Re: [OE-core] [PATCH V3 1/2] rust: Use clang instead of rust-llvm

[Khem Raj <raj.khem@gmail.com>]

On 9/26/25 3:24 AM, Varatharajan, Deepesh via lists.openembedded.org wrote:
> From: Deepesh Varatharajan <Deepesh.Varatharajan@windriver.com>
> 
> Updated the Rust build to depend on Clang 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 clang's 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 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 clang
> -DLLVM_ENABLE_FFI=ON \
> -DLLVM_ENABLE_ZSTD=ON \
> 
> *When multilibs enabled:
> 
> llvm-config expects static libraries to be located in the lib directory rather than
> lib64. However, since LLVM is built as a non-multilib component, the lib directory
> doesn't contain any library files. To accommodate this without breaking multilib
> behavior, we copy the required library files appropriately.
> 
> 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 ${libdir}/llvm-rust, which included
> its own bin and lib directories. Thus, llvm-config located in bin would correctly find
> the libraries in the adjacent lib directory.
> 
> Even when multilib was enabled or not, llvm-config would still look for libraries under
> lib in this structure, so everything functioned as expected.
> 
> *Changes needs to be done when llvm splits from clang:
> In rust recipe:
> Update the dependency from:
> DEPENDS += "ninja-native clang" to DEPENDS += "ninja-native llvm"
> 
> In llvm recipe:
> Apply the same changes that were made in the Clang recipe, as those
> configurations have now been moved to the LLVM recipe after the split.
> 
> Signed-off-by: Deepesh Varatharajan <Deepesh.Varatharajan@windriver.com>
> ---
>   meta/recipes-devtools/clang/clang_git.bb     |  4 ++--
>   meta/recipes-devtools/clang/common-clang.inc |  6 +++---
>   meta/recipes-devtools/rust/rust_1.90.0.bb    | 18 ++++++++++++++----
>   3 files changed, 19 insertions(+), 9 deletions(-)
> 
> diff --git a/meta/recipes-devtools/clang/clang_git.bb b/meta/recipes-devtools/clang/clang_git.bb
> index 53bca1c24f..3e117b308b 100644
> --- a/meta/recipes-devtools/clang/clang_git.bb
> +++ b/meta/recipes-devtools/clang/clang_git.bb
> @@ -83,7 +83,6 @@ OECMAKE_SOURCEPATH = "${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 ?= ""
>   
> @@ -107,6 +106,7 @@ EXTRA_OECMAKE += "-DLLVM_ENABLE_ASSERTIONS=OFF \
>                     -DLLVM_ENABLE_PIC=ON \
>                     -DCLANG_DEFAULT_PIE_ON_LINUX=ON \
>                     -DLLVM_BINDINGS_LIST='' \
> +		  -DLLVM_INSTALL_UTILS=ON \
>                     -DLLVM_ENABLE_FFI=ON \
>                     -DLLVM_ENABLE_ZSTD=ON \
>                     -DFFI_INCLUDE_DIR=$(pkg-config --variable=includedir libffi) \
> @@ -137,7 +137,7 @@ EXTRA_OECMAKE:append:class-target = "\
>                     -DCMAKE_AR=${STAGING_BINDIR_TOOLCHAIN}/${TARGET_PREFIX}llvm-ar \
>                     -DCMAKE_NM=${STAGING_BINDIR_TOOLCHAIN}/${TARGET_PREFIX}llvm-nm \
>                     -DCMAKE_STRIP=${STAGING_BINDIR_TOOLCHAIN}/${TARGET_PREFIX}llvm-strip \
> -                  -DLLVM_TARGET_ARCH=${HOST_ARCH} \
> +                  -DLLVM_TARGET_ARCH=${@get_clang_target_arch(bb, d)} \
>                     -DLLVM_DEFAULT_TARGET_TRIPLE=${TARGET_SYS}${HF} \
>                     -DLLVM_HOST_TRIPLE=${TARGET_SYS}${HF} \
>                     -DLLVM_LIBDIR_SUFFIX=${LLVM_LIBDIR_SUFFIX} \
> diff --git a/meta/recipes-devtools/clang/common-clang.inc b/meta/recipes-devtools/clang/common-clang.inc
> index bf3a63914a..c22e3c1b19 100644
> --- a/meta/recipes-devtools/clang/common-clang.inc
> +++ b/meta/recipes-devtools/clang/common-clang.inc
> @@ -30,10 +30,10 @@ def get_clang_arch(bb, d, arch_var):
>       elif re.match('aarch64$', a):                      return 'AArch64'
>       elif re.match('aarch64_be$', a):                   return 'AArch64'
>       elif re.match('mips(isa|)(32|64|)(r6|)(el|)$', a): return 'Mips'
> -    elif re.match('riscv32$', a):                      return 'RISCV'
> -    elif re.match('riscv64$', a):                      return 'RISCV'
> +    elif re.match('riscv32$', a):                      return 'riscv32'
> +    elif re.match('riscv64$', a):                      return 'riscv64'
>       elif re.match('p(pc|owerpc)(|64)', a):             return 'PowerPC'
> -    elif re.match('loongarch64$', a):                  return 'LoongArch'
> +    elif re.match('loongarch64$', a):                  return 'loongarch64'
>       else:
>           bb.fatal("Unhandled architecture %s" % arch_val)
>           return ""
> diff --git a/meta/recipes-devtools/rust/rust_1.90.0.bb b/meta/recipes-devtools/rust/rust_1.90.0.bb
> index 5d804c7398..c2cb8f8829 100644
> --- a/meta/recipes-devtools/rust/rust_1.90.0.bb
> +++ b/meta/recipes-devtools/rust/rust_1.90.0.bb
> @@ -7,7 +7,7 @@ LIC_FILES_CHKSUM = "file://COPYRIGHT;md5=11a3899825f4376896e438c8c753f8dc"
>   inherit rust
>   inherit cargo_common
>   
> -DEPENDS += "rust-llvm"
> +DEPENDS += "ninja-native clang"
>   # native rust uses cargo/rustc from binary snapshots to bootstrap
>   # but everything else should use our native builds
>   DEPENDS:append:class-target = " cargo-native rust-native"
> @@ -28,8 +28,8 @@ PV .= "${@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 +188,16 @@ python do_configure() {
>       bb.build.exec_func("setup_cargo_environment", d)
>   }
>   
> +#llvm-config expecting static libraries in 'lib' instead of 'lib64'.
> +#Since LLVM is built as a non-multilib component, the 'lib' directory
> +#doesn't have any library files when multilibs enabled. So, copying
> +#library files without impacting multilib behavior.
> +do_compile:append:class-target() {
> +if [ -d ${STAGING_DIR_TARGET}/usr/lib64 ]; then
> +    cp ${STAGING_DIR_TARGET}/usr/lib64/libLLVM*.a ${STAGING_DIR_TARGET}/usr/lib/.
> +fi
> +}

llvm-config can deduce paths to be self-relative, You might try to 
create a copy of native llvm-config in STAGING_LIBDIR or STAGING_BINDIR 
and that might help

> +
>   rust_runx () {
>       echo "COMPILE ${PN}" "$@"
>   
> @@ -199,7 +209,7 @@ 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!
> 
> 
> 
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