Index: libcxx/cmake/caches/Armv7M-picolibc.cmake
===================================================================
--- /dev/null
+++ 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 "")
Index: libcxx/docs/index.rst
===================================================================
--- libcxx/docs/index.rst
+++ libcxx/docs/index.rst
@@ -124,16 +124,17 @@
 
 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
Index: libcxx/test/configs/armv7m-libc++.cfg.in
===================================================================
--- /dev/null
+++ libcxx/test/configs/armv7m-libc++.cfg.in
@@ -0,0 +1,41 @@
+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
+))
+
+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
+)
Index: libcxx/test/libcxx/clang_modules_include.gen.py
===================================================================
--- libcxx/test/libcxx/clang_modules_include.gen.py
+++ libcxx/test/libcxx/clang_modules_include.gen.py
@@ -39,6 +39,9 @@
 // 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}>
Index: libcxx/test/libcxx/selftest/dsl/dsl.sh.py
===================================================================
--- libcxx/test/libcxx/selftest/dsl/dsl.sh.py
+++ libcxx/test/libcxx/selftest/dsl/dsl.sh.py
@@ -6,6 +6,9 @@
 #
 # ===----------------------------------------------------------------------===##
 
+# Picolibc test executor is `true` at present.
+# XFAIL: LIBCXX-PICOLIBC-FIXME
+
 # Note: We prepend arguments with 'x' to avoid thinking there are too few
 #       arguments in case an argument is an empty string.
 # RUN: %{python} %s x%S x%T x%{substitutions}
Index: libcxx/test/libcxx/selftest/pass.cpp/run-error.pass.cpp
===================================================================
--- libcxx/test/libcxx/selftest/pass.cpp/run-error.pass.cpp
+++ libcxx/test/libcxx/selftest/pass.cpp/run-error.pass.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
 // XFAIL: *
 
 // Make sure the test DOES NOT pass if it fails at runtime.
Index: libcxx/test/libcxx/selftest/pass.mm/run-error.pass.mm
===================================================================
--- libcxx/test/libcxx/selftest/pass.mm/run-error.pass.mm
+++ libcxx/test/libcxx/selftest/pass.mm/run-error.pass.mm
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: objective-c++
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
 
 // XFAIL: *
 
Index: libcxx/test/libcxx/system_reserved_names.gen.py
===================================================================
--- libcxx/test/libcxx/system_reserved_names.gen.py
+++ libcxx/test/libcxx/system_reserved_names.gen.py
@@ -101,7 +101,10 @@
 # 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
Index: libcxx/test/std/atomics/atomics.types.generic/integral.pass.cpp
===================================================================
--- libcxx/test/std/atomics/atomics.types.generic/integral.pass.cpp
+++ libcxx/test/std/atomics/atomics.types.generic/integral.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: !has-64-bit-atomics
+
 // <atomic>
 
 // template <>
Index: libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add.pass.cpp
===================================================================
--- libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add.pass.cpp
+++ 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>
Index: libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add_explicit.pass.cpp
===================================================================
--- libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add_explicit.pass.cpp
+++ 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>
Index: libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_and.pass.cpp
===================================================================
--- libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_and.pass.cpp
+++ 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>
Index: libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_and_explicit.pass.cpp
===================================================================
--- libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_and_explicit.pass.cpp
+++ 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>
Index: libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_or.pass.cpp
===================================================================
--- libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_or.pass.cpp
+++ 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>
Index: libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_or_explicit.pass.cpp
===================================================================
--- libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_or_explicit.pass.cpp
+++ 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>
Index: libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub.pass.cpp
===================================================================
--- libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub.pass.cpp
+++ 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>
Index: libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub_explicit.pass.cpp
===================================================================
--- libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub_explicit.pass.cpp
+++ 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>
Index: libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_xor.pass.cpp
===================================================================
--- libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_xor.pass.cpp
+++ 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>
Index: libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_xor_explicit.pass.cpp
===================================================================
--- libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_xor_explicit.pass.cpp
+++ 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>
Index: libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/ctor.pass.cpp
===================================================================
--- libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/ctor.pass.cpp
+++ 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)
Index: libcxx/test/std/depr/depr.c.headers/fenv_h.compile.pass.cpp
===================================================================
--- libcxx/test/std/depr/depr.c.headers/fenv_h.compile.pass.cpp
+++ 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>
Index: libcxx/test/std/depr/depr.c.headers/stdio_h.compile.pass.cpp
===================================================================
--- libcxx/test/std/depr/depr.c.headers/stdio_h.compile.pass.cpp
+++ libcxx/test/std/depr/depr.c.headers/stdio_h.compile.pass.cpp
@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+// stderr et al are not macros in picolibc's tinystdio.
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // test <stdio.h>
 
 #include <stdio.h>
Index: libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp
===================================================================
--- libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp
+++ 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>
Index: libcxx/test/std/experimental/simd/simd.reference/reference_assignment.pass.cpp
===================================================================
--- libcxx/test/std/experimental/simd/simd.reference/reference_assignment.pass.cpp
+++ libcxx/test/std/experimental/simd/simd.reference/reference_assignment.pass.cpp
@@ -11,6 +11,9 @@
 // FIXME: Timeouts.
 // UNSUPPORTED: sanitizer-new-delete
 
+// The machine emulated in tests does not have enough memory for code.
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
+
 // <experimental/simd>
 //
 // [simd.reference]
Index: libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/cerr.sh.cpp
===================================================================
--- libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/cerr.sh.cpp
+++ libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/cerr.sh.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
+
 // <iostream>
 
 // ostream cerr;
Index: libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/clog.sh.cpp
===================================================================
--- libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/clog.sh.cpp
+++ libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/clog.sh.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
+
 // <iostream>
 
 // ostream clog;
Index: libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/cout.sh.cpp
===================================================================
--- libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/cout.sh.cpp
+++ libcxx/test/std/input.output/iostream.objects/narrow.stream.objects/cout.sh.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
+
 // <iostream>
 
 // ostream cout;
Index: libcxx/test/std/iterators/iterator.primitives/iterator.traits/cxx20_iterator_traits.compile.pass.cpp
===================================================================
--- libcxx/test/std/iterators/iterator.primitives/iterator.traits/cxx20_iterator_traits.compile.pass.cpp
+++ 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;
 
Index: libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/sized_delete_array14.pass.cpp
===================================================================
--- libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/sized_delete_array14.pass.cpp
+++ 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.
Index: libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/sized_delete14.pass.cpp
===================================================================
--- libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/sized_delete14.pass.cpp
+++ 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.
Index: libcxx/test/std/language.support/support.runtime/ctime.timespec.compile.pass.cpp
===================================================================
--- libcxx/test/std/language.support/support.runtime/ctime.timespec.compile.pass.cpp
+++ libcxx/test/std/language.support/support.runtime/ctime.timespec.compile.pass.cpp
@@ -13,6 +13,9 @@
 
 // XFAIL: LIBCXX-AIX-FIXME
 
+// picolibc doesn't define TIME_UTC.
+// XFAIL: LIBCXX-PICOLIBC-FIXME
+
 // ::timespec_get is provided by the C library, but it's marked as
 // unavailable until macOS 10.15
 // XFAIL: stdlib=apple-libc++ && target={{.+}}-apple-macosx10.{{9|10|11|12|13|14}}
Index: libcxx/test/std/numerics/cfenv/cfenv.syn/cfenv.pass.cpp
===================================================================
--- libcxx/test/std/numerics/cfenv/cfenv.syn/cfenv.pass.cpp
+++ 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>
Index: libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp
===================================================================
--- libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp
+++ 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>
Index: libcxx/test/std/time/time.clock/time.clock.hires/now.pass.cpp
===================================================================
--- libcxx/test/std/time/time.clock/time.clock.hires/now.pass.cpp
+++ 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
Index: libcxx/test/std/time/time.clock/time.clock.file/to_from_sys.pass.cpp
===================================================================
--- libcxx/test/std/time/time.clock/time.clock.file/to_from_sys.pass.cpp
+++ 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
Index: libcxx/test/std/time/time.clock/time.clock.system/from_time_t.pass.cpp
===================================================================
--- libcxx/test/std/time/time.clock/time.clock.system/from_time_t.pass.cpp
+++ 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
Index: libcxx/test/std/time/time.clock/time.clock.system/now.pass.cpp
===================================================================
--- libcxx/test/std/time/time.clock/time.clock.system/now.pass.cpp
+++ 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
Index: libcxx/test/std/time/time.clock/time.clock.system/to_time_t.pass.cpp
===================================================================
--- libcxx/test/std/time/time.clock/time.clock.system/to_time_t.pass.cpp
+++ 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
Index: libcxx/test/std/time/time.point/time.point.nonmember/op_-duration.pass.cpp
===================================================================
--- libcxx/test/std/time/time.point/time.point.nonmember/op_-duration.pass.cpp
+++ 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
Index: libcxx/utils/ci/BOT_OWNERS.txt
===================================================================
--- libcxx/utils/ci/BOT_OWNERS.txt
+++ libcxx/utils/ci/BOT_OWNERS.txt
@@ -10,7 +10,7 @@
 
 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
Index: libcxx/utils/ci/build-picolibc.sh
===================================================================
--- /dev/null
+++ libcxx/utils/ci/build-picolibc.sh
@@ -0,0 +1,109 @@
+#!/usr/bin/env bash
+#===----------------------------------------------------------------------===##
+#
+# Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+# See https://llvm.org/LICENSE.txt for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+#
+#===----------------------------------------------------------------------===##
+
+#
+# This script builds picolibc (https://github.com/picolibc/picolibc) from
+# source to facilitate building libc++ against it.
+#
+
+set -e
+
+PROGNAME="$(basename "${0}")"
+
+function error() { printf "error: %s\n" "$*" >&2; exit 1; }
+
+function usage() {
+cat <<EOF
+Usage:
+${PROGNAME} [options]
+
+[-h|--help]                  Display this help and exit.
+
+--build-dir <DIR>            Path to the directory to use for building.
+
+--install-dir <DIR>          Path to the directory to install the library to.
+EOF
+}
+
+while [[ $# -gt 0 ]]; do
+    case ${1} in
+        -h|--help)
+            usage
+            exit 0
+            ;;
+        --build-dir)
+            build_dir="${2}"
+            shift; shift
+            ;;
+        --install-dir)
+            install_dir="${2}"
+            shift; shift
+            ;;
+        --target)
+            target="${2}"
+            shift; shift
+            ;;
+        *)
+            error "Unknown argument '${1}'"
+            ;;
+    esac
+done
+
+for arg in build_dir install_dir target; do
+    if [ -z ${!arg+x} ]; then
+        error "Missing required argument '--${arg//_/-}'"
+    elif [ "${!arg}" == "" ]; then
+        error "Argument to --${arg//_/-} must not be empty"
+    fi
+done
+
+
+echo "--- Downloading picolibc"
+picolibc_source_dir="${build_dir}/picolibc-source"
+picolibc_build_dir="${build_dir}/picolibc-build"
+mkdir -p "${picolibc_source_dir}"
+mkdir -p "${picolibc_build_dir}"
+# Download the version of picolibc that was the latest at the time this script was written.
+# The picolibc linker script was changed after version 1.8.2 to work with ld.lld so a
+# more recent version is required, which at the time of writing is not yet released.
+picolibc_commit="8dd225aa1469c03805617106020d494912c6d265"
+curl -L "https://github.com/picolibc/picolibc/archive/${picolibc_commit}.zip" --output "${picolibc_source_dir}/picolibc.zip"
+unzip -q "${picolibc_source_dir}/picolibc.zip" -d "${picolibc_source_dir}"
+mv "${picolibc_source_dir}/picolibc-${picolibc_commit}"/* "${picolibc_source_dir}"
+rm -rf "${picolibc_source_dir}/picolibc-${picolibc_commit}"
+
+cat <<EOF > "${picolibc_build_dir}/meson-cross-build.txt"
+[binaries]
+c = ['${CC:-cc}', '--target=${target}', '-mfloat-abi=soft', '-nostdlib']
+ar = 'llvm-ar'
+as = 'llvm-as'
+ld = 'lld'
+strip = 'llvm-strip'
+[host_machine]
+system = 'none'
+cpu_family = 'arm'
+cpu = 'arm'
+endian = 'little'
+[properties]
+skip_sanity_check = true
+EOF
+
+venv_dir="${build_dir}/meson-venv"
+python3 -m venv "${venv_dir}"
+# Install the version of meson that was the latest at the time this script was written.
+"${venv_dir}/bin/pip" install "meson==1.1.1"
+
+"${venv_dir}/bin/meson" setup \
+  -Dincludedir=include -Dlibdir=lib -Dspecsdir=none -Dmultilib=false -Dpicoexit=false \
+  --prefix "${install_dir}" \
+  --cross-file "${picolibc_build_dir}/meson-cross-build.txt" \
+  "${picolibc_build_dir}" \
+  "${picolibc_source_dir}"
+
+"${venv_dir}/bin/meson" install -C "${picolibc_build_dir}"
Index: libcxx/utils/ci/buildkite-pipeline.yml
===================================================================
--- libcxx/utils/ci/buildkite-pipeline.yml
+++ libcxx/utils/ci/buildkite-pipeline.yml
@@ -1038,6 +1038,20 @@
             limit: 2
       timeout_in_minutes: 120
 
+    - label: "Armv7-M picolibc"
+      command: "libcxx/utils/ci/run-buildbot armv7m-picolibc"
+      artifact_paths:
+        - "**/test-results.xml"
+        - "**/*.abilist"
+      agents:
+        queue: "libcxx-builders-linaro-arm"
+        arch: "aarch64"
+      retry:
+        automatic:
+          - exit_status: -1  # Agent was lost
+            limit: 2
+      timeout_in_minutes: 120
+
   - group: "AIX"
     steps:
     - label: "AIX (32-bit)"
Index: libcxx/utils/ci/run-buildbot
===================================================================
--- libcxx/utils/ci/run-buildbot
+++ libcxx/utils/ci/run-buildbot
@@ -621,6 +621,41 @@
     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
Index: libcxx/utils/libcxx/test/features.py
===================================================================
--- libcxx/utils/libcxx/test/features.py
+++ libcxx/utils/libcxx/test/features.py
@@ -110,6 +110,17 @@
         name="verify-support",
         when=lambda cfg: hasCompileFlag(cfg, "-Xclang -verify-ignore-unexpected"),
     ),
+    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; }
+          """,
+        ),
+    ),
     Feature(
         name="non-lockfree-atomics",
         when=lambda cfg: sourceBuilds(
@@ -215,7 +226,8 @@
             #include <unistd.h>
             #include <sys/wait.h>
             int main(int, char**) {
-              return 0;
+              int fd[2];
+              return pipe(fd);
             }
           """,
         ),
@@ -418,6 +430,19 @@
         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.
Index: libcxxabi/test/configs/armv7m-libc++abi.cfg.in
===================================================================
--- /dev/null
+++ libcxxabi/test/configs/armv7m-libc++abi.cfg.in
@@ -0,0 +1,39 @@
+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
+))
+
+# LIBCXX-PICOLIBC-FIXME is the feature name used to XFAIL the
+# initial picolibc failures until they can be properly diagnosed
+# and fixed. This allows easier detection of new test failures
+# and regressions. Note: New failures should not be suppressed
+# using this feature.
+config.available_features.add('LIBCXX-PICOLIBC-FIXME')
+
+import os, site
+site.addsitedir(os.path.join('@LIBCXXABI_LIBCXX_PATH@', 'utils'))
+import libcxx.test.params, libcxx.test.config
+libcxx.test.config.configure(
+    libcxx.test.params.DEFAULT_PARAMETERS,
+    libcxx.test.features.DEFAULT_FEATURES,
+    config,
+    lit_config
+)
Index: libcxxabi/test/test_demangle.pass.cpp
===================================================================
--- libcxxabi/test/test_demangle.pass.cpp
+++ 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}}
Index: libunwind/test/configs/armv7m-libunwind.cfg.in
===================================================================
--- /dev/null
+++ libunwind/test/configs/armv7m-libunwind.cfg.in
@@ -0,0 +1,32 @@
+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
+))
+
+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
+)