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

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

On 26-09-2025 23:00, 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}"
>
> compiler we build is building all architectures once because it then
> reuses same compiler for all cross compilers just by creating canonical
> symlinks, this change is not going to work.
We removed this line because, for the clang class-target build, LLVM was 
only compiling
libraries for the target architecture and GPU targets and kept inside 
the target
sysroot. Since we use the llvm-config built for the class-target during 
the Rust target build,
it was failing due to the absence of static libraries for other 
architectures in the target sysroot.
Here the target is x86 arch. So, it compiled libraries for x86 arch only 
and we can see the error
for missing libraries for other archs as below:

|   --- stderr
|   llvm-config: error: missing: 
poky/build/tmp/work/x86-64-v3-poky-linux/rust/1.90.0/recipe-sysroot/usr/lib/libLLVMAArch64Info.a
|   llvm-config: error: missing: 
poky/build/tmp/work/x86-64-v3-poky-linux/rust/1.90.0/recipe-sysroot/usr/lib/libLLVMLoongArchInfo.a
|   llvm-config: error: missing: 
poky/build/tmp/work/x86-64-v3-poky-linux/rust/1.90.0/recipe-sysroot/usr/lib/libLLVMLoongArchDisassembler.a
|   llvm-config: error: missing: 
poky/build/tmp/work/x86-64-v3-poky-linux/rust/1.90.0/recipe-sysroot/usr/lib/libLLVMMipsInfo.a
|   llvm-config: error: missing: 
poky/build/tmp/work/x86-64-v3-poky-linux/rust/1.90.0/recipe-sysroot/usr/lib/libLLVMPowerPCInfo.a
|   llvm-config: error: missing: 
poky/build/tmp/work/x86-64-v3-poky-linux/rust/1.90.0/recipe-sysroot/usr/lib/libLLVMRISCVTargetMCA.a
.
.
.
|   thread 'main' panicked at compiler/rustc_llvm/build.rs:277:16:
|   command did not execute successfully: 
"poky/build/tmp/work/x86-64-v3-poky-linux/rust/1.90.0/recipe-sysroot/usr/lib/llvm-config" 

"--link-static" "--libs" "aarch64" "amdgpu" "arm" "asmparser" 
"bitreader" "bitwriter" "bpf" "coverage" "instrumentation" "ipo" "linker"
"loongarch" "lto" "mips" "nvptx" "powerpc" "riscv" "x86"
|   expected success, got: exit status: 1

Maybe instead of removing. Can we change
-LLVM_TARGETS_TO_BUILD:class-target ?= "${@get_clang_host_arch(bb, 
d)};BPF;${LLVM_TARGETS_GPU}"
to
-LLVM_TARGETS_TO_BUILD:class-target ?= 
"AArch64;ARM;BPF;Mips;PowerPC;RISCV;X86;LoongArch;;BPF;${LLVM_TARGETS_GPU}" 
?
>
>>
>>   LLVM_EXPERIMENTAL_TARGETS_TO_BUILD ?= ""
>>
>> @@ -107,6 +106,7 @@ EXTRA_OECMAKE += "-DLLVM_ENABLE_ASSERTIONS=OFF \
>>                     -DLLVM_ENABLE_PIC=ON \
>>                     -DCLANG_DEFAULT_PIE_ON_LINUX=ON \
>>                     -DLLVM_BINDINGS_LIST='' \
>> +               -DLLVM_INSTALL_UTILS=ON \
>
> tabs vs spaces inconsistenty. Please fix it.
Sure.
>
>>                     -DLLVM_ENABLE_FFI=ON \
>>                     -DLLVM_ENABLE_ZSTD=ON \
>>                     -DFFI_INCLUDE_DIR=$(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)} \
>
> why is this change needed ?
>
I made this change to ensure that LLVM_TARGET_ARCH is set correctly to 
the actual target architecture.
However, it appears that even without this change, the build completes 
successfully for the target. I checked
and printed the value of HOST_ARCH and realized it was already set 
correctly my mistake. This change is
not required.
>> -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'
>
> This is representing LLVM backend name and not normal arch and clang
> uses RISCV for both rv64 and rv32
>
After this change were made,
-                  -DLLVM_TARGET_ARCH=${HOST_ARCH} \
+                  -DLLVM_TARGET_ARCH=${@get_clang_target_arch(bb, d)} \
do_configure of clang failed
with the following error:

| CMake Error at cmake/config-ix.cmake:585 (message):
|   Unknown architecture riscv

Since this change is irrelevant
-                  -DLLVM_TARGET_ARCH=${HOST_ARCH} \
+                  -DLLVM_TARGET_ARCH=${@get_clang_target_arch(bb, d)} \

The below changes are also not needed.
  -    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('loongarch64$', a):                  return 'LoongArch'
+    elif re.match('loongarch64$', a):                  return 
'loongarch64'
>>       elif re.match('p(pc|owerpc)(|64)', a):             return 
>> 'PowerPC'
>> -    elif re.match('loongarch64$', a):                  return 
>> 'LoongArch'
>> +    elif re.match('loongarch64$', a):                  return 
>> 'loongarch64'
>
> same problem as above here
>
>>       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"
>
> I think using 'llvm' instead of 'clang' here is more appropriate to
> represent the dependency, clang provides llvm as well, it also ensures
> that when we split llvm out of clang recipe then you do not need to
> change it.
Sure will do that.
>
>>   # 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
>> +}
>> +
>>   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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