[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

654

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

70

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

70

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

70

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

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

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

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

21

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

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

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

21

Agreed

[michaelplatings]

libcxx/utils/libcxx/test/features.py

207

(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

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

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

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

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

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

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

318–323 ↗

(On Diff #557024)

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

What is the relationship between picolibc and _NEWLIB_VERSION?

libcxx/utils/ci/run-buildbot

639

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

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

207

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

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

639

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

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

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

3

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

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

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

 
 Libc++ also supports common platforms and architectures:
 
-=============== ========================= ============================
-Target platform Target architecture       Notes
-=============== ========================= ============================
-macOS 10.13+    i386, x86_64, arm64
-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
-Android 5.0+    i386, x86_64, arm, arm64
-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 10.13+          i386, x86_64, arm64
+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
+Android 5.0+          i386, x86_64, arm, arm64
+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

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

+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'
+
+    # Disable warnings in cxx_atomic_impl.h:
+    # "large atomic operation may incur significant performance penalty; the
+    # access size (4 bytes) exceeds the max lock-free size (0  bytes)"
+    ' -Wno-atomic-alignment'
+
+    # Various libc++ headers check for the definition of _NEWLIB_VERSION
+    # which for picolibc is defined in picolibc.h.
+    ' -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
+))
+config.available_features.add('libcxx-fake-executor')
+
+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/atomics/atomics.types.generic/atomics.types.float/lockfree.pass.cpp

 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: target={{.+}}-windows-gnu
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 
 //   static constexpr bool is_always_lock_free = implementation-defined;
 //   bool is_lock_free() const volatile noexcept;

libcxx/test/libcxx/clang_modules_include.gen.py

 // 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/selftest/dsl/dsl.sh.py

 #
 # ===----------------------------------------------------------------------===##
 
+# XFAIL: libcxx-fake-executor
+
 # 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}

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

 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: libcxx-fake-executor
 // XFAIL: *
 
 // Make sure the test DOES NOT pass if it fails at runtime.

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

 //===----------------------------------------------------------------------===//
 
 // REQUIRES: objective-c++
+// UNSUPPORTED: libcxx-fake-executor
 
 // XFAIL: *
 

libcxx/test/libcxx/system_reserved_names.gen.py

 //--- {header}.compile.pass.cpp
 {lit_header_restrictions.get(header, '')}
 
+// for "_NEWLIB_VERSION" macro
+#include <string.h>
+
 #define SYSTEM_RESERVED_NAME This name should not be used in libc++
 
 // libc++ does not use single-letter names as a matter of principle.

 #endif
 
 // Test that libc++ doesn't use names that collide with FreeBSD system macros.
-#ifndef __FreeBSD__
+// newlib and picolibc also define these macros
+#if !defined(__FreeBSD__) && !defined(_NEWLIB_VERSION)
 #  define __null_sentinel SYSTEM_RESERVED_NAME
 #  define __generic SYSTEM_RESERVED_NAME
 #endif

 # define __pre SYSTEM_RESERVED_NAME
 #endif
 
-#define __input SYSTEM_RESERVED_NAME
+// Newlib & picolibc use __input as a parameter name of a64l & l64a
+#ifndef _NEWLIB_VERSION
+# define __input SYSTEM_RESERVED_NAME
+#endif
 #define __output SYSTEM_RESERVED_NAME
 
 #define __acquire SYSTEM_RESERVED_NAME

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/assign.pass.cpp

 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: target={{.+}}-windows-gnu
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 
 //  floating-point-type operator=(floating-point-type) volatile noexcept;
 //  floating-point-type operator=(floating-point-type) noexcept;

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/compare_exchange_strong.pass.cpp

 //===----------------------------------------------------------------------===//
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 
 // bool compare_exchange_strong(T& expected, T desired,
 //                            memory_order success, memory_order failure) volatile noexcept;

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/compare_exchange_weak.pass.cpp

 //===----------------------------------------------------------------------===//
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 
 // bool compare_exchange_weak(T& expected, T desired,
 //                            memory_order success, memory_order failure) volatile noexcept;

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/ctor.pass.cpp

 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: target={{.+}}-windows-gnu
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 
 // constexpr atomic() noexcept;
 // constexpr atomic(floating-point-type) noexcept;

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/exchange.pass.cpp

 // XFAIL: tsan
 // XFAIL: target={{x86_64-.*}} && msan
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 
 //  T exchange(T, memory_order = memory_order::seq_cst) volatile noexcept;
 //  T exchange(T, memory_order = memory_order::seq_cst) noexcept;

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/fetch_add.pass.cpp

 // Hangs with msan.
 // UNSUPPORTED: msan
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 
 // floating-point-type fetch_add(floating-point-type,
 //                               memory_order = memory_order::seq_cst) volatile noexcept;

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/fetch_sub.pass.cpp

 // Hangs with msan.
 // UNSUPPORTED: msan
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 
 // floating-point-type fetch_sub(floating-point-type,
 //                               memory_order = memory_order::seq_cst) volatile noexcept;

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/load.pass.cpp

 // Clang's support for atomic operations on long double is broken. See https://github.com/llvm/llvm-project/issues/72893
 // XFAIL: tsan
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 
 //  floating-point-type load(memory_order = memory_order::seq_cst) volatile noexcept;
 //  floating-point-type load(memory_order = memory_order::seq_cst) noexcept;

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/lockfree.pass.cpp

 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: target={{.+}}-windows-gnu
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 
 //   static constexpr bool is_always_lock_free = implementation-defined;
 //   bool is_lock_free() const volatile noexcept;

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/notify_all.pass.cpp

 // XFAIL: availability-synchronization_library-missing
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 
 //  void notify_all() volatile noexcept;
 //  void notify_all() noexcept;

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/notify_one.pass.cpp

 // XFAIL: availability-synchronization_library-missing
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 
 //  void notify_one() volatile noexcept;
 //  void notify_one() noexcept;

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.float.pass.cpp

 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: target={{.+}}-windows-gnu
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 
 //  operator floating-point-type() volatile noexcept;
 //  operator floating-point-type() noexcept;

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.minus_equals.pass.cpp

 // UNSUPPORTED: target={{.+}}-windows-gnu
 // XFAIL: LIBCXX-AIX-FIXME
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 // Hangs with msan.
 // UNSUPPORTED: msan
 

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.plus_equals.pass.cpp

 // UNSUPPORTED: target={{.+}}-windows-gnu
 // XFAIL: LIBCXX-AIX-FIXME
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 // Hangs with msan.
 // UNSUPPORTED: msan
 

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/store.pass.cpp

 // Clang's support for atomic operations on long double is broken. See https://github.com/llvm/llvm-project/issues/72893
 // XFAIL: tsan
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 
 // void store(floating-point-type, memory_order = memory_order::seq_cst) volatile noexcept;
 // void store(floating-point-type, memory_order = memory_order::seq_cst) noexcept;

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/wait.pass.cpp

 // XFAIL: tsan
 // XFAIL: target={{x86_64-.*}} && msan
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 
 // void wait(T old, memory_order order = memory_order::seq_cst) const volatile noexcept;
 // void wait(T old, memory_order order = memory_order::seq_cst) const noexcept;

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

 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template <>

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

 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>

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

 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>

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

 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>

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

 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>

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

 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>

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

 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>

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

 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>

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

 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>

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

 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>

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

 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>

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

 
 // UNSUPPORTED: c++03
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // constexpr atomic<T>::atomic(T value)

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

 //
 //===----------------------------------------------------------------------===//
 
+// Floating point exceptions are required for the FE_... macros to be defined.
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <fenv.h>
 
 #include <fenv.h>

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

 // 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>

libcxx/test/std/experimental/simd/simd.reference/reference_assignment.pass.cpp

 
 // UNSUPPORTED: c++03, c++11, c++14
 
+// The machine emulated in tests does not have enough memory for code.
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
+
 // <experimental/simd>
 //
 // [simd.reference]

libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/cerr.sh.cpp

 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
+
 // <iostream>
 
 // ostream cerr;

libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/clog.sh.cpp

 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
+
 // <iostream>
 
 // ostream clog;

libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/cout.sh.cpp

 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
+
 // <iostream>
 
 // ostream cout;

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

 // 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;
 

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

 // 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.

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

 // 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.

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

 
 // UNSUPPORTED: c++03, c++11, c++14
 
+// 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}}

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

 //
 //===----------------------------------------------------------------------===//
 
+// Floating point exceptions are required for the FE_... macros to be defined.
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <cfenv>
 
 #include <cfenv>

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

 // 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>

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

 //
 //===----------------------------------------------------------------------===//
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 
 // high_resolution_clock

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

 
 // UNSUPPORTED: availability-filesystem-missing
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 //
 // file_clock

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

 //
 //===----------------------------------------------------------------------===//
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 
 // system_clock

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

 //
 //===----------------------------------------------------------------------===//
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 
 // system_clock

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

 //
 //===----------------------------------------------------------------------===//
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 
 // system_clock

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

 //
 //===----------------------------------------------------------------------===//
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 
 // time_point

libcxx/utils/ci/BOT_OWNERS.txt

 
 N: Linaro Toolchain Working Group
 E: [e-mail]
-D: Armv7, Armv8, AArch64
+D: Arm platforms
 
 N: LLVM on Power
 E: [e-mail]

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.
+# Following changes are required and were introduced after version 1.8.5:
+# - updated semihost arguments handling,
+# - added missing macros in stdio.h
+# Version following 1.8.5, was not released by the time of writing.
+picolibc_commit="4b9b8ef01d18cf0563348ad4af4ae89a7277e7ca"
+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

     <<: *arm_agent_armv8l
     <<: *common
 
+  - label: Armv7-M picolibc
+    command: libcxx/utils/ci/run-buildbot armv7m-picolibc
+    <<: *arm_agent_aarch64
+    <<: *common
+
 - group: AIX
   steps:
   - label: AIX (32-bit)

libcxx/utils/ci/run-buildbot

     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

libcxx/utils/libcxx/test/features.py

             #include <unistd.h>
             #include <sys/wait.h>
             int main(int, char**) {
-              return 0;
+              int fd[2];
+              return pipe(fd);
             }
           """,
         ),

     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),
+    # this feature has to be checked after the "gcc" feature, so that the
+    # "-D_LIBCPP_DISABLE_DEPRECATION_WARNINGS" flag is already in place
+    Feature(
+        name="has-64-bit-atomics",
+        when=lambda cfg: sourceBuilds(
+            cfg,
+            """
+            #include <atomic>
+            std::atomic_uint64_t x;
+            int main(int, char**) { (void)x.load(); return 0; }
+          """,
+        ),
+    ),
 ]
 
 # Deduce and add the test features that that are implied by the #defines in

         name="LIBCXX-FREEBSD-FIXME",
         when=lambda cfg: "__FreeBSD__" in compilerMacros(cfg),
     ),
+    Feature(
+        name="LIBCXX-PICOLIBC-FIXME",
+        when=lambda cfg: sourceBuilds(
+            cfg,
+            """
+            #include <string.h>
+            #ifndef __PICOLIBC__
+            #error not picolibc
+            #endif
+            int main(int, char**) { return 0; }
+          """,
+        ),
+    ),
 ]
 
 # Add features representing the build host platform name.

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

+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'
+))
+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
+))
+config.available_features.add('libcxx-fake-executor')
+
+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
+)

libcxxabi/test/test_demangle.pass.cpp

 // The demangler does not pass all these tests with the system dylibs on macOS.
 // XFAIL: stdlib=apple-libc++ && target={{.+}}-apple-macosx10.{{9|10|11|12|13|14|15}}
 
+// This test is too big for most embedded devices.
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // https://llvm.org/PR51407 was not fixed in some previously-released
 // demanglers, which causes them to run into the infinite loop.
 // UNSUPPORTED: stdlib=apple-libc++ && target={{.+}}-apple-macosx10.{{9|10|11|12|13|14|15}}

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

+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}'
+))
+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
+))
+config.available_features.add('libcxx-fake-executor')
+
+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
+)