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

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

On 26-09-2025 22:20, Khem Raj wrote:
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> 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
The llvm-config binary built for the target is already available in 
STAGING_BINDIR
  and we're using that for the Rust target build. I also tried copying 
the natively built
llvm-config into STAGING_LIBDIR and adjusted the Rust target build to 
depend on it,
but the build still failed. The issue was that it continued to look in 
the lib directory
instead of lib64 when multilibs are enabled. However, this can be 
addressed in an
alternative way as below:

-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"

Currently I am making the above changes. Since as you suggested to copy 
and use
natively built llvm-config, I'm wondering would it be better to simply 
make the
Rust target build depend directly on the native llvm-config as below.

-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 ?= "${STAGING_BINDIR_NATIVE}/llvm-config"
+RUST_ALTERNATE_EXE_PATH_NATIVE = "${STAGING_BINDIR_NATIVE}/llvm-config"

>
>> +
>>   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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