[libc++] Add initial support for picolibc

Picolibc is a C Standard Library that is commonly used in embedded
environments. This patch adds initial support for this configuration
along with pre-commit CI. As of this patch, the test suite only builds
the tests and nothing is run. A follow-up patch will make the test suite
actually run the tests.

Differential Revision: https://reviews.llvm.org/D154246

63 files changed, 441 insertions, 14 deletions

diff --git a/libcxx/cmake/caches/Armv7M-picolibc.cmake b/libcxx/cmake/caches/Armv7M-picolibc.cmake
new file mode 100644
index 0000000..6ed1866
--- /dev/null
+++ b/libcxx/cmake/caches/Armv7M-picolibc.cmake
@@ -0,0 +1,38 @@
+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 "")
diff --git a/libcxx/docs/index.rst b/libcxx/docs/index.rst
index c4199dc..7bb5512 100644
--- a/libcxx/docs/index.rst
+++ b/libcxx/docs/index.rst
@@ -124,16 +124,17 @@ GCC          12              In C++11 or later only     latest stable release pe
 
 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
diff --git a/libcxx/test/configs/armv7m-picolibc-libc++.cfg.in b/libcxx/test/configs/armv7m-picolibc-libc++.cfg.in
new file mode 100644
index 0000000..c0d23f1
--- /dev/null
+++ b/libcxx/test/configs/armv7m-picolibc-libc++.cfg.in
@@ -0,0 +1,42 @@
+lit_config.load_config(config, '@CMAKE_CURRENT_BINARY_DIR@/cmake-bridge.cfg')
+
+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 {}'.format(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
+)
diff --git a/libcxx/test/libcxx/atomics/atomics.types.generic/atomics.types.float/lockfree.pass.cpp b/libcxx/test/libcxx/atomics/atomics.types.generic/atomics.types.float/lockfree.pass.cpp
index 4651153..222c6d0 100644
--- a/libcxx/test/libcxx/atomics/atomics.types.generic/atomics.types.float/lockfree.pass.cpp
+++ b/libcxx/test/libcxx/atomics/atomics.types.generic/atomics.types.float/lockfree.pass.cpp
@@ -8,6 +8,7 @@
 // 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;
diff --git a/libcxx/test/libcxx/clang_modules_include.gen.py b/libcxx/test/libcxx/clang_modules_include.gen.py
index 04c8e03..26ef207 100644
--- a/libcxx/test/libcxx/clang_modules_include.gen.py
+++ b/libcxx/test/libcxx/clang_modules_include.gen.py
@@ -39,6 +39,9 @@ for header in public_headers:
 // 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}>
diff --git a/libcxx/test/libcxx/selftest/dsl/dsl.sh.py b/libcxx/test/libcxx/selftest/dsl/dsl.sh.py
index dfe290b..6c695a0d 100644
--- a/libcxx/test/libcxx/selftest/dsl/dsl.sh.py
+++ b/libcxx/test/libcxx/selftest/dsl/dsl.sh.py
@@ -6,6 +6,8 @@
 #
 # ===----------------------------------------------------------------------===##
 
+# 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}
diff --git a/libcxx/test/libcxx/selftest/pass.cpp/run-error.pass.cpp b/libcxx/test/libcxx/selftest/pass.cpp/run-error.pass.cpp
index eac7d88..1df98ef 100644
--- a/libcxx/test/libcxx/selftest/pass.cpp/run-error.pass.cpp
+++ b/libcxx/test/libcxx/selftest/pass.cpp/run-error.pass.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: libcxx-fake-executor
 // XFAIL: *
 
 // Make sure the test DOES NOT pass if it fails at runtime.
diff --git a/libcxx/test/libcxx/selftest/pass.mm/run-error.pass.mm b/libcxx/test/libcxx/selftest/pass.mm/run-error.pass.mm
index 22e17666..02e8321 100644
--- a/libcxx/test/libcxx/selftest/pass.mm/run-error.pass.mm
+++ b/libcxx/test/libcxx/selftest/pass.mm/run-error.pass.mm
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: objective-c++
+// UNSUPPORTED: libcxx-fake-executor
 
 // XFAIL: *
 
diff --git a/libcxx/test/libcxx/system_reserved_names.gen.py b/libcxx/test/libcxx/system_reserved_names.gen.py
index e2aa8fc..8c4be97 100644
--- a/libcxx/test/libcxx/system_reserved_names.gen.py
+++ b/libcxx/test/libcxx/system_reserved_names.gen.py
@@ -63,7 +63,8 @@ for header in public_headers:
 #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
@@ -101,7 +102,10 @@ for header in public_headers:
 # 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
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/assign.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/assign.pass.cpp
index 8efb556..3bdd8ae 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/assign.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/assign.pass.cpp
@@ -8,6 +8,7 @@
 // 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;
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/compare_exchange_strong.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/compare_exchange_strong.pass.cpp
index 839d79d..b9e2fe4 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/compare_exchange_strong.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/compare_exchange_strong.pass.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 // 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;
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/compare_exchange_weak.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/compare_exchange_weak.pass.cpp
index 097210c..f3ac52f 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/compare_exchange_weak.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/compare_exchange_weak.pass.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 // 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;
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/ctor.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/ctor.pass.cpp
index febabb4..d2f4a2b 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/ctor.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/ctor.pass.cpp
@@ -8,6 +8,7 @@
 // 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;
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/exchange.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/exchange.pass.cpp
index a830630..1807e48 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/exchange.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/exchange.pass.cpp
@@ -11,6 +11,7 @@
 // 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;
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/fetch_add.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/fetch_add.pass.cpp
index 0c59b50..a73c5f2 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/fetch_add.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/fetch_add.pass.cpp
@@ -13,6 +13,7 @@
 // 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;
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/fetch_sub.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/fetch_sub.pass.cpp
index 4435e3f..d21a42e 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/fetch_sub.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/fetch_sub.pass.cpp
@@ -13,6 +13,7 @@
 // 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;
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/load.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/load.pass.cpp
index fa08527..2901af0 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/load.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/load.pass.cpp
@@ -10,6 +10,7 @@
 // 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;
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/lockfree.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/lockfree.pass.cpp
index 447e0f8..7619d11 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/lockfree.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/lockfree.pass.cpp
@@ -8,6 +8,7 @@
 // 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;
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/notify_all.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/notify_all.pass.cpp
index e2320ed..143a034 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/notify_all.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/notify_all.pass.cpp
@@ -9,6 +9,7 @@
 // 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;
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/notify_one.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/notify_one.pass.cpp
index 058424f..debc9dd 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/notify_one.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/notify_one.pass.cpp
@@ -9,6 +9,7 @@
 // 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;
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.float.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.float.pass.cpp
index 54c62ba..e0da122 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.float.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.float.pass.cpp
@@ -8,6 +8,7 @@
 // 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;
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.minus_equals.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.minus_equals.pass.cpp
index 23ad0c5..0ee02327 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.minus_equals.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.minus_equals.pass.cpp
@@ -9,6 +9,7 @@
 // UNSUPPORTED: target={{.+}}-windows-gnu
 // XFAIL: LIBCXX-AIX-FIXME
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 // Hangs with msan.
 // UNSUPPORTED: msan
 
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.plus_equals.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.plus_equals.pass.cpp
index 0676645..1cdafaa 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.plus_equals.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.plus_equals.pass.cpp
@@ -9,6 +9,7 @@
 // UNSUPPORTED: target={{.+}}-windows-gnu
 // XFAIL: LIBCXX-AIX-FIXME
 // ADDITIONAL_COMPILE_FLAGS(has-latomic): -latomic
+// XFAIL: !has-64-bit-atomics
 // Hangs with msan.
 // UNSUPPORTED: msan
 
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/store.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/store.pass.cpp
index 0bb1f00..957dbbe 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/store.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/store.pass.cpp
@@ -9,6 +9,7 @@
 // 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;
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/wait.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/wait.pass.cpp
index 15f3613..6c4f65d 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/wait.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/wait.pass.cpp
@@ -11,6 +11,7 @@
 // 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;
diff --git a/libcxx/test/std/atomics/atomics.types.generic/integral.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/integral.pass.cpp
index 2695ff9..bb88f55 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/integral.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/integral.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template <>
diff --git a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add.pass.cpp b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add.pass.cpp
index 3156378..069b64b 100644
--- a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>
diff --git a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add_explicit.pass.cpp b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add_explicit.pass.cpp
index 747a62a..2b75e91 100644
--- a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add_explicit.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add_explicit.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>
diff --git a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_and.pass.cpp b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_and.pass.cpp
index 04b279c..e106e1a 100644
--- a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_and.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_and.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>
diff --git a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_and_explicit.pass.cpp b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_and_explicit.pass.cpp
index f6887a2..ed878e2 100644
--- a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_and_explicit.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_and_explicit.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>
diff --git a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_or.pass.cpp b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_or.pass.cpp
index 76a66dc..01e73f4 100644
--- a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_or.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_or.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>
diff --git a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_or_explicit.pass.cpp b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_or_explicit.pass.cpp
index 7316123..f8ae8fb 100644
--- a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_or_explicit.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_or_explicit.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>
diff --git a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub.pass.cpp b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub.pass.cpp
index 37a9586..9212f03 100644
--- a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>
diff --git a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub_explicit.pass.cpp b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub_explicit.pass.cpp
index 1924096..628b815 100644
--- a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub_explicit.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub_explicit.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>
diff --git a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_xor.pass.cpp b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_xor.pass.cpp
index 3875f9e..0c648fc 100644
--- a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_xor.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_xor.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>
diff --git a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_xor_explicit.pass.cpp b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_xor_explicit.pass.cpp
index 7521a02..4bdad43f 100644
--- a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_xor_explicit.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_xor_explicit.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template<class T>
diff --git a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/ctor.pass.cpp b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/ctor.pass.cpp
index 32baca4..3ee44a1 100644
--- a/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/ctor.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/ctor.pass.cpp
@@ -8,6 +8,8 @@
 
 // UNSUPPORTED: c++03
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // constexpr atomic<T>::atomic(T value)
diff --git a/libcxx/test/std/depr/depr.c.headers/fenv_h.compile.pass.cpp b/libcxx/test/std/depr/depr.c.headers/fenv_h.compile.pass.cpp
index fda5e18..dcc9757 100644
--- a/libcxx/test/std/depr/depr.c.headers/fenv_h.compile.pass.cpp
+++ b/libcxx/test/std/depr/depr.c.headers/fenv_h.compile.pass.cpp
@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+// Floating point exceptions are required for the FE_... macros to be defined.
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <fenv.h>
 
 #include <fenv.h>
diff --git a/libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp b/libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp
index 2b64554..a1560c8 100644
--- a/libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp
+++ b/libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp
@@ -11,6 +11,9 @@
 // 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>
diff --git a/libcxx/test/std/experimental/simd/simd.reference/reference_assignment.pass.cpp b/libcxx/test/std/experimental/simd/simd.reference/reference_assignment.pass.cpp
index 0486250..d37b4af 100644
--- a/libcxx/test/std/experimental/simd/simd.reference/reference_assignment.pass.cpp
+++ b/libcxx/test/std/experimental/simd/simd.reference/reference_assignment.pass.cpp
@@ -8,6 +8,9 @@
 
 // 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]
diff --git a/libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/cerr.sh.cpp b/libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/cerr.sh.cpp
index b9e274a..9a4b437 100644
--- a/libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/cerr.sh.cpp
+++ b/libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/cerr.sh.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
+
 // <iostream>
 
 // ostream cerr;
diff --git a/libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/clog.sh.cpp b/libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/clog.sh.cpp
index 476addb..783be3a 100644
--- a/libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/clog.sh.cpp
+++ b/libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/clog.sh.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
+
 // <iostream>
 
 // ostream clog;
diff --git a/libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/cout.sh.cpp b/libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/cout.sh.cpp
index b8d3193..1234da3 100644
--- a/libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/cout.sh.cpp
+++ b/libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/cout.sh.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
+
 // <iostream>
 
 // ostream cout;
diff --git a/libcxx/test/std/iterators/iterator.primitives/iterator.traits/cxx20_iterator_traits.compile.pass.cpp b/libcxx/test/std/iterators/iterator.primitives/iterator.traits/cxx20_iterator_traits.compile.pass.cpp
index f6b7f6f..f56750b 100644
--- a/libcxx/test/std/iterators/iterator.primitives/iterator.traits/cxx20_iterator_traits.compile.pass.cpp
+++ b/libcxx/test/std/iterators/iterator.primitives/iterator.traits/cxx20_iterator_traits.compile.pass.cpp
@@ -11,6 +11,11 @@
 // 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;
 
diff --git a/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/sized_delete_array14.pass.cpp b/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/sized_delete_array14.pass.cpp
index 21663cd..901f2cc 100644
--- a/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/sized_delete_array14.pass.cpp
+++ b/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/sized_delete_array14.pass.cpp
@@ -9,6 +9,7 @@
 // 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.
diff --git a/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/sized_delete14.pass.cpp b/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/sized_delete14.pass.cpp
index a8701ce..d0b10b6 100644
--- a/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/sized_delete14.pass.cpp
+++ b/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/sized_delete14.pass.cpp
@@ -9,6 +9,7 @@
 // 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.
diff --git a/libcxx/test/std/language.support/support.runtime/ctime.timespec.compile.pass.cpp b/libcxx/test/std/language.support/support.runtime/ctime.timespec.compile.pass.cpp
index 04c4c77..12b1c38 100644
--- a/libcxx/test/std/language.support/support.runtime/ctime.timespec.compile.pass.cpp
+++ b/libcxx/test/std/language.support/support.runtime/ctime.timespec.compile.pass.cpp
@@ -11,6 +11,9 @@
 
 // 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}}
diff --git a/libcxx/test/std/numerics/cfenv/cfenv.syn/cfenv.pass.cpp b/libcxx/test/std/numerics/cfenv/cfenv.syn/cfenv.pass.cpp
index 557a93c..7b3b490 100644
--- a/libcxx/test/std/numerics/cfenv/cfenv.syn/cfenv.pass.cpp
+++ b/libcxx/test/std/numerics/cfenv/cfenv.syn/cfenv.pass.cpp
@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+// Floating point exceptions are required for the FE_... macros to be defined.
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <cfenv>
 
 #include <cfenv>
diff --git a/libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp b/libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp
index db00cbd..2076384 100644
--- a/libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp
+++ b/libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp
@@ -11,6 +11,9 @@
 // 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>
diff --git a/libcxx/test/std/time/time.clock/time.clock.file/to_from_sys.pass.cpp b/libcxx/test/std/time/time.clock/time.clock.file/to_from_sys.pass.cpp
index 5b1f465..b1031c8 100644
--- a/libcxx/test/std/time/time.clock/time.clock.file/to_from_sys.pass.cpp
+++ b/libcxx/test/std/time/time.clock/time.clock.file/to_from_sys.pass.cpp
@@ -10,6 +10,9 @@
 
 // UNSUPPORTED: availability-filesystem-missing
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 //
 // file_clock
diff --git a/libcxx/test/std/time/time.clock/time.clock.hires/now.pass.cpp b/libcxx/test/std/time/time.clock/time.clock.hires/now.pass.cpp
index db1fb55..8625ac5 100644
--- a/libcxx/test/std/time/time.clock/time.clock.hires/now.pass.cpp
+++ b/libcxx/test/std/time/time.clock/time.clock.hires/now.pass.cpp
@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 
 // high_resolution_clock
diff --git a/libcxx/test/std/time/time.clock/time.clock.system/from_time_t.pass.cpp b/libcxx/test/std/time/time.clock/time.clock.system/from_time_t.pass.cpp
index 70dd811..5ff6674 100644
--- a/libcxx/test/std/time/time.clock/time.clock.system/from_time_t.pass.cpp
+++ b/libcxx/test/std/time/time.clock/time.clock.system/from_time_t.pass.cpp
@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 
 // system_clock
diff --git a/libcxx/test/std/time/time.clock/time.clock.system/now.pass.cpp b/libcxx/test/std/time/time.clock/time.clock.system/now.pass.cpp
index dade6ba..70fbe98 100644
--- a/libcxx/test/std/time/time.clock/time.clock.system/now.pass.cpp
+++ b/libcxx/test/std/time/time.clock/time.clock.system/now.pass.cpp
@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 
 // system_clock
diff --git a/libcxx/test/std/time/time.clock/time.clock.system/to_time_t.pass.cpp b/libcxx/test/std/time/time.clock/time.clock.system/to_time_t.pass.cpp
index bf4339c..f3238f7 100644
--- a/libcxx/test/std/time/time.clock/time.clock.system/to_time_t.pass.cpp
+++ b/libcxx/test/std/time/time.clock/time.clock.system/to_time_t.pass.cpp
@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 
 // system_clock
diff --git a/libcxx/test/std/time/time.point/time.point.nonmember/op_-duration.pass.cpp b/libcxx/test/std/time/time.point/time.point.nonmember/op_-duration.pass.cpp
index 80e9d04..199bdec 100644
--- a/libcxx/test/std/time/time.point/time.point.nonmember/op_-duration.pass.cpp
+++ b/libcxx/test/std/time/time.point/time.point.nonmember/op_-duration.pass.cpp
@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+// "unable to find library from dependent library specifier: rt"
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // <chrono>
 
 // time_point
diff --git a/libcxx/utils/ci/BOT_OWNERS.txt b/libcxx/utils/ci/BOT_OWNERS.txt
index 721b19e..de575c5 100644
--- a/libcxx/utils/ci/BOT_OWNERS.txt
+++ b/libcxx/utils/ci/BOT_OWNERS.txt
@@ -10,7 +10,7 @@ least the (N), (E) and (D) fields.
 
 N: Linaro Toolchain Working Group
 E: linaro-toolchain@lists.linaro.org
-D: Armv7, Armv8, AArch64
+D: Arm platforms
 
 N: LLVM on Power
 E: powerllvm@ca.ibm.com
diff --git a/libcxx/utils/ci/build-picolibc.sh b/libcxx/utils/ci/build-picolibc.sh
new file mode 100755
index 0000000..078c2bd
--- /dev/null
+++ b/libcxx/utils/ci/build-picolibc.sh
@@ -0,0 +1,111 @@
+#!/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}"
diff --git a/libcxx/utils/ci/buildkite-pipeline.yml b/libcxx/utils/ci/buildkite-pipeline.yml
index 3c49247..06b0c55 100644
--- a/libcxx/utils/ci/buildkite-pipeline.yml
+++ b/libcxx/utils/ci/buildkite-pipeline.yml
@@ -227,6 +227,11 @@ steps:
     <<: *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)
diff --git a/libcxx/utils/ci/run-buildbot b/libcxx/utils/ci/run-buildbot
index 58c4c2a..445b1be 100755
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -610,6 +610,41 @@ armv7-no-exceptions)
     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 "${MONOREPO_ROOT}/libcxx/cmake/caches/Armv7M-picolibc.cmake" \
+        -DLIBCXX_TEST_CONFIG="armv7m-picolibc-libc++.cfg.in" \
+        -DLIBCXXABI_TEST_CONFIG="armv7m-picolibc-libc++abi.cfg.in" \
+        -DLIBUNWIND_TEST_CONFIG="armv7m-picolibc-libunwind.cfg.in" \
+        -DCMAKE_C_FLAGS="${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
diff --git a/libcxx/utils/libcxx/test/features.py b/libcxx/utils/libcxx/test/features.py
index e1cfa03..ab5a24d 100644
--- a/libcxx/utils/libcxx/test/features.py
+++ b/libcxx/utils/libcxx/test/features.py
@@ -190,6 +190,17 @@ DEFAULT_FEATURES = [
           """,
         ),
     ),
+    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; }
+          """,
+        ),
+    ),
     # TODO: Remove this feature once compiler-rt includes __atomic_is_lockfree()
     # on all supported platforms.
     Feature(
@@ -265,7 +276,8 @@ DEFAULT_FEATURES = [
             #include <unistd.h>
             #include <sys/wait.h>
             int main(int, char**) {
-              return 0;
+              int fd[2];
+              return pipe(fd);
             }
           """,
         ),
@@ -413,6 +425,19 @@ DEFAULT_FEATURES += [
         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.
diff --git a/libcxxabi/test/configs/armv7m-picolibc-libc++abi.cfg.in b/libcxxabi/test/configs/armv7m-picolibc-libc++abi.cfg.in
new file mode 100644
index 0000000..cb5ede3
--- /dev/null
+++ b/libcxxabi/test/configs/armv7m-picolibc-libc++abi.cfg.in
@@ -0,0 +1,33 @@
+lit_config.load_config(config, '@CMAKE_CURRENT_BINARY_DIR@/cmake-bridge.cfg')
+
+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 {}'.format(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
+)
diff --git a/libcxxabi/test/test_demangle.pass.cpp b/libcxxabi/test/test_demangle.pass.cpp
index 67d5208..b7e4109 100644
--- a/libcxxabi/test/test_demangle.pass.cpp
+++ b/libcxxabi/test/test_demangle.pass.cpp
@@ -9,6 +9,9 @@
 // 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}}
diff --git a/libunwind/test/configs/armv7m-picolibc-libunwind.cfg.in b/libunwind/test/configs/armv7m-picolibc-libunwind.cfg.in
new file mode 100644
index 0000000..c2dd320
--- /dev/null
+++ b/libunwind/test/configs/armv7m-picolibc-libunwind.cfg.in
@@ -0,0 +1,33 @@
+lit_config.load_config(config, '@CMAKE_CURRENT_BINARY_DIR@/cmake-bridge.cfg')
+
+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 {}'.format(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
+)