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

[Deepesh Varatharajan <deepesh.varatharajan@windriver.com>]

On 17-10-2025 11:31, Khem Raj wrote:
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>
> 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"
Hi Khem,

WIth the above mentioned conf in local.conf, I tried building rust for 
qemuarm and qemuarm64 quite
few times and the build completed successfully without any issues for 
both native and target. Do I need
to add any other layers on top of poky ? Do you have any branch where I 
can clone and reproduce the
issue ? Am I missing any step in reproducing the issue ?

Regards,
Deepesh
>
> 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
>>
>>
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