[libcxx][CI] Prepare ARM Picolibc builds for upgrade to clang 21 #158300
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Some time between Clang 19 and Clang 21, changes were made to the bare metal toolchain driver, and on top of that we (Linaro) updated our host OS to Ubuntu Jammy. This has caused the tests to not build with clang-21. 1. The host version of ld (2.38) not being able to parse certain DWARFv5 constructs. Normally I would switch to lld instead. However when I did this, I found that Picolib's ARM linker script uses ALIGN_WITH_INPUT, which lld does not support. So instead I'm sticking with ld and falling back to DWARFv4. Until such time as we upgrade the host to Ubuntu Noble. (if you're wondering how we can use ld in the first place, given the host is AArch64, the AArch64 ld is also capable of linking ARM 32-bit binaries) 2. Many missing symbol errors. This is solved in two parts: * Adding libunwind to the testing linker flags. * Putting the libraries in a group so that the ordering does not matter. And finally for exception_guard.odr.sh.cpp, I have moved the object files to before the linker flags. ld is sensitive to the order of objects on the command line, and this should not break other configurations. I've tested these with 19.1.7 too so my plan is to land these changes first, then proceed with upgrading to 21.x.
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@llvm/pr-subscribers-libunwind @llvm/pr-subscribers-libcxx Author: David Spickett (DavidSpickett) ChangesSome time between Clang 19 and Clang 21, changes were made to the bare metal toolchain driver, and on top of that we (Linaro) updated our host OS to Ubuntu Jammy. This has caused the tests to not build with clang-21.
Normally I would switch to lld instead. However when I did this, I found that Picolib's ARM linker script uses ALIGN_WITH_INPUT, which lld does not support. So instead I'm sticking with ld and falling back to DWARFv4. Until such time as we upgrade the host to Ubuntu Noble. (if you're wondering how we can use ld in the first place, given the host is AArch64, the AArch64 ld is also capable of linking ARM 32-bit binaries)
This is solved in two parts:
And finally for exception_guard.odr.sh.cpp, I have moved the object files to before the linker flags. ld is sensitive to the order of objects on the command line, and this should not break other configurations. I've tested these with 19.1.7 too so my plan is to land these changes first, then proceed with upgrading to 21.x. Full diff: https://github.com/llvm/llvm-project/pull/158300.diff 6 Files Affected:
diff --git a/libcxx/cmake/caches/Armv7M-picolibc.cmake b/libcxx/cmake/caches/Armv7M-picolibc.cmake
index 0f8189b457285..7ad2bb6db9856 100644
--- a/libcxx/cmake/caches/Armv7M-picolibc.cmake
+++ b/libcxx/cmake/caches/Armv7M-picolibc.cmake
@@ -1,8 +1,8 @@
set(CMAKE_ASM_COMPILER_TARGET "armv7m-none-eabi" CACHE STRING "")
set(CMAKE_CXX_COMPILER_TARGET "armv7m-none-eabi" CACHE STRING "")
-set(CMAKE_CXX_FLAGS "-mfloat-abi=soft" CACHE STRING "")
+set(CMAKE_CXX_FLAGS "-mfloat-abi=soft -gdwarf-4" CACHE STRING "")
set(CMAKE_C_COMPILER_TARGET "armv7m-none-eabi" CACHE STRING "")
-set(CMAKE_C_FLAGS "-mfloat-abi=soft" CACHE STRING "")
+set(CMAKE_C_FLAGS "-mfloat-abi=soft -gdwarf-4" CACHE STRING "")
set(CMAKE_SYSTEM_NAME Generic CACHE STRING "")
set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY CACHE STRING "")
set(COMPILER_RT_BAREMETAL_BUILD ON CACHE BOOL "")
diff --git a/libcxx/test/configs/armv7m-picolibc-libc++.cfg.in b/libcxx/test/configs/armv7m-picolibc-libc++.cfg.in
index 9bff5021494ef..c1a1a99972a22 100644
--- a/libcxx/test/configs/armv7m-picolibc-libc++.cfg.in
+++ b/libcxx/test/configs/armv7m-picolibc-libc++.cfg.in
@@ -5,7 +5,7 @@ libc_linker_script = '@CMAKE_INSTALL_PREFIX@/lib/picolibcpp.ld'
config.substitutions.append(('%{flags}', '--sysroot=@CMAKE_INSTALL_PREFIX@'))
config.substitutions.append(('%{compile_flags}',
- '-nostdinc++ -I %{include-dir} -I %{target-include-dir} -I %{libcxx-dir}/test/support'
+ '-nostdinc++ -I %{include-dir} -I %{target-include-dir} -I %{libcxx-dir}/test/support -gdwarf-4'
# Disable warnings in the <atomic> implementation:
# "large atomic operation may incur significant performance penalty; the
@@ -13,8 +13,8 @@ config.substitutions.append(('%{compile_flags}',
' -Wno-atomic-alignment'
))
config.substitutions.append(('%{link_flags}',
- '-nostdlib -nostdlib++ -L %{lib-dir} -lc++ -lc++abi'
- ' -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost' +
+ '-nostdlib -nostdlib++ -L %{lib-dir} -Wl,--start-group -lc++ -lc++abi'
+ ' -lunwind -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost -Wl,--end-group' +
' -T {}'.format(libc_linker_script) +
' -Wl,--defsym=__flash=0x0'
' -Wl,--defsym=__flash_size=0x400000'
diff --git a/libcxx/test/libcxx/utilities/exception_guard.odr.sh.cpp b/libcxx/test/libcxx/utilities/exception_guard.odr.sh.cpp
index f6b8a29d36821..a1a4ad84e61d9 100644
--- a/libcxx/test/libcxx/utilities/exception_guard.odr.sh.cpp
+++ b/libcxx/test/libcxx/utilities/exception_guard.odr.sh.cpp
@@ -14,7 +14,7 @@
// RUN: %{cxx} %s %{flags} %{compile_flags} -c -o %t.except.o -O1 -fexceptions
// RUN: %{cxx} %s %{flags} %{compile_flags} -c -o %t.noexcept.o -O1 -fno-exceptions
-// RUN: %{cxx} %{flags} %{link_flags} -o %t.exe %t.except.o %t.noexcept.o
+// RUN: %{cxx} %{flags} -o %t.exe %t.except.o %t.noexcept.o %{link_flags}
// RUN: %{run}
#include <__utility/exception_guard.h>
diff --git a/libcxx/utils/ci/build-picolibc.sh b/libcxx/utils/ci/build-picolibc.sh
index 521c1bef9fc7e..f480368dacbf1 100755
--- a/libcxx/utils/ci/build-picolibc.sh
+++ b/libcxx/utils/ci/build-picolibc.sh
@@ -78,7 +78,7 @@ rm -rf "${picolibc_source_dir}/picolibc-${picolibc_commit}"
cat <<EOF > "${picolibc_build_dir}/meson-cross-build.txt"
[binaries]
-c = ['${CC:-cc}', '--target=${target}', '-mfloat-abi=soft', '-nostdlib']
+c = ['${CC:-cc}', '--target=${target}', '-mfloat-abi=soft', '-nostdlib', '-gdwarf-4']
ar = 'llvm-ar'
as = 'llvm-as'
ld = 'lld'
diff --git a/libcxxabi/test/configs/armv7m-picolibc-libc++abi.cfg.in b/libcxxabi/test/configs/armv7m-picolibc-libc++abi.cfg.in
index b4744f935ad85..12e15fbfc6efe 100644
--- a/libcxxabi/test/configs/armv7m-picolibc-libc++abi.cfg.in
+++ b/libcxxabi/test/configs/armv7m-picolibc-libc++abi.cfg.in
@@ -5,11 +5,11 @@ libc_linker_script = '@CMAKE_INSTALL_PREFIX@/lib/picolibcpp.ld'
config.substitutions.append(('%{flags}', '--sysroot=@CMAKE_INSTALL_PREFIX@'))
config.substitutions.append(('%{compile_flags}',
- '-nostdinc++ -I %{include} -I %{cxx-include} -I %{cxx-target-include} %{maybe-include-libunwind} -I %{libcxx}/test/support -I %{libcxx}/src -D_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS'
+ '-nostdinc++ -I %{include} -I %{cxx-include} -I %{cxx-target-include} %{maybe-include-libunwind} -I %{libcxx}/test/support -I %{libcxx}/src -D_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS -gdwarf-4'
))
config.substitutions.append(('%{link_flags}',
- '-nostdlib -nostdlib++ -L %{lib} -lc++ -lc++abi'
- ' -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost' +
+ '-nostdlib -nostdlib++ -L %{lib} -Wl,--start-group -lc++ -lc++abi'
+ ' -lunwind -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost -Wl,--end-group' +
' -T {}'.format(libc_linker_script) +
' -Wl,--defsym=__flash=0x0'
' -Wl,--defsym=__flash_size=0x400000'
diff --git a/libunwind/test/configs/armv7m-picolibc-libunwind.cfg.in b/libunwind/test/configs/armv7m-picolibc-libunwind.cfg.in
index e8f68a51fc53f..494fb13945f56 100644
--- a/libunwind/test/configs/armv7m-picolibc-libunwind.cfg.in
+++ b/libunwind/test/configs/armv7m-picolibc-libunwind.cfg.in
@@ -5,11 +5,11 @@ libc_linker_script = '@CMAKE_INSTALL_PREFIX@/lib/picolibcpp.ld'
config.substitutions.append(('%{flags}', '--sysroot=@CMAKE_INSTALL_PREFIX@'))
config.substitutions.append(('%{compile_flags}',
- '-nostdinc++ -I %{include}'
+ '-nostdinc++ -I %{include} -gdwarf-4'
))
config.substitutions.append(('%{link_flags}',
- '-nostdlib -nostdlib++ -L %{lib} -lunwind'
- ' -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost' +
+ '-nostdlib -nostdlib++ -L %{lib} -Wl,--start-group -lunwind'
+ ' -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost -Wl,--end-group' +
' -T {}'.format(libc_linker_script) +
' -Wl,--defsym=__flash=0x0'
' -Wl,--defsym=__flash_size=0x400000'
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Will get someone from Arm to review as well since they are consumers of Picolib and libcxx. |
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LGTM. Just a few typos: it's "picolibc" rather than "picolib". Missing the final 'c'. |
vhscampos
approved these changes
Sep 12, 2025
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Thanks for reviewing! I've fixed the spellings. |
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Arm colleagues alerted me to the fact that picolibc does have some code to handle this To keep each change simple, I'm going ahead with this PR, then I'll work out if we can in fact get lld working and send another PR if so. |
I added LLD support to picolibc in picolibc/picolibc#376, so it should all work out of the box. |
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Some time between Clang 19 and Clang 21, changes were made to the bare metal toolchain driver, and on top of that we (Linaro) updated our host OS to Ubuntu Jammy. This has caused the tests to not build with clang-21.
Normally I would switch to lld instead. However when I did this, I found that Picolibc's ARM linker script uses ALIGN_WITH_INPUT, which lld does not support.
So instead I'm sticking with ld and falling back to DWARFv4. Until such time as we upgrade the host to Ubuntu Noble.
(if you're wondering how we can use ld in the first place, given the host is AArch64, the AArch64 ld is also capable of linking ARM 32-bit binaries)
This is solved in two parts:
And finally for exception_guard.odr.sh.cpp, I have moved the object files to before the linker flags. ld is sensitive to the order of objects on the command line, and this should not break other configurations.
I've tested these with 19.1.7 too so my plan is to land these changes first, then proceed with upgrading to 21.x.