[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

[ldionne]

libcxx/utils/ci/run-buildbot

617

I think we should just use a pragma to disable the warning in that function.

625

We're not running the tests? I would expect

echo "+++ Running the libc++ tests"
${NINJA} -vC "${BUILD_DIR}" check-cxx check-cxxabi check-unwind

651

Should we add something to libcxx/docs/index.rst in the platform support table that mentions that picolibc is supported?

[michaelplatings]

Apply @ldionne's suggestions

[michaelplatings]

libcxx/utils/ci/run-buildbot

617

I've now found that the warning pops up in all the __c11_atomic_... invocations so I've surrounded all the invocations with a pragma disable.

625

Yes I'd also like to get the tests passing or xfailed but that's going to require a fair bit of work. Off the top of my head:

• Add linker scripts.
• Figure out how to run the tests via emulation in the Docker containers.
• Xfail all the tests that rely on features that picolibc doesn't support.

All of these have been done already with various hacks in various downstream repositories, but finding a suitable way to do them properly in the libcxx sources is something I anticipate could take months.

For now I just want to set a baseline of keeping libcxx+picolibc building. Once that's in place we can start ratcheting up the quality bar.

[DavidSpickett]

libcxx/utils/ci/build-picolibc.sh

70

clang -> cc so it can work on our container.

[DavidSpickett]

I got this to build on Linaro's container.

clang 15.0.0 (our current compiler) fails with errors about not finding types that should be in locale.h. The files are all in the right place, and moving to clang 16 fixes this. So I assume some driver changes did that.

clang 16.0.4 failed with this on one part of the build:

clang-16: error: argument unused during compilation: '-G R-' [-Werror,-Wunused-command-line-argument]

Turns out that -GR- is the equivalent of /GR- which is the msvc option to disable RTTI (https://learn.microsoft.com/en-us/cpp/build/reference/gr-enable-run-time-type-information?view=msvc-170).

My guess is that some part of clang checked for this -GR- or -fno-rtti and forgot to claim the other one. This was fixed in https://github.com/llvm/llvm-project/commit/cd18efb61d759405956dbd30e4b5f2720d8e1783 but we will have to wait until 17.0.0 to have that, and I don't want to be building a custom clang for Linaro's bots.

So I think the sensible thing to do right now is add -Wno-unused-command-line-argument to the build flags and I will update the container's clang to 16.0.4. Which we would have to do eventually anyway.

I'll let you know once that's done so you can update this diff and see a green CI build before landing.

libcxx/utils/ci/run-buildbot

616

flags="--sysroot=${INSTALL_DIR} -Wno-unused-command-line-argument"

For reasons explained in the top level comment.

[simon_tatham]

In D154246#4474116, @DavidSpickett wrote:

clang-16: error: argument unused during compilation: '-G R-' [-Werror,-Wunused-command-line-argument]

Turns out that -GR- is the equivalent of /GR- which is the msvc option to disable RTTI

That's familiar – I ran into that in another context last month. libc++ contains this in its cmake files:

function(cxx_add_rtti_flags target)
  if (NOT LIBCXX_ENABLE_RTTI)
    target_add_compile_flags_if_supported(${target} PUBLIC -GR-)
    target_add_compile_flags_if_supported(${target} PUBLIC -fno-rtti)
  endif()
endfunction()

and that "add_compile_flags_if_supported" is somehow misidentifying whether it's supported. Possibly because -GR- is possible to re-parse in a non-msvc-style command line as the completely different option -G with argument word R-. The fix might be to explicitly query the command-line flavour, via if (CMAKE_C_COMPILER_ID MATCHES "MSVC" OR CMAKE_C_COMPILER_FRONTEND_VARIANT MATCHES "MSVC") or something along those lines, and use that to condition the use of that MSVC option.

[simon_tatham]

Aha, even better, check LIBCXX_TARGETING_MSVC which the same CMakeLists has already defined.

[DavidSpickett]

https://reviews.llvm.org/D154582 to do that.

[michaelplatings]

clang -> $CC

[DavidSpickett]

Production containers are updated.

I've done a build on one, with the cc setting fixed, and it works. So in theory, update this one more time and we'll get a green build.

libcxx/utils/ci/build-picolibc.sh

69

Needs to literally be cc. As /usr/local/bin/cc is linked to clang-16.0.4 in our images, we don't set any env vars.

I know it's not great for anyone reproducing, and an env var would probably be better but cross that road when it's needed.

[michaelplatings]

Default to compiling picolibc with cc

[michaelplatings]

libcxx/utils/ci/build-picolibc.sh

69

Thanks for explaining. Now if CC isn't set then cc will be used.

[DavidSpickett]

We got a green build - https://buildkite.com/llvm-project/libcxx-ci/builds/27739#_

All the infrastructure changes are done, so please libcxx folks proceed with the review as usual.

libcxx/utils/ci/build-picolibc.sh

69

Nice, that's much more convenient.

[Mordante]

libcxx/docs/index.rst

132

The CI is green. Is this work-in-progress or working?

libcxx/utils/ci/build-picolibc.sh

10

Can you add a short comment what this file does?

[michaelplatings]

Add summary comment to build-picolibc.sh

libcxx/docs/index.rst

132

At present all this job will do is check that libc++ can build against picolibc. It doesn't yet run any tests so there's not yet any guarantee that the build output actually works. Therefore work-in-progress. Once tests are running and green this note should be removed.

[ldionne]

libcxx/utils/ci/run-buildbot

625

For now I just want to set a baseline of keeping libcxx+picolibc building. Once that's in place we can start ratcheting up the quality bar.

Ratcheting the quality bar is usually done by first setting up the test suite and adding a bunch of XFAIL and UNSUPPORTED markup that is then gradually removed. In this case, we're not running the test suite at all, and that's insufficient to claim support for it. I hate to be that person, but this patch needs to include whatever's necessary to build and run the test suite before it is merged.

One thing we could discuss is actually only compiling and linking all the tests, but not running them right away since that might require more complicated setup. I'm not sure we want to set that precedent, but I'd definitely be open to the discussion if we had everything compiling and linking. If we were to do that, we'd replace the %{exec} substitution by something like true and that would cause the tests not to be executed for real.

[Mordante]

libcxx/utils/ci/run-buildbot

625

I too would like to see XFAIL and UNSUPPORTED. That gives us more insights in which parts work and which parts don't work. This also helps with future patches to decide how much effort we should spend on getting it to work with picolib. For example getting locales tested properly may take quite a bit of effort; at least that has been my experience. When adding a new locale test it's easier to justify to add a new XFAIL/UNSUPPORTED.

[michaelplatings]

Compile and link tests

[michaelplatings]

libcxx/utils/ci/run-buildbot

625

OK I've updated it to compile & link all the tests, using true for %{exec} as suggested.

[philnik]

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

9 ↗

(On Diff #540496)

I guess this is caused by a missing libatomic?

libcxx/utils/ci/run-buildbot

608

Personally I'm not super concerned about building picolibc every time. I don't expect it to be a long build (probably similar to libc++), and testing always the latest version seems like a nice thing to me.

[arichardson]

libcxx/utils/libcxx/test/features.py

207 ↗

(On Diff #540496)

Why pipe? Is that function actually called inside libc++?

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

20 ↗

(On Diff #540496)

There should probably be a comment here such as todo: run using qemu-system-arm?

[michaelplatings]

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

9 ↗

(On Diff #540496)

Yes, there's no libatomic or equivalent. Perhaps something like REQUIRES: atomic would be better here.

libcxx/utils/ci/run-buildbot

608

Yes, the time to build picolibc is seconds - negligible compared to the testing time. The revision of picolibc is pinned, but doing it this way does make it trivial to update to the latest version.

libcxx/utils/libcxx/test/features.py

207 ↗

(On Diff #540496)

check_assertion.h uses pipe, fork, exec etc. Picolibc declares these in unistd.h but doesn't implement them. Therefore this feature check was previously passing with picolibc, but now fails at link time.

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

20 ↗

(On Diff #540496)

Agreed

[michaelplatings]

libcxx/utils/libcxx/test/features.py

207 ↗

(On Diff #540496)

(Despite the name "has-unix-headers", the check is mostly (always?) used to xfail tests that use check_assertion.h)

[Mordante]

I want to have a closer look when the CI is green. But it seems the number of disabled tests is quite small :-)

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

9 ↗

(On Diff #540496)

When there are no plans to add that to picolib soon I think we indeed should add something like //UNSUPPRTED: no-libatomic (maybe needs a better name). The LIBCXX-PICOLIBC-FIXME should be used for things that can be fixed.

[michaelplatings]

Add "atomics" feature, run git-clang-format, and other minor tweaks

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

9 ↗

(On Diff #540496)

For consistency with the existing non-lockfree-atomics feature I've gone with the name atomics.

[philnik]

LGTM. Leaving final approval to @ldionne, since he knows the CI best.

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

9 ↗

(On Diff #541255)

I would just go with // REQUIRES: atomics. That's a lot less confusing that the double negation.

[michaelplatings]

XFAIL: !atomics -> REQUIRES: atomics

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

9 ↗

(On Diff #541255)

I went with XFAIL for consistency with non-lockfree-atomics. REQUIRES means the tests won't be run at all, but I'm OK with that.

[michaelplatings]

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

9 ↗

(On Diff #541255)

The CI results with XFAIL: !atomics suggest that the code to detect atomics support was incorrect. REQUIRES wouldn't have picked that up so I feel XFAIL is the better choice.

[michaelplatings]

Change how atomics feature is detected and change atomic REQUIRES back to negated XFAIL

[michaelplatings]

Use 16MB area of RAM to enable running bigger tests

[arichardson]

libcxx/utils/libcxx/test/features.py

102 ↗

(On Diff #541465)

This is effectively checking for availability of 64-bit atomics (since uintmax_t essentially has to be the same as uint64_t for ABI reasons). Maybe something like has-64-bit-atomics is more accurate? I believe this will fail for 32 bit arm picolibc since that needs a libatomic runtime function. The other larger libatomic requirements should already be handled by non-lockfree-atomics.

[michaelplatings]

atomics -> has-64-bit-atomics

[michaelplatings]

Rebase after -Wcast-qual error was fixed

[ldionne]

Is this ready for review? I see that the CI is failing. This would need to be rebased and the CI to be green.

Once that's done, please ping me on Discord. We're trying to drain the review queue here on Phabricator as part of our transition to Github PRs.

[michaelplatings]

Rebase

[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

1–2 ↗

(On Diff #557024)

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

12 ↗

(On Diff #557024)

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

615

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

104 ↗

(On Diff #557024)

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

207 ↗

(On Diff #540496)

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

12 ↗

(On Diff #557024)

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

615

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

17 ↗

(On Diff #557476)

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

38 ↗

(On Diff #557476)

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

17 ↗

(On Diff #557476)

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.

38 ↗

(On Diff #557476)

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

17 ↗

(On Diff #557476)

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?

38 ↗

(On Diff #557476)

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

17 ↗

(On Diff #557476)

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.

38 ↗

(On Diff #557476)

Thanks! Applied in the new diff.

[arichardson]

One suggestion otherwise looking good to me

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

17 ↗

(On Diff #557476)

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

29 ↗

(On Diff #557983)

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

29 ↗

(On Diff #557983)

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 ↗

(On Diff #558024)

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 ↗

(On Diff #558024)

Same here

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

10 ↗

(On Diff #558024)

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

3 ↗

(On Diff #558169)

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)

17 ↗

(On Diff #558169)

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 ↗

(On Diff #558169)

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 ↗

(On Diff #558169)

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

294 ↗

(On Diff #558169)

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/Armv6M-picolibc.cmake

+set(CMAKE_ASM_COMPILER_TARGET "armv6m-none-eabi" CACHE STRING "")
+set(CMAKE_CXX_COMPILER_TARGET "armv6m-none-eabi" CACHE STRING "")
+set(CMAKE_CXX_FLAGS "-mfloat-abi=soft" CACHE STRING "")
+set(CMAKE_C_COMPILER_TARGET "armv6m-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(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 "")
+set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR 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/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
+#
+#===----------------------------------------------------------------------===##
+
+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="${2}"
+            shift; shift
+            ;;
+        *)
+            error "Unknown argument '${1}'"
+            ;;
+    esac
+done
+
+for arg in build_dir install_dir; 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.
+curl -L "https://github.com/picolibc/picolibc/archive/refs/tags/1.8.2.tar.gz" | tar -xz --strip-components=1 -C "${picolibc_source_dir}"
+
+
+cat <<EOF > "${picolibc_build_dir}/meson-cross-build.txt"
+[binaries]
+c = ['$CC', '--target=armv6m-none-eabi', '-mfloat-abi=soft', '-nostdlib']
+ar = 'llvm-ar'
+as = 'llvm-as'
+ld = 'lld'
+strip = 'llvm-strip'
+[host_machine]
+system = 'none'
+cpu_family = 'arm'
+cpu = 'arm'
+endian = 'little'
+[properties]
+skip_sanity_check = true
+EOF
+
+venv_dir="${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"
+
+"${venv_dir}/bin/meson" setup \
+  -Dincludedir=include -Dlibdir=lib -Dspecsdir=none -Dmultilib=false \
+  --prefix "${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: "Armv6-M picolibc"
+      command: "libcxx/utils/ci/run-buildbot armv6m-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
 ;;
+armv6m-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 "${INSTALL_DIR}"
+
+    flags="--sysroot=${INSTALL_DIR}"
+    generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Armv6M-picolibc.cmake" \
+          -DCMAKE_C_FLAGS="${flags}" \
+          -DCMAKE_CXX_FLAGS="${flags}"
+
+    echo "--- Installing libc++, libc++abi and libunwind to a fake location"
+    ${NINJA} -vC "${BUILD_DIR}" install-cxx install-cxxabi install-unwind
+;;
 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.
     generate-cmake-libcxx-win -DLIBCXX_TEST_PARAMS="enable_experimental=False"
     echo "+++ Running the libc++ tests"
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.