of the 'lib64'
> +# directory. To ensure llvm-config can locate the necessary libraries, create a symlink from 'lib'
> +do_compile:append:class-target() {
> + # Ensure llvm-config can find static libraries in multilib setup
> + lib64_dir="${STAGING_DIR_TARGET}/usr/lib64"
> + lib_dir="${STAGING_DIR_TARGET}/usr/lib"
> +
> + if [ -d "$lib64_dir" ]; then
> + # If lib does not exist, symlink it to lib64
> + if [ ! -e "$lib_dir" ]; then
> + ln -s lib64 "$lib_dir"
> + fi
> +
> + # Only do per-file symlinking if lib is a real directory (not symlink)
> + if [ -d "$lib_dir" ] && [ ! -L "$lib_dir" ]; then
> + for lib64_file in "${lib64_dir}"/libLLVM*.a; do
> + if [ -e "$lib64_file" ]; then
> + lib_name=$(basename "${lib64_file}")
> + target_link="${lib_dir}/${lib_name}"
> +
> + if [ ! -e "${target_link}" ]; then
> + ln -s "../lib64/${lib_name}" "${target_link}"
> + fi
> + fi
> + done
> + fi
> + fi
> +}
> +
> rust_runx () {
> echo "COMPILE ${PN}" "$@"
>
> @@ -199,7 +230,7 @@ rust_runx () {
> unset CXXFLAGS
> unset CPPFLAGS
>
> - export RUSTFLAGS="${RUST_DEBUG_REMAP}"
> + export RUSTFLAGS="${RUST_DEBUG_REMAP} -Clink-arg=-lz -Clink-arg=-lzstd"
>
> # Copy the natively built llvm-config into the target so we can run it. Horrible,
> # but works!
> --
> 2.49.0
>
>
> -=-=-=-=-=-=-=-=-=-=-=-
> Links: You receive all messages sent to this group.
> View/Reply Online (#224888): https://lists.openembedded.org/g/openembedded-core/message/224888
> Mute This Topic: https://lists.openembedded.org/mt/115768136/1997914
> Group Owner: openembedded-core+owner@lists.openembedded.org
> Unsubscribe: https://lists.openembedded.org/g/openembedded-core/unsub [raj.khem@gmail.com]
> -=-=-=-=-=-=-=-=-=-=-=-
>