[libc++] Add check for building with picolibc

[domin144]

Summary

This is intended to identify changes that would fail to build on
embedded platforms e.g. D152382

[michaelplatings]

In D154246#4647568, @ldionne wrote:

Is this ready for review?

Yes this is rebased, ready for review and CI is looking good now. The same goes for D155521. I've pinged you on Discord as requested.

[michaelplatings]

Ping

[ldionne]

This looks pretty clean, a lot cleaner than I would have expected for an embedded c library. I have a few questions and comments but I don't see major blockers at this point.

libcxx/include/__atomic/cxx_atomic_impl.h

320–325

I am not certain we want to disable this warning here. At the end of the day, it seems like the compiler is warning us about something legitimate, no? We could perhaps silence that warning in the armv7m testing configuration instead?

libcxx/test/configs/armv7m-libc++.cfg.in

2–3

This comment seems copy-pasted (same for the libc++abi one and probably the libunwind one too).

13

Why is that include needed?

libcxx/test/libcxx/system_reserved_names.gen.py

105 ↗

(On Diff #557024)

What is the relationship between picolibc and _NEWLIB_VERSION?

libcxx/utils/ci/run-buildbot

622

It's weird that you're passing this here but also hardcoding it in your Armv7M-picolibc.cmake cache file. IMO it would make more sense *not* to hardcode it in the cache.

libcxx/utils/libcxx/test/features.py

108

Otherwise this test will fail in -ffreestanding mode, where main() isn't special.

218

Yeah it's pretty lame, we need to change that.

[michaelplatings]

Make changes requested by @ldionne and rebase.

[michaelplatings]

libcxx/test/configs/armv7m-libc++.cfg.in

13

Added comment explaining it.

libcxx/test/libcxx/system_reserved_names.gen.py

105 ↗

(On Diff #557024)

Picolibc is derived from Newlib, so defines _NEWLIB_VERSION. I've updated the comment to make it clear that it applies to both libraries.

libcxx/utils/ci/run-buildbot

622

Yes I'd prefer not to put the target in the CMake cache file, but that I think that would cause confusion because it would be inconsistent with all the other CMake cache files.

[arichardson]

Would very much like to see this land soon. Two small comments

libcxx/test/configs/armv7m-libc++.cfg.in

18

Doesn't this mean the library can't be used without adding this include to any build system that is trying to use libc++ with picolibc? I think it would be a lot cleaner if we could just add the required includes to the libc++ source code?

As far as I recall from the last time I tried to use libc++ with picolibc this was not needed but I may have been building a slightly different configuration

39

Could this be autodetected instead by checking for the presence of the picolibc header? Then it would also work out of the box with the default config file.

[michaelplatings]

libcxx/test/configs/armv7m-libc++.cfg.in

18

Doesn't this mean the library can't be used without adding this include to any build system that is trying to use libc++ with picolibc?

Most libc++ headers already indirectly include picolibc.h so most of the time this isn't a problem in practice. It's a minority of tests that fail without this include.

I think it would be a lot cleaner if we could just add the required includes to the libc++ source code?

I agree with you that it would be cleaner to remove this line and instead change the libc++ headers. However I can imagine that adding extra includes could cause undesirable knock-on effects. I think it's best to limit the scope of this change to only adding the test setup and not touching libc++ itself.

39

I don't think we can infer that we're targeting picolibc from the presence of the header. It might also happen to be on an include path in a scenario in which another platform is targeted. I think it's best to be explicit here.

[michaelplatings]

@ldionne ping

[domin144]

@ldionne ping

[arichardson]

libcxx/test/configs/armv7m-libc++.cfg.in

18

Do you have a list of tests that fail? If it's just a few maybe the better solution is to add the fixme to REQUIRES?

39

I meant trying to compile something like the following:

#include <stdio.h>
#ifndef __PICOLIBC__
#error not picolibc
#endif

[domin144]

This comment was removed by domin144.

[domin144]

rebase (minor conflict in doc formatting)
move LIBCXX-PICOLIBC-FIXME derivation to features.py (as suggested in comment)

[domin144]

libcxx/test/configs/armv7m-libc++.cfg.in

18

There are 138 tests that fail without the ' -include picolibc.h'.
I haven't reviewed them all, but most of them fall into two categories:

• use of __input reserved name in stdlib.h (e.g. algorithm.compile.pass.cpp)
• ld.lld: error: undefined symbol: std::__1::basic_streambuf<char, std::__1::char_traits<char>>::seekoff(long long, std::__1::ios_base::seekdir, unsigned int) (e.g. eof.pass.cpp)

I would prefer to solve it in a separate patch as this might need modification to libc++ headers.

39

Thanks! Applied in the new diff.

[arichardson]

One suggestion otherwise looking good to me

libcxx/test/configs/armv7m-libc++.cfg.in

18

If it's so many tests then yes that sounds good to me.

libcxx/test/libcxx/system_reserved_names.gen.py

105 ↗

(On Diff #557024)

So it seems this header needs the picolibc.h include? Could we just add something like #include <stdlib.h> to the top of this file?

libcxxabi/test/configs/armv7m-libc++abi.cfg.in

30

I believe this is not needed anymore since it's in features.oy now?

[domin144]

Applied suggestions from review:

• remove redundant LIBCXX-PICOLIBC-FIXME definition
• add "#include <string.h>" to make sure the _NEWLIB_VERSION is available

[domin144]

libcxx/test/libcxx/system_reserved_names.gen.py

105 ↗

(On Diff #557024)

Done.

libcxxabi/test/configs/armv7m-libc++abi.cfg.in

30

Good spot, I removed this part in the libc++ version and missed it here. Thanks.

[arichardson]

Not quite sure what is going on with the CI here, it sounds like GCC is happy with those tests even though the has-64-bit-atomics feature check fails. Should be good to land once that has been fixed and once @ldionne has approved this review.

libcxx/test/libcxx/selftest/dsl/dsl.sh.py

10

To be pedantic, this is not really a problem with picolibc but rather with using an executor stub. Maybe XFAIL: libcxx-fake-executor and defining that feature in the .cfg?

libcxx/test/libcxx/selftest/pass.cpp/run-error.pass.cpp

9

Same here

libcxx/test/libcxx/selftest/pass.mm/run-error.pass.mm

10

And here

[domin144]

Applied comments and fixed gcc-13 failure:

• add libcxx-fake-executor feature,
• move "has-64-bit-atomics" after "gcc" feature

[domin144]

The gcc-13 error in "has-64-bit-atomics" detection is:

/home/domwoj01/sources/llvm-project/builds/libcxx-gcc-13-test/build/include/c++/v1/limits:529:69: error: ‘float_denorm_style’ is deprecated [-Werror=deprecated-declarations]
  529 |     _LIBCPP_CONSTEXPR const float_denorm_style numeric_limits<_Tp>::has_denorm;
      |                                                                     ^~~~~~~~~~

The tests are run with the flag below, while the "has-64-bit-atomics" are not.

AddCompileFlag("-D_LIBCPP_DISABLE_DEPRECATION_WARNINGS"),

I moved the "has-64-bit-atomics" check to the end, so it also gets the flag.

[domin144]

@ldionne ping

[domin144]

Rebase to resolve conflict
Update picolibc, so XFAIL for stdio.h is no longer needed.

[domin144]

resolve conflict

[domin144]

Rebase, add XFAIL for new tests of floating point atomics as they need support for 64-bit atomic types.

[ldionne]

libcxx/test/configs/armv7m-libc++.cfg.in

4

This doesn't have to be a Lit substitution. You could just do this instead:

libc_linker_script = '@CMAKE_INSTALL_PREFIX@/lib/picolibcpp.ld'

# and then below:

' -T {}'.format(libc_linker_script)

18

Let's name the configuration files armv6m-picolibc-libc++.cfg.in since this is not a generic armv7m configuration, it's tied to picolibc.

libcxx/test/libcxx/system_reserved_names.gen.py

23–24 ↗

(On Diff #558169)

We shouldn't be including a file here at all, since this breaks the intent of the test (which is to check that every individual header doesn't use poisoned identifiers).

For example if <string.h> has #undef __allocator, we'd now fail to have coverage for e.g. vector using __allocator as a name, since <string.h> would always undef it.

I think we should rely on the -include picolibc.h part for now. It seems better to only affect the picolibc configuration with this "workaround" than affect the correctness of this test for everyone else.

libcxx/test/std/iterators/iterator.primitives/iterator.traits/cxx20_iterator_traits.compile.pass.cpp

17

I'm pretty sure this is a real bug in our test suite. Fine with this being a FIXME for now, but we should really investigate and fix it for everyone.

libcxx/utils/libcxx/test/features.py

291–292

This comment becomes unnecessary once I move the compiler detection features above. Still not great to rely on implicit ordering but 🤷🏼‍♂️

294

Just to record what we're going to do: we're going to leave this feature as-is and then change non-lockfree-atomics to be something like

Feature(
    name="has-128-bits-atomics",
    when=lambda cfg: sourceBuilds(
        cfg,
        """
        #include <atomic>
        struct Large { __int128_t storage; };
        std::atomic<Large> x;
        int main(int, char**) { (void)x.load(); return 0; }
      """,
    ),
),

[domin144]

Applied changes from comments.

[ldionne]

LGTM, thanks! It's nice to have official support for a more embedded libc.

libcxx/cmake/caches/Armv7M-picolibc.cmake

+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_C_COMPILER_TARGET "armv7m-none-eabi" CACHE STRING "")
+set(CMAKE_C_FLAGS "-mfloat-abi=soft" 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 "")
+set(COMPILER_RT_BUILD_LIBFUZZER OFF CACHE BOOL "")
+set(COMPILER_RT_BUILD_PROFILE OFF CACHE BOOL "")
+set(COMPILER_RT_BUILD_SANITIZERS OFF CACHE BOOL "")
+set(COMPILER_RT_BUILD_XRAY OFF CACHE BOOL "")
+set(COMPILER_RT_DEFAULT_TARGET_ONLY ON CACHE BOOL "")
+set(LIBCXXABI_BAREMETAL ON CACHE BOOL "")
+set(LIBCXXABI_ENABLE_ASSERTIONS OFF CACHE BOOL "")
+set(LIBCXXABI_ENABLE_EXCEPTIONS OFF CACHE BOOL "")
+set(LIBCXXABI_ENABLE_SHARED OFF CACHE BOOL "")
+set(LIBCXXABI_ENABLE_STATIC ON CACHE BOOL "")
+set(LIBCXXABI_ENABLE_THREADS OFF CACHE BOOL "")
+set(LIBCXXABI_USE_COMPILER_RT ON CACHE BOOL "")
+set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
+set(LIBCXX_ENABLE_EXCEPTIONS OFF CACHE BOOL "")
+set(LIBCXX_ENABLE_FILESYSTEM OFF CACHE STRING "")
+set(LIBCXX_ENABLE_MONOTONIC_CLOCK OFF CACHE BOOL "")
+set(LIBCXX_ENABLE_RANDOM_DEVICE OFF CACHE BOOL "")
+set(LIBCXX_ENABLE_RTTI OFF CACHE BOOL "")
+set(LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
+set(LIBCXX_ENABLE_STATIC ON CACHE BOOL "")
+set(LIBCXX_ENABLE_THREADS OFF CACHE BOOL "")
+set(LIBCXX_ENABLE_WIDE_CHARACTERS OFF CACHE BOOL "")
+set(LIBCXX_INCLUDE_BENCHMARKS OFF CACHE BOOL "")
+set(LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
+set(LIBUNWIND_ENABLE_SHARED OFF CACHE BOOL "")
+set(LIBUNWIND_ENABLE_STATIC ON CACHE BOOL "")
+set(LIBUNWIND_ENABLE_THREADS OFF CACHE BOOL "")
+set(LIBUNWIND_IS_BAREMETAL ON CACHE BOOL "")
+set(LIBUNWIND_REMEMBER_HEAP_ALLOC ON CACHE BOOL "")
+set(LIBUNWIND_USE_COMPILER_RT ON CACHE BOOL "")

libcxx/docs/index.rst

 Clang        15, 16, 17-git                             latest two stable releases per `LLVM's release page <https://releases.llvm.org>`_ and the development version
 AppleClang   14                                         latest stable release per `Xcode's release page <https://developer.apple.com/documentation/xcode-release-notes>`_
 Open XL      17.1 (AIX)                                 latest stable release per `Open XL's documentation page <https://www.ibm.com/docs/en/openxl-c-and-cpp-aix>`_
 GCC          12              In C++11 or later only     latest stable release per `GCC's release page <https://gcc.gnu.org/releases.html>`_
 ============ =============== ========================== =====================
 
 Libc++ also supports common platforms and architectures:
 
-=============== ========================= ============================
-Target platform Target architecture       Notes
-=============== ========================= ============================
-macOS 10.9+     i386, x86_64, arm64       Building the shared library itself requires targetting macOS 10.13+
-FreeBSD 12+     i386, x86_64, arm
-Linux           i386, x86_64, arm, arm64  Only glibc-2.24 and later and no other libc is officially supported
-Windows         i386, x86_64              Both MSVC and MinGW style environments, ABI in MSVC environments is :doc:`unstable <DesignDocs/ABIVersioning>`
-AIX 7.2TL5+     powerpc, powerpc64
-=============== ========================= ============================
+===================== ========================= ============================
+Target platform       Target architecture       Notes
+===================== ========================= ============================
+macOS 10.9+           i386, x86_64, arm64       Building the shared library itself requires targetting macOS 10.13+
+FreeBSD 12+           i386, x86_64, arm
+Linux                 i386, x86_64, arm, arm64  Only glibc-2.24 and later and no other libc is officially supported
+Windows               i386, x86_64              Both MSVC and MinGW style environments, ABI in MSVC environments is :doc:`unstable <DesignDocs/ABIVersioning>`
+AIX 7.2TL5+           powerpc, powerpc64
+Embedded (picolibc)   arm                       Support for building with picolibc is currently work-in-progress
+===================== ========================= ============================
 
 Generally speaking, libc++ should work on any platform that provides a fairly complete
 C Standard Library. It is also possible to turn off parts of the library for use on
 systems that provide incomplete support.
 
 However, libc++ aims to provide a high-quality implementation of the C++ Standard
 Library, especially when it comes to correctness. As such, we aim to have test coverage
 for all the platforms and compilers that we claim to support. If a platform or compiler

libcxx/include/__atomic/cxx_atomic_impl.h

 #endif // _LIBCPP_CXX03_LANG
   _LIBCPP_CONSTEXPR explicit __cxx_atomic_base_impl(_Tp __value) _NOEXCEPT
     : __a_value(__value) {}
   _LIBCPP_DISABLE_EXTENSION_WARNING _Atomic(_Tp) __a_value;
 };
 
 #define __cxx_atomic_is_lock_free(__s) __c11_atomic_is_lock_free(__s)
 
+_LIBCPP_DIAGNOSTIC_PUSH
+#  ifdef __arm__
+// Disable warning "large atomic operation may incur significant performance
+// penalty; the access size (4 bytes) exceeds the max lock-free size (0  bytes)"
+_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Watomic-alignment")
+#  endif
+
 _LIBCPP_HIDE_FROM_ABI inline
 void __cxx_atomic_thread_fence(memory_order __order) _NOEXCEPT {
     __c11_atomic_thread_fence(static_cast<__memory_order_underlying_t>(__order));
 }
 
 _LIBCPP_HIDE_FROM_ABI inline
 void __cxx_atomic_signal_fence(memory_order __order) _NOEXCEPT {
     __c11_atomic_signal_fence(static_cast<__memory_order_underlying_t>(__order));

 }
 template<class _Tp>
 _LIBCPP_HIDE_FROM_ABI
 _Tp __cxx_atomic_fetch_xor(__cxx_atomic_base_impl<_Tp> * __a, _Tp __pattern, memory_order __order) _NOEXCEPT {
   return __c11_atomic_fetch_xor(
       std::addressof(__a->__a_value), __pattern, static_cast<__memory_order_underlying_t>(__order));
 }
 
+_LIBCPP_DIAGNOSTIC_POP
+
 #endif // _LIBCPP_HAS_GCC_ATOMIC_IMP, _LIBCPP_HAS_C_ATOMIC_IMP
 
 #ifdef _LIBCPP_ATOMIC_ONLY_USE_BUILTINS
 
 template<typename _Tp>
 struct __cxx_atomic_lock_impl {
 
   _LIBCPP_HIDE_FROM_ABI

libcxx/test/configs/armv7m-libc++.cfg.in

+# This testing configuration handles running the test suite against LLVM's libc++
+# using a static library.
+
+lit_config.load_config(config, '@CMAKE_CURRENT_BINARY_DIR@/cmake-bridge.cfg')
+
+config.substitutions.append(('%{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 %{target-include} -I %{libcxx}/test/support'
+    ' -include picolibc.h'
+))
+config.substitutions.append(('%{link_flags}',
+    '-nostdlib -nostdlib++ -L %{lib} -lc++ -lc++abi'
+    ' -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost'
+    ' -T %{libc-linker-script}'
+    ' -Wl,--defsym=__flash=0x0'
+    ' -Wl,--defsym=__flash_size=0x400000'
+    ' -Wl,--defsym=__ram=0x21000000'
+    ' -Wl,--defsym=__ram_size=0x1000000'
+    ' -Wl,--defsym=__stack_size=0x1000'
+))
+config.substitutions.append(('%{exec}',
+    'true' # TODO use qemu-system-arm
+))
+
+# LIBCXX-PICOLIBC-FIXME is the feature name used to XFAIL the
+# initial picolibc failures until they can be properly diagnosed
+# and fixed. This allows easier detection of new test failures
+# and regressions. Note: New failures should not be suppressed
+# using this feature.
+config.available_features.add('LIBCXX-PICOLIBC-FIXME')
+
+import os, site
+site.addsitedir(os.path.join('@LIBCXX_SOURCE_DIR@', 'utils'))
+import libcxx.test.params, libcxx.test.config
+libcxx.test.config.configure(
+    libcxx.test.params.DEFAULT_PARAMETERS,
+    libcxx.test.features.DEFAULT_FEATURES,
+    config,
+    lit_config
+)

libcxx/test/libcxx/modules_include.gen.py

 // UNSUPPORTED{BLOCKLIT}: buildhost=windows
 
 // The Android headers don't appear to be compatible with modules yet
 // XFAIL{BLOCKLIT}: LIBCXX-ANDROID-FIXME
 
 // TODO: Investigate this failure
 // UNSUPPORTED{BLOCKLIT}: LIBCXX-FREEBSD-FIXME
 
+// TODO: Investigate this failure
+// UNSUPPORTED{BLOCKLIT}: LIBCXX-PICOLIBC-FIXME
+
 {lit_header_restrictions.get(header, '')}
 
 #include <{header}>
 """)

libcxx/test/libcxx/nasty_macros.gen.py

 # define __inout NASTY_MACRO
 # define __nz NASTY_MACRO
 # define __out NASTY_MACRO
 # define __part NASTY_MACRO
 # define __post NASTY_MACRO
 # define __pre NASTY_MACRO
 #endif
 
-#define __input NASTY_MACRO
+// picolibc uses __input as a parameter name of a64l & l64a
+#ifndef _NEWLIB_VERSION
+# define __input NASTY_MACRO
+#endif
 #define __output NASTY_MACRO
 
 #define __acquire NASTY_MACRO
 #define __release NASTY_MACRO
 
 // These names are not reserved, so the user can macro-define them.
 // These are intended to find improperly _Uglified template parameters.
 #define A NASTY_MACRO

libcxx/test/libcxx/selftest/dsl/dsl.sh.py

 # ===----------------------------------------------------------------------===##
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 # ===----------------------------------------------------------------------===##
 
+# Picolibc test executor is `true` at present.
+# XFAIL: LIBCXX-PICOLIBC-FIXME
+
 # Note: We prepend arguments with 'x' to avoid thinking there are too few
 #       arguments in case an argument is an empty string.
 # RUN: %{python} %s x%S x%T x%{substitutions}
 
 import base64
 import copy
 import os
 import pickle

libcxx/test/libcxx/selftest/pass.cpp/run-error.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
 // XFAIL: *
 
 // Make sure the test DOES NOT pass if it fails at runtime.
 
 int main(int, char**) {
     return 1;
 }

libcxx/test/libcxx/selftest/pass.mm/run-error.pass.mm

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: objective-c++
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
 
 // XFAIL: *
 
 // Make sure the test DOES NOT pass if it fails at runtime.
 
 int main(int, char**) {
     return 1;
 }

libcxx/test/std/atomics/atomics.types.generic/integral.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !atomics
+
 // <atomic>
 
 // template <>
 // struct atomic<integral>
 // {
 //     bool is_lock_free() const volatile;
 //     bool is_lock_free() const;
 //     void store(integral desr, memory_order m = memory_order_seq_cst) volatile;

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !atomics
+
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_add(volatile atomic<T>* obj, atomic<T>::difference_type) noexcept;
 //
 // template<class T>
 //     T

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !atomics
+
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_add_explicit(volatile atomic<T>*, atomic<T>::difference_type,
 //                               memory_order) noexcept;
 //
 // template<class T>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_and.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !atomics
+
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_and(volatile atomic<T>*, atomic<T>::value_type) noexcept;
 //
 // template<class T>
 //     T

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_and_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !atomics
+
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_and_explicit(volatile atomic<T>*, atomic<T>::value_type,
 //                               memory_order) noexcept;
 //
 // template<class T>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_or.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !atomics
+
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_or(volatile atomic<T>*, atomic<T>::value_type) noexcept;
 //
 // template<class T>
 //     T

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_or_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !atomics
+
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_or_explicit(volatile atomic<T>*, atomic<T>::value_type,
 //                              memory_order) noexcept;
 //
 // template<class T>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !atomics
+
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_sub(volatile atomic<T>*, atomic<T>::difference_type) noexcept;
 //
 // template<class T>
 //     T

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !atomics
+
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_sub_explicit(volatile atomic<T>*, atomic<T>::difference_type,
 //                               memory_order) noexcept;
 //
 // template<class T>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_xor.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !atomics
+
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_xor(volatile atomic<T>*, atomic<T>::value_type) noexcept;
 //
 // template<class T>
 //     T

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_xor_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !atomics
+
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_xor_explicit(volatile atomic<T>*, atomic<T>::value_type,
 //                               memory_order) noexcept;
 //
 // template<class T>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/ctor.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// XFAIL: !atomics
+
 // <atomic>
 
 // constexpr atomic<T>::atomic(T value)
 
 #include <atomic>
 #include <type_traits>
 #include <cassert>
 

libcxx/test/std/depr/depr.c.headers/fenv_h.compile.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// Floating point exceptions are required for the FE_... macros to be defined.
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <fenv.h>
 
 #include <fenv.h>
 
 #include "test_macros.h"
 
 #ifndef FE_DIVBYZERO
 #error FE_DIVBYZERO not defined

libcxx/test/std/depr/depr.c.headers/stdio_h.compile.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// stderr et al are not macros in picolibc's tinystdio.
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // test <stdio.h>
 
 #include <stdio.h>
 #include <stdarg.h>
 
 #include "test_macros.h"
 
 #ifdef getc

libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
 // Apple platforms don't provide <uchar.h> yet, so these tests fail.
 // XFAIL: target={{.+}}-apple-{{.+}}
 
+// mbrtoc16 not defined.
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <uchar.h>
 
 #include <uchar.h>
 
 #include "test_macros.h"
 
 // __STDC_UTF_16__ may or may not be defined by the C standard library
 // __STDC_UTF_32__ may or may not be defined by the C standard library

libcxx/test/std/iterators/iterator.primitives/iterator.traits/cxx20_iterator_traits.compile.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
 // This test uses iterator types from std::filesystem
 // XFAIL: availability-filesystem-missing
 
+// std::same_as<typename Traits::difference_type, DiffType> failed.
+// The former was long and the latter was long long.
+// Possibly related to "using streamoff = long int" in ios.h.
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // template<class T>
 // struct iterator_traits;
 
 #include <iterator>
 
 #include <array>
 #include <concepts>
 #include <cstddef>

libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/sized_delete_array14.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // test sized operator delete[] replacement.
 
 // UNSUPPORTED: sanitizer-new-delete, c++03, c++11
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
 
 // NOTE: Clang does not enable sized-deallocation in C++14 and beyond by
 // default. It is only enabled when -fsized-deallocation is given.
 // XFAIL: clang, apple-clang
 
 #include <new>
 #include <cstddef>
 #include <cstdlib>

libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/sized_delete14.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // test sized operator delete replacement.
 
 // UNSUPPORTED: sanitizer-new-delete, c++03, c++11
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
 
 // NOTE: Clang does not enable sized-deallocation in C++14 and beyond by
 // default. It is only enabled when -fsized-deallocation is given.
 // XFAIL: clang, apple-clang
 
 #include <new>
 #include <cstddef>
 #include <cstdlib>

libcxx/test/std/language.support/support.runtime/ctime.timespec.compile.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // test <ctime>
 // std::timespec and std::timespec_get
 
 // UNSUPPORTED: c++03, c++11, c++14
 
 // XFAIL: LIBCXX-AIX-FIXME
 
+// picolibc doesn't define TIME_UTC.
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // ::timespec_get is provided by the C library, but it's marked as
 // unavailable until macOS 10.15
 // XFAIL: stdlib=apple-libc++ && target={{.+}}-apple-macosx10.{{9|10|11|12|13|14}}
 
 // ::timespec_get is available starting with Android Q (API 29)
 // XFAIL: target={{.+}}-android{{(eabi)?(21|22|23|24|25|26|27|28)}}
 
 #include <ctime>
 #include <type_traits>
 
 #ifndef TIME_UTC
 #error TIME_UTC not defined
 #endif
 
 std::timespec tmspec = {};
 static_assert(std::is_same<decltype(std::timespec_get(&tmspec, 0)), int>::value, "");

libcxx/test/std/numerics/cfenv/cfenv.syn/cfenv.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// Floating point exceptions are required for the FE_... macros to be defined.
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <cfenv>
 
 #include <cfenv>
 #include <type_traits>
 
 #include "test_macros.h"
 
 #ifndef FE_DIVBYZERO

libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
 // Apple platforms don't provide <uchar.h> yet, so these tests fail.
 // XFAIL: target={{.+}}-apple-{{.+}}
 
+// mbrtoc16 not defined.
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <cuchar>
 
 #include <cuchar>
 
 #include "test_macros.h"
 
 // __STDC_UTF_16__ may or may not be defined by the C standard library
 // __STDC_UTF_32__ may or may not be defined by the C standard library

libcxx/test/std/time/time.clock/time.clock.hires/now.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 
 // high_resolution_clock
 
 // static time_point now();
 
 #include <chrono>
 #include <cassert>

libcxx/test/std/time/time.clock/time.clock.file/to_from_sys.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
 // UNSUPPORTED: availability-filesystem-missing
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 //
 // file_clock
 //
 // template<class Duration>
 // static sys_time<see-below> to_sys(const file_time<Duration>&);
 //
 // template<class Duration>

libcxx/test/std/time/time.clock/time.clock.system/from_time_t.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 
 // system_clock
 
 // static time_point from_time_t(time_t t);
 
 #include <chrono>
 #include <ctime>

libcxx/test/std/time/time.clock/time.clock.system/now.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 
 // system_clock
 
 // static time_point now();
 
 #include <chrono>
 #include <cassert>

libcxx/test/std/time/time.clock/time.clock.system/to_time_t.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 
 // system_clock
 
 // time_t to_time_t(const time_point& t);
 
 #include <chrono>
 #include <ctime>

libcxx/test/std/time/time.point/time.point.nonmember/op_-duration.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 
 // time_point
 
 // template <class Clock, class Duration1, class Rep2, class Period2>
 //   time_point<Clock, typename common_type<Duration1, duration<Rep2, Period2>>::type>
 //   operator-(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);
 

libcxx/utils/ci/BOT_OWNERS.txt

 This file is a list of the people or groups responsible for ensuring that bots
 are running correctly. They will also deal with platform specific issues that
 cannot be figured out by the community alone.
 
 The list is sorted by surname and formatted to allow easy grepping and
 beautification by scripts. The fields are: name (N), email (E), web-address
 (W), PGP key ID and fingerprint (P), description (D), snail-mail address
 (S), Phabricator handle (H) and IRC handle (I). Each entry should contain at
 least the (N), (E) and (D) fields.
 
 N: Linaro Toolchain Working Group
 E: [e-mail]
-D: Armv7, Armv8, AArch64
+D: Arm platforms
 
 N: LLVM on Power
 E: [e-mail]
 D: AIX, ppc64le

libcxx/utils/ci/build-picolibc.sh

+#!/usr/bin/env bash
+#===----------------------------------------------------------------------===##
+#
+# Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+# See https://llvm.org/LICENSE.txt for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+#
+#===----------------------------------------------------------------------===##
+
+#
+# This script builds picolibc (https://github.com/picolibc/picolibc) from
+# source to facilitate building libc++ against it.
+#
+
+set -e
+
+PROGNAME="$(basename "${0}")"
+
+function error() { printf "error: %s\n" "$*" >&2; exit 1; }
+
+function usage() {
+cat <<EOF
+Usage:
+${PROGNAME} [options]
+
+[-h|--help]                  Display this help and exit.
+
+--build-dir <DIR>            Path to the directory to use for building.
+
+--install-dir <DIR>          Path to the directory to install the library to.
+EOF
+}
+
+while [[ $# -gt 0 ]]; do
+    case ${1} in
+        -h|--help)
+            usage
+            exit 0
+            ;;
+        --build-dir)
+            build_dir="${2}"
+            shift; shift
+            ;;
+        --install-dir)
+            install_dir="${2}"
+            shift; shift
+            ;;
+        --target)
+            target="${2}"
+            shift; shift
+            ;;
+        *)
+            error "Unknown argument '${1}'"
+            ;;
+    esac
+done
+
+for arg in build_dir install_dir target; do
+    if [ -z ${!arg+x} ]; then
+        error "Missing required argument '--${arg//_/-}'"
+    elif [ "${!arg}" == "" ]; then
+        error "Argument to --${arg//_/-} must not be empty"
+    fi
+done
+
+
+echo "--- Downloading picolibc"
+picolibc_source_dir="${build_dir}/picolibc-source"
+picolibc_build_dir="${build_dir}/picolibc-build"
+mkdir -p "${picolibc_source_dir}"
+mkdir -p "${picolibc_build_dir}"
+# Download the version of picolibc that was the latest at the time this script was written.
+# The picolibc linker script was changed after version 1.8.2 to work with ld.lld so a
+# more recent version is required, which at the time of writing is not yet released.
+picolibc_commit="8dd225aa1469c03805617106020d494912c6d265"
+curl -L "https://github.com/picolibc/picolibc/archive/${picolibc_commit}.zip" --output "${picolibc_source_dir}/picolibc.zip"
+unzip -q "${picolibc_source_dir}/picolibc.zip" -d "${picolibc_source_dir}"
+mv "${picolibc_source_dir}/picolibc-${picolibc_commit}"/* "${picolibc_source_dir}"
+rm -rf "${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']
+ar = 'llvm-ar'
+as = 'llvm-as'
+ld = 'lld'
+strip = 'llvm-strip'
+[host_machine]
+system = 'none'
+cpu_family = 'arm'
+cpu = 'arm'
+endian = 'little'
+[properties]
+skip_sanity_check = true
+EOF
+
+venv_dir="${build_dir}/meson-venv"
+python3 -m venv "${venv_dir}"
+# Install the version of meson that was the latest at the time this script was written.
+"${venv_dir}/bin/pip" install "meson==1.1.1"
+
+"${venv_dir}/bin/meson" setup \
+  -Dincludedir=include -Dlibdir=lib -Dspecsdir=none -Dmultilib=false -Dpicoexit=false \
+  --prefix "${install_dir}" \
+  --cross-file "${picolibc_build_dir}/meson-cross-build.txt" \
+  "${picolibc_build_dir}" \
+  "${picolibc_source_dir}"
+
+"${venv_dir}/bin/meson" install -C "${picolibc_build_dir}"

libcxx/utils/ci/buildkite-pipeline.yml

- label: "Armv7 -fno-exceptions"
         queue: "libcxx-builders-linaro-arm"
         arch: "armv8l" # Compiling for v7, running on v8 hardware
       retry:
         automatic:
           - exit_status: -1  # Agent was lost
             limit: 2
       timeout_in_minutes: 120
 
+    - label: "Armv7-M picolibc"
+      command: "libcxx/utils/ci/run-buildbot armv7m-picolibc"
+      artifact_paths:
+        - "**/test-results.xml"
+        - "**/*.abilist"
+      agents:
+        queue: "libcxx-builders-linaro-arm"
+        arch: "aarch64"
+      retry:
+        automatic:
+          - exit_status: -1  # Agent was lost
+            limit: 2
+      timeout_in_minutes: 120
+
   - group: "AIX"
     steps:
     - label: "AIX (32-bit)"
       command: "libcxx/utils/ci/run-buildbot aix"
       artifact_paths:
         - "**/test-results.xml"
         - "**/*.abilist"
       env:

libcxx/utils/ci/run-buildbot

armv7)
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Armv7Arm.cmake"
     check-runtimes
 ;;
 armv7-no-exceptions)
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Armv7Thumb-no-exceptions.cmake"
     check-runtimes
 ;;
+armv7m-picolibc)
+    clean
+
+    # To make it easier to get this builder up and running, build picolibc
+    # from scratch. Anecdotally, the build-picolibc script takes about 16 seconds.
+    # This could be optimised by building picolibc into the Docker container.
+    ${MONOREPO_ROOT}/libcxx/utils/ci/build-picolibc.sh \
+        --build-dir "${BUILD_DIR}" \
+        --install-dir "${INSTALL_DIR}" \
+        --target armv7m-none-eabi
+
+    echo "--- Generating CMake"
+    flags="--sysroot=${INSTALL_DIR}"
+    ${CMAKE} \
+        -S "${MONOREPO_ROOT}/compiler-rt" \
+        -B "${BUILD_DIR}/compiler-rt" \
+        -GNinja -DCMAKE_MAKE_PROGRAM="${NINJA}" \
+        -DCMAKE_BUILD_TYPE=RelWithDebInfo \
+        -DCMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
+        -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Armv7M-picolibc.cmake" \
+        -DCMAKE_C_FLAGS="${flags}" \
+        -DCMAKE_CXX_FLAGS="${flags}" \
+        -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON
+    generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Armv7M-picolibc.cmake" \
+        -DLIBCXX_TEST_CONFIG="armv7m-libc++.cfg.in" \
+        -DLIBCXXABI_TEST_CONFIG="armv7m-libc++abi.cfg.in" \
+        -DLIBUNWIND_TEST_CONFIG="armv7m-libunwind.cfg.in" \
+        -DCMAKE_C_FLAGS="${flags}" \
+        -DCMAKE_CXX_FLAGS="${flags}"
+
+    ${NINJA} -vC "${BUILD_DIR}/compiler-rt" install
+    mv "${BUILD_DIR}/install/lib/armv7m-none-eabi"/* "${BUILD_DIR}/install/lib"
+
+    check-runtimes
+;;
 clang-cl-dll)
     clean
     # TODO: Currently, building with the experimental library breaks running
     # tests (the test linking look for the c++experimental library with the
     # wrong name, and the statically linked c++experimental can't be linked
     # correctly when libc++ visibility attributes indicate dllimport linkage
     # anyway), thus just disable the experimental library. Remove this
     # setting when cmake and the test driver does the right thing automatically.
clang-cl-no-vcruntime)
     clean
     # Building libc++ in the same way as in clang-cl-dll above, but running
     # tests with -D_HAS_EXCEPTIONS=0, which users might set in certain
     # translation units while using libc++, even if libc++ is built with
     # exceptions enabled.
     generate-cmake-libcxx-win -DLIBCXX_TEST_PARAMS="enable_experimental=False" \
                               -DLIBCXX_TEST_CONFIG="llvm-libc++-shared-no-vcruntime-clangcl.cfg.in"
     echo "+++ Running the libc++ tests"
     ${NINJA} -vC "${BUILD_DIR}" check-cxx
 ;;
 mingw-dll)
     clean
     # Explicitly specify the compiler with a triple prefix. The CI
     # environment has got two installations of Clang; the default one
     # defaults to MSVC mode, while there's an installation of llvm-mingw
     # further back in PATH. By calling the compiler with an explicit
     # triple prefix, we use the one that is bundled with a mingw sysroot.

libcxx/utils/libcxx/test/features.py

Feature(
         name="objective-c++",
         when=lambda cfg: hasCompileFlag(cfg, "-xobjective-c++ -fobjc-arc"),
     ),
     Feature(
         name="verify-support",
         when=lambda cfg: hasCompileFlag(cfg, "-Xclang -verify-ignore-unexpected"),
     ),
     Feature(
+        name="atomics",
+        when=lambda cfg: sourceBuilds(
+            cfg,
+            """
+            #include <atomic>
+            std::atomic_uintmax_t x;
+            int main(int, char**) { (void)x.load(); }
+          """,
+        ),
+    ),
+    Feature(
         name="non-lockfree-atomics",
         when=lambda cfg: sourceBuilds(
             cfg,
             """
             #include <atomic>
             struct Large { int storage[100]; };
             std::atomic<Large> x;
             int main(int, char**) { (void)x.load(); return 0; }
DEFAULT_FEATURES = [
     Feature(
         name="has-unix-headers",
         when=lambda cfg: sourceBuilds(
             cfg,
             """
             #include <unistd.h>
             #include <sys/wait.h>
             int main(int, char**) {
-              return 0;
+              int fd[2];
+              return pipe(fd);
             }
           """,
         ),
     ),
     # Whether Bash can run on the executor.
     # This is not always the case, for example when running on embedded systems.
     #
     # For the corner case of bash existing, but it being missing in the path
DEFAULT_FEATURES = [
     ),
     Feature(
         name=lambda cfg: "gcc-{__GNUC__}".format(**compilerMacros(cfg)), when=_isGCC
     ),
     Feature(
         name=lambda cfg: "gcc-{__GNUC__}.{__GNUC_MINOR__}".format(**compilerMacros(cfg)),
         when=_isGCC,
     ),
     Feature(
         name=lambda cfg: "gcc-{__GNUC__}.{__GNUC_MINOR__}.{__GNUC_PATCHLEVEL__}".format(**compilerMacros(cfg)),
         when=_isGCC,
     ),
     Feature(name="msvc", when=_isMSVC),
     Feature(name=lambda cfg: "msvc-{}".format(*_msvcVersion(cfg)), when=_isMSVC),
     Feature(name=lambda cfg: "msvc-{}.{}".format(*_msvcVersion(cfg)), when=_isMSVC),
 ]
 
 # Deduce and add the test features that that are implied by the #defines in
 # the <__config_site> header.
 #

libcxxabi/test/configs/armv7m-libc++abi.cfg.in

+# This testing configuration handles running the test suite against LLVM's libc++
+# using a static library.
+
+lit_config.load_config(config, '@CMAKE_CURRENT_BINARY_DIR@/cmake-bridge.cfg')
+
+config.substitutions.append(('%{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'
+    ' -include picolibc.h'
+))
+config.substitutions.append(('%{link_flags}',
+    '-nostdlib -nostdlib++ -L %{lib} -lc++ -lc++abi'
+    ' -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost'
+    ' -T %{libc-linker-script}'
+    ' -Wl,--defsym=__flash=0x0'
+    ' -Wl,--defsym=__flash_size=0x400000'
+    ' -Wl,--defsym=__ram=0x21000000'
+    ' -Wl,--defsym=__ram_size=0x1000000'
+    ' -Wl,--defsym=__stack_size=0x1000'
+))
+config.substitutions.append(('%{exec}',
+    'true' # TODO use qemu-system-arm
+))
+
+# LIBCXX-PICOLIBC-FIXME is the feature name used to XFAIL the
+# initial picolibc failures until they can be properly diagnosed
+# and fixed. This allows easier detection of new test failures
+# and regressions. Note: New failures should not be suppressed
+# using this feature.
+config.available_features.add('LIBCXX-PICOLIBC-FIXME')
+
+import os, site
+site.addsitedir(os.path.join('@LIBCXXABI_LIBCXX_PATH@', 'utils'))
+import libcxx.test.params, libcxx.test.config
+libcxx.test.config.configure(
+    libcxx.test.params.DEFAULT_PARAMETERS,
+    libcxx.test.features.DEFAULT_FEATURES,
+    config,
+    lit_config
+)

libunwind/test/configs/armv7m-libunwind.cfg.in

+# This testing configuration handles running the test suite against LLVM's libc++
+# using a static library.
+
+lit_config.load_config(config, '@CMAKE_CURRENT_BINARY_DIR@/cmake-bridge.cfg')
+
+config.substitutions.append(('%{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}'
+    ' -include picolibc.h'
+))
+config.substitutions.append(('%{link_flags}',
+    '-nostdlib -nostdlib++ -L %{lib} -lunwind'
+    ' -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost'
+    ' -T %{libc-linker-script}'
+    ' -Wl,--defsym=__flash=0x0'
+    ' -Wl,--defsym=__flash_size=0x400000'
+    ' -Wl,--defsym=__ram=0x21000000'
+    ' -Wl,--defsym=__ram_size=0x1000000'
+    ' -Wl,--defsym=__stack_size=0x1000'
+))
+config.substitutions.append(('%{exec}',
+    'true' # TODO use qemu-system-arm
+))
+
+import os, site
+site.addsitedir(os.path.join('@LIBUNWIND_LIBCXX_PATH@', 'utils'))
+import libcxx.test.params, libcxx.test.config
+libcxx.test.config.configure(
+    libcxx.test.params.DEFAULT_PARAMETERS,
+    libcxx.test.features.DEFAULT_FEATURES,
+    config,
+    lit_config
+)