diff --git a/CMakeLists.txt b/CMakeLists.txt
index 73cd4246..dd5c96c0 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -140,7 +140,7 @@ set(LLVM_TOOLCHAIN_C_LIBRARY
     "Which C library to use."
 )
 set_property(CACHE LLVM_TOOLCHAIN_C_LIBRARY
-    PROPERTY STRINGS picolibc newlib)
+    PROPERTY STRINGS picolibc newlib llvmlibc)
 
 # Previously, the LLVM_TOOLCHAIN_LIBRARY_OVERLAY_INSTALL option was
 # called LLVM_TOOLCHAIN_NEWLIB_OVERLAY_INSTALL. Detect a setting of
@@ -153,7 +153,7 @@ else()
   set(overlay_install_default OFF)
 endif()
 option(LLVM_TOOLCHAIN_LIBRARY_OVERLAY_INSTALL
-    "Make cpack build an overlay package that can be unpacked over the main toolchain to install a secondary set of libraries based on newlib."
+    "Make cpack build an overlay package that can be unpacked over the main toolchain to install a secondary set of libraries based on newlib or llvm-libc."
     ${overlay_install_default})
 if(LLVM_TOOLCHAIN_LIBRARY_OVERLAY_INSTALL)
   if(LLVM_TOOLCHAIN_C_LIBRARY STREQUAL "picolibc")
@@ -268,7 +268,9 @@ function(read_repo_version output_variable_prefix repo)
     set(${output_variable_prefix}_SHALLOW "${shallow}" PARENT_SCOPE)
 endfunction()
 read_repo_version(llvmproject llvm-project)
-read_repo_version(${LLVM_TOOLCHAIN_C_LIBRARY} ${LLVM_TOOLCHAIN_C_LIBRARY})
+if(NOT (LLVM_TOOLCHAIN_C_LIBRARY STREQUAL llvmlibc)) # libc in a separate repo?
+  read_repo_version(${LLVM_TOOLCHAIN_C_LIBRARY} ${LLVM_TOOLCHAIN_C_LIBRARY})
+endif()
 
 # The patches are generated from custom branch, with followin command:
 #   git format-patch -k origin/main
@@ -319,7 +321,9 @@ FetchContent_Declare(newlib
 )
 
 FetchContent_MakeAvailable(llvmproject)
-FetchContent_MakeAvailable(${LLVM_TOOLCHAIN_C_LIBRARY})
+if(NOT (LLVM_TOOLCHAIN_C_LIBRARY STREQUAL llvmlibc)) # libc in a separate repo?
+  FetchContent_MakeAvailable(${LLVM_TOOLCHAIN_C_LIBRARY})
+endif()
 
 # We generally want to install to a local directory to see what the
 # output will look like rather than install into the system, so change
@@ -334,6 +338,10 @@ if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
     )
 endif()
 
+# Whether to try to build C++ libraries. (We can't currently do this
+# for all choices of C library.)
+set(CXX_LIBS ON)
+
 if(LLVM_TOOLCHAIN_C_LIBRARY STREQUAL newlib)
     install(
         FILES
@@ -350,6 +358,42 @@ install(
     COMPONENT llvm-toolchain-omax-cfg
 )
 
+if(LLVM_TOOLCHAIN_C_LIBRARY STREQUAL llvmlibc)
+    install(
+        FILES
+        ${CMAKE_CURRENT_SOURCE_DIR}/llvmlibc.cfg
+        DESTINATION bin
+        COMPONENT llvm-toolchain-llvmlibc-configs
+    )
+
+    # We aren't yet able to build C++ libraries to go with llvm-libc
+    set(CXX_LIBS OFF)
+
+    # We need to build libc-hdrgen
+    ExternalProject_Add(
+        libc_hdrgen
+        SOURCE_DIR ${llvmproject_SOURCE_DIR}/llvm
+        DEPENDS ${lib_tool_dependencies}
+        CMAKE_ARGS
+            -DLLVM_ENABLE_RUNTIMES=libc
+            -DLLVM_LIBC_FULL_BUILD=ON
+            -DCMAKE_BUILD_TYPE=Debug
+        STEP_TARGETS build install
+        BUILD_COMMAND ${CMAKE_COMMAND} --build . --target libc-hdrgen
+        INSTALL_COMMAND ${CMAKE_COMMAND} -E true
+        # Always run the build command so that incremental builds are correct.
+        BUILD_ALWAYS TRUE
+        CONFIGURE_HANDLED_BY_BUILD TRUE
+    )
+    ExternalProject_Get_property(libc_hdrgen BINARY_DIR)
+    set(LIBC_HDRGEN ${BINARY_DIR}/bin/libc-hdrgen${CMAKE_EXECUTABLE_SUFFIX})
+
+    # Add an empty check target, to simplify the logic below that expects to
+    # find one for every libc type. We have no current setup to run the LLVM
+    # libc test suite.
+    add_custom_target(check-llvmlibc)
+endif()
+
 add_subdirectory(
     ${llvmproject_SOURCE_DIR}/llvm llvm
 )
@@ -396,7 +440,7 @@ set(CPACK_ARCHIVE_COMPONENT_INSTALL TRUE)
 # Don't create a separate archive for each component.
 set(CPACK_COMPONENTS_GROUPING ALL_COMPONENTS_IN_ONE)
 # When extracting the files put them in an ArmCompiler-.../ directory.
-# Exception: the newlib overlay package does not do this, because it has
+# Exception: the overlay packages do not do this, because they have
 # to be able to unpack over the top of an existing installation on all
 # platforms, and each platform has a different top-level directory name.
 if(LLVM_TOOLCHAIN_LIBRARY_OVERLAY_INSTALL)
@@ -479,7 +523,7 @@ add_custom_target(
     -DLLVMEmbeddedToolchainForArm_VERSION=${LLVMEmbeddedToolchainForArm_VERSION}
     -DLLVMEmbeddedToolchainForArm_SOURCE_DIR=${CMAKE_CURRENT_SOURCE_DIR}
     -Dllvmproject_SOURCE_DIR=${llvmproject_SOURCE_DIR}
-    # only one of picolibc and newlib source dirs is needed, but easiest to
+    # at most one of picolibc and newlib source dirs is needed, but easiest to
     # specify both definitions
     -Dpicolibc_SOURCE_DIR=${picolibc_SOURCE_DIR}
     -Dnewlib_SOURCE_DIR=${newlib_SOURCE_DIR}
@@ -817,6 +861,112 @@ function(
     )
 endfunction()
 
+function(
+    add_llvmlibc
+    directory
+    variant
+    target_triple
+    flags
+    test_executor_params
+    default_boot_flash_addr
+    default_boot_flash_size
+    default_flash_addr
+    default_flash_size
+    default_ram_addr
+    default_ram_size
+    default_stack_size
+)
+    get_runtimes_flags("${directory}" "${flags}")
+
+    set(runtimes_flags "${runtimes_flags} -Wno-error=atomic-alignment")
+
+    set(common_cmake_args
+        -DCMAKE_AR=${LLVM_BINARY_DIR}/bin/llvm-ar${CMAKE_EXECUTABLE_SUFFIX}
+        -DCMAKE_ASM_COMPILER=${LLVM_BINARY_DIR}/bin/clang${CMAKE_EXECUTABLE_SUFFIX}
+        -DCMAKE_ASM_COMPILER_TARGET=${target_triple}
+        -DCMAKE_ASM_FLAGS=${runtimes_flags}
+        -DCMAKE_BUILD_TYPE=Release
+        -DCMAKE_CXX_COMPILER=${LLVM_BINARY_DIR}/bin/clang++${CMAKE_EXECUTABLE_SUFFIX}
+        -DCMAKE_CXX_COMPILER_TARGET=${target_triple}
+        -DCMAKE_CXX_FLAGS=${runtimes_flags}
+        -DCMAKE_C_COMPILER=${LLVM_BINARY_DIR}/bin/clang${CMAKE_EXECUTABLE_SUFFIX}
+        -DCMAKE_C_COMPILER_TARGET=${target_triple}
+        -DCMAKE_C_FLAGS=${runtimes_flags}
+        -DCMAKE_INSTALL_MESSAGE=${CMAKE_INSTALL_MESSAGE}
+        -DCMAKE_INSTALL_PREFIX=<INSTALL_DIR>
+        -DCMAKE_NM=${LLVM_BINARY_DIR}/bin/llvm-nm${CMAKE_EXECUTABLE_SUFFIX}
+        -DCMAKE_RANLIB=${LLVM_BINARY_DIR}/bin/llvm-ranlib${CMAKE_EXECUTABLE_SUFFIX}
+        # Let CMake know we're cross-compiling
+        -DCMAKE_SYSTEM_NAME=Generic
+        -DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY
+    )
+
+    ExternalProject_Add(
+        llvmlibc_${variant}
+        SOURCE_DIR ${llvmproject_SOURCE_DIR}/runtimes
+        PREFIX llvmlibc/${variant}
+        INSTALL_DIR llvmlibc/${variant}/install
+        DEPENDS ${lib_tool_dependencies} ${libc_target} libc_hdrgen
+        CMAKE_ARGS
+        ${common_cmake_args}
+        -DLIBC_TARGET_TRIPLE=${target_triple}
+        -DLIBC_HDRGEN_EXE=${LIBC_HDRGEN}
+        -DLIBC_TARGET_OS=baremetal
+        -DLLVM_CMAKE_DIR=${LLVM_BINARY_DIR}/lib/cmake/llvm
+        -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON
+        -DLLVM_ENABLE_RUNTIMES=libc
+        -DLLVM_INCLUDE_TESTS=OFF # I haven't yet got the tests to build
+        -DLLVM_LIBC_FULL_BUILD=ON
+        STEP_TARGETS build install
+        USES_TERMINAL_CONFIGURE FALSE
+        USES_TERMINAL_BUILD TRUE
+        USES_TERMINAL_INSTALL TRUE
+        USES_TERMINAL_TEST TRUE
+        LIST_SEPARATOR ,
+        # Always run the build command so that incremental builds are correct.
+        BUILD_ALWAYS TRUE
+        CONFIGURE_HANDLED_BY_BUILD TRUE
+        INSTALL_COMMAND ${CMAKE_COMMAND} --install .
+        # Copy llvm-libc lib directory, moving libraries out of their
+        # target-specific subdirectory.
+        COMMAND
+            ${CMAKE_COMMAND}
+            -E copy_directory
+            <INSTALL_DIR>/lib/${target_triple}
+            "${LLVM_BINARY_DIR}/${directory}/lib"
+        # And copy the include directory, which is already arranged right.
+        COMMAND
+            ${CMAKE_COMMAND}
+            -E copy_directory
+            <INSTALL_DIR>/include
+            "${LLVM_BINARY_DIR}/${directory}/include"
+    )
+
+    ExternalProject_Add(
+        llvmlibc-support_${variant}
+        SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/llvmlibc-support
+        PREFIX llvmlibc-support/${variant}
+        INSTALL_DIR "${LLVM_BINARY_DIR}/${directory}"
+        DEPENDS ${lib_tool_dependencies} llvmlibc_${variant}-install
+        CMAKE_ARGS ${common_cmake_args}
+        STEP_TARGETS build install
+        USES_TERMINAL_CONFIGURE FALSE
+        USES_TERMINAL_BUILD TRUE
+        USES_TERMINAL_INSTALL TRUE
+        USES_TERMINAL_TEST TRUE
+        LIST_SEPARATOR ,
+        # Always run the build command so that incremental builds are correct.
+        BUILD_ALWAYS TRUE
+        CONFIGURE_HANDLED_BY_BUILD TRUE
+    )
+
+    add_dependencies(
+        llvm-toolchain-runtimes
+        llvmlibc_${variant}
+        llvmlibc-support_${variant}
+    )
+endfunction()
+
 macro(
     add_libc
     directory
@@ -869,6 +1019,21 @@ macro(
             "${default_ram_size}"
             "${default_stack_size}"
         )
+    elseif(LLVM_TOOLCHAIN_C_LIBRARY STREQUAL llvmlibc)
+        add_llvmlibc(
+            "${directory}"
+            "${variant}"
+            "${target_triple}"
+            "${flags}"
+            "${test_executor_params}"
+            "${default_boot_flash_addr}"
+            "${default_boot_flash_size}"
+            "${default_flash_addr}"
+            "${default_flash_size}"
+            "${default_ram_addr}"
+            "${default_ram_size}"
+            "${default_stack_size}"
+        )
     endif()
 endmacro()
 
@@ -1231,25 +1396,29 @@ function(add_library_variant target_arch)
             "${VARIANT_COMPILE_FLAGS}"
             "${lit_test_executor}"
             "${LLVM_TOOLCHAIN_C_LIBRARY}_${variant}-install"
-        )
-        add_libcxx_libcxxabi_libunwind(
-            "${directory}"
-            "${variant}"
-            "${target_triple}"
-            "${VARIANT_COMPILE_FLAGS}"
-            "${lit_test_executor}"
-            "${LLVM_TOOLCHAIN_C_LIBRARY}_${variant}-install"
-            "${${LLVM_TOOLCHAIN_C_LIBRARY}_specific_runtimes_options}"
-            ${VARIANT_ENABLE_EXCEPTIONS}
-            ${VARIANT_ENABLE_RTTI}
-        )
+            )
+        if(CXX_LIBS)
+            add_libcxx_libcxxabi_libunwind(
+                "${directory}"
+                "${variant}"
+                "${target_triple}"
+                "${VARIANT_COMPILE_FLAGS}"
+                "${lit_test_executor}"
+                "${LLVM_TOOLCHAIN_C_LIBRARY}_${variant}-install"
+                "${${LLVM_TOOLCHAIN_C_LIBRARY}_specific_runtimes_options}"
+                ${VARIANT_ENABLE_EXCEPTIONS}
+                ${VARIANT_ENABLE_RTTI}
+            )
+        endif()
         if(VARIANT_COMPILE_FLAGS MATCHES "-march=armv8")
             message("C++ runtime libraries tests disabled for ${variant}")
         else()
             add_custom_target(check-llvm-toolchain-runtimes-${variant})
             add_dependencies(check-llvm-toolchain-runtimes check-llvm-toolchain-runtimes-${variant})
             add_compiler_rt_tests("${variant}")
-            add_libcxx_libcxxabi_libunwind_tests("${variant}")
+            if(CXX_LIBS)
+                add_libcxx_libcxxabi_libunwind_tests("${variant}")
+            endif()
         endif()
     endif()
 
@@ -1333,21 +1502,25 @@ set(multilib_yaml_content "")
 # For most variants, the "flash" memory is placed in address range, where
 # simulated boards have RAM. This is because code for some tests does not fit
 # the real flash.
-add_library_variants_for_cpu(
-    aarch64
-    COMPILE_FLAGS "-march=armv8-a"
-    MULTILIB_FLAGS "--target=aarch64-unknown-none-elf"
-    PICOLIBC_BUILD_TYPE "release"
-    QEMU_MACHINE "virt"
-    QEMU_CPU "cortex-a57"
-    BOOT_FLASH_ADDRESS 0x40000000
-    BOOT_FLASH_SIZE 0x1000
-    FLASH_ADDRESS 0x40001000
-    FLASH_SIZE 0xfff000
-    RAM_ADDRESS 0x41000000
-    RAM_SIZE 0x1000000
-    STACK_SIZE 8K
-)
+if(NOT (LLVM_TOOLCHAIN_C_LIBRARY STREQUAL llvmlibc))
+    # llvm-libc doesn't have a bare-metal configuration for AArch64, so we
+    # leave out the AArch64 library if we're building that libc.
+    add_library_variants_for_cpu(
+        aarch64
+        COMPILE_FLAGS "-march=armv8-a"
+        MULTILIB_FLAGS "--target=aarch64-unknown-none-elf"
+        PICOLIBC_BUILD_TYPE "release"
+        QEMU_MACHINE "virt"
+        QEMU_CPU "cortex-a57"
+        BOOT_FLASH_ADDRESS 0x40000000
+        BOOT_FLASH_SIZE 0x1000
+        FLASH_ADDRESS 0x40001000
+        FLASH_SIZE 0xfff000
+        RAM_ADDRESS 0x41000000
+        RAM_SIZE 0x1000000
+        STACK_SIZE 8K
+    )
+endif()
 # For AArch32, clang uses different defaults for FPU selection than GCC, both
 # when "+fp" or "+fp.dp" are used and when no FPU specifier is provided in
 # "-march=". Using "-mfpu=" explicitly.
diff --git a/README.md b/README.md
index 802085fc..0bdb97b8 100644
--- a/README.md
+++ b/README.md
@@ -7,7 +7,7 @@ bare-metal LLVM based toolchain targeting Arm based on:
 * libc++abi
 * libc++
 * compiler-rt
-* picolibc, or optionally newlib
+* picolibc, or optionally newlib or LLVM's libc
 
 ## Goal
 
diff --git a/cmake/generate_version_txt.cmake b/cmake/generate_version_txt.cmake
index b36460cb..a3f70327 100644
--- a/cmake/generate_version_txt.cmake
+++ b/cmake/generate_version_txt.cmake
@@ -23,12 +23,14 @@ execute_process(
     OUTPUT_STRIP_TRAILING_WHITESPACE
     COMMAND_ERROR_IS_FATAL ANY
 )
-execute_process(
-    COMMAND git -C ${${LLVM_TOOLCHAIN_C_LIBRARY}_SOURCE_DIR} rev-parse HEAD
-    OUTPUT_VARIABLE ${LLVM_TOOLCHAIN_C_LIBRARY}_COMMIT
-    OUTPUT_STRIP_TRAILING_WHITESPACE
-    COMMAND_ERROR_IS_FATAL ANY
-)
+if(NOT (LLVM_TOOLCHAIN_C_LIBRARY STREQUAL llvmlibc)) # libc in a separate repo?
+    execute_process(
+        COMMAND git -C ${${LLVM_TOOLCHAIN_C_LIBRARY}_SOURCE_DIR} rev-parse HEAD
+        OUTPUT_VARIABLE ${LLVM_TOOLCHAIN_C_LIBRARY}_COMMIT
+        OUTPUT_STRIP_TRAILING_WHITESPACE 
+        COMMAND_ERROR_IS_FATAL ANY
+    )
+endif()
 
 configure_file(
     ${CMAKE_CURRENT_LIST_DIR}/VERSION.txt.in
diff --git a/docs/llvmlibc.md b/docs/llvmlibc.md
new file mode 100644
index 00000000..b9cc6fc8
--- /dev/null
+++ b/docs/llvmlibc.md
@@ -0,0 +1,66 @@
+# Experimental LLVM libc support
+
+LLVM Embedded Toolchain for Arm uses
+[`picolibc`](https://github.com/picolibc/picolibc) as the standard C
+library. For experimental and evaluation purposes, you can instead
+choose to use the LLVM project's own C library.
+
+> **NOTE:** `llvmlibc` support in LLVM Embedded Toolchain for Arm is
+> an experimental technology preview, with significant limitations.
+
+## Building the toolchain with LLVM libc
+
+> **NOTE:** Building the LLVM libc package is only supported on Linux
+> and macOS.
+
+Configure the toolchain with the CMake setting
+`-DLLVM_TOOLCHAIN_C_LIBRARY=llvmlibc` to build a version of the
+toolchain based on LLVM libc.
+
+If you also add `-DLLVM_TOOLCHAIN_LIBRARY_OVERLAY_INSTALL=on` then the
+`package-llvm-toolchain` CMake target will generate an overlay package
+similar to the [newlib overlay
+package](https://github.com/ARM-software/LLVM-embedded-toolchain-for-Arm/blob/main/docs/newlib.md).
+If you unpack this over an existing installation of the toolchain,
+then you can switch to LLVM libc by adding `--config=llvmlibc.cfg` on
+the command line.
+
+## Using LLVM libc
+
+To compile a program with this LLVM libc, you must provide the
+following command line options, in addition to `--target`, `-march` or
+`-mcpu`, and the input and output files:
+
+* `--config=llvmlibc.cfg` if you are using LLVM libc as an overlay
+  package (but you do not need this if you have built the whole
+  toolchain with only LLVM libc)
+
+* `-lcrt0` to include a library defining the `_start` symbol (or else
+  provide that symbol yourself)
+
+* `-lsemihost` to include a library that implements porting functions
+  in LLVM's libc in terms of the Arm semihosting API (or else provide
+  an alternative implementation of those functions yourself)
+
+* `-Wl,--defsym=__stack=0x`_nnnnnn_ to define the starting value of
+  your stack pointer. Alternatively, use a linker script that defines
+  the symbol `__stack` in addition to whatever other memory layout you
+  want.
+
+For example:
+
+```
+clang --config=llvmlibc.cfg --target=arm-none-eabi -march=armv7m -o hello hello.c -lsemihost -lcrt0 -Wl,--defsym=__stack=0x200000
+```
+
+## Limitations of LLVM libc in LLVM Embedded Toolchain for Arm
+
+At present, this toolchain only builds LLVM libc for AArch32, not for
+AArch64.
+
+At present, this toolchain does not build any C++ libraries to go with
+LLVM libc.
+
+At the time of writing this (2024-07), LLVM libc is a work in
+progress. It is incomplete: not all standard C library functionality
+is provided.
diff --git a/llvmlibc-support/CMakeLists.txt b/llvmlibc-support/CMakeLists.txt
new file mode 100644
index 00000000..0e61b37f
--- /dev/null
+++ b/llvmlibc-support/CMakeLists.txt
@@ -0,0 +1,46 @@
+#
+# Copyright (c) 2022, Arm Limited and affiliates.
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+# This directory builds two additional library files to go with llvm-libc:
+#
+# libcrt0.a contains an implementation of the _start() default entry point,
+# which sets up the stack and does any necessary initialization before
+# calling main().
+#
+# libsemihost.a implements llvm-libc's porting function API such as
+# __llvm_libc_stdio_write, in terms of the Arm semihosting system.
+#
+# To use LLVM libc in a semihosting context, include both of these
+# libraries. To use it in a different context, you will need to
+# reimplement the same functions that libsemihost.a provides, but
+# libcrt0.a might still be useful.
+
+cmake_minimum_required(VERSION 3.20.0)
+project(llvmlibc-support LANGUAGES C ASM)
+
+add_library(semihost STATIC
+  init.c
+  exit.c
+  stdio_read.c
+  stdio_write.c
+)
+
+add_library(crt0 STATIC
+  crt0.c
+)
+
+install(TARGETS semihost crt0)
diff --git a/llvmlibc-support/crt0.c b/llvmlibc-support/crt0.c
new file mode 100644
index 00000000..b2f11693
--- /dev/null
+++ b/llvmlibc-support/crt0.c
@@ -0,0 +1,34 @@
+//
+// Copyright (c) 2022, Arm Limited and affiliates.
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#include <stddef.h>
+#include <stdlib.h>
+
+#include "platform.h"
+
+int main(int, char **);
+
+__attribute__((used)) static void c_startup(void) {
+  _platform_init();
+  _Exit(main(0, NULL));
+}
+
+extern long __stack[];
+__attribute__((naked)) void _start(void) {
+  __asm__("mov sp, %0" : : "r"(__stack));
+  __asm__("b c_startup");
+}
diff --git a/llvmlibc-support/exit.c b/llvmlibc-support/exit.c
new file mode 100644
index 00000000..d784a597
--- /dev/null
+++ b/llvmlibc-support/exit.c
@@ -0,0 +1,32 @@
+//
+// Copyright (c) 2022, Arm Limited and affiliates.
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#include "semihost.h"
+
+void __llvm_libc_exit(int status) {
+
+#if defined(__ARM_64BIT_STATE) && __ARM_64BIT_STATE
+  size_t block[2];
+  block[0] = ADP_Stopped_ApplicationExit;
+  block[1] = code;
+  semihosting_call(SYS_EXIT, block);
+#else
+  semihosting_call(SYS_EXIT, (const void *)ADP_Stopped_ApplicationExit);
+#endif
+
+  __builtin_unreachable(); /* semihosting call doesn't return */
+}
diff --git a/llvmlibc-support/init.c b/llvmlibc-support/init.c
new file mode 100644
index 00000000..bbe8d3c8
--- /dev/null
+++ b/llvmlibc-support/init.c
@@ -0,0 +1,39 @@
+//
+// Copyright (c) 2022, Arm Limited and affiliates.
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#include <stddef.h>
+
+#include "platform.h"
+#include "semihost.h"
+
+struct __llvm_libc_stdio_cookie __llvm_libc_stdin_cookie;
+struct __llvm_libc_stdio_cookie __llvm_libc_stdout_cookie;
+struct __llvm_libc_stdio_cookie __llvm_libc_stderr_cookie;
+
+static void stdio_open(struct __llvm_libc_stdio_cookie *cookie, int mode) {
+  size_t args[3];
+  args[0] = (size_t) ":tt";
+  args[1] = (size_t)mode;
+  args[2] = (size_t)3; /* name length */
+  cookie->handle = semihosting_call(SYS_OPEN, args);
+}
+
+void _platform_init(void) {
+  stdio_open(&__llvm_libc_stdin_cookie, OPENMODE_R);
+  stdio_open(&__llvm_libc_stdout_cookie, OPENMODE_W);
+  stdio_open(&__llvm_libc_stderr_cookie, OPENMODE_W);
+}
diff --git a/llvmlibc-support/platform.h b/llvmlibc-support/platform.h
new file mode 100644
index 00000000..668c3304
--- /dev/null
+++ b/llvmlibc-support/platform.h
@@ -0,0 +1,34 @@
+//
+// Copyright (c) 2022, Arm Limited and affiliates.
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+// This header file defines the interface between libcrt0.a, which defines
+// the program entry point, and libsemihost.a, which implements the
+// LLVM-libc porting functions in terms of semihosting. If you replace
+// libsemihost.a with something else, this header file shows how to make
+// that work with libcrt0.a.
+
+#ifndef LLVMET_LLVMLIBC_SUPPORT_PLATFORM_H
+#define LLVMET_LLVMLIBC_SUPPORT_PLATFORM_H
+
+// libcrt0.a will call this function after the stack pointer is
+// initialized. If any setup specific to the libc porting layer is
+// needed, this is where to do it. For example, in semihosting, the
+// standard I/O handles must be opened via the SYS_OPEN operation, and
+// this function is where libsemihost.a does it.
+void _platform_init(void);
+
+#endif // LLVMET_LLVMLIBC_SUPPORT_PLATFORM_H
diff --git a/llvmlibc-support/semihost.h b/llvmlibc-support/semihost.h
new file mode 100644
index 00000000..71c80949
--- /dev/null
+++ b/llvmlibc-support/semihost.h
@@ -0,0 +1,117 @@
+//
+// Copyright (c) 2022, Arm Limited and affiliates.
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+// This header file provides internal definitions for libsemihost.a,
+// including an inline function to make a semihosting call, and a lot
+// of constant definitions.
+
+#ifndef LLVMET_LLVMLIBC_SUPPORT_SEMIHOST_H
+#define LLVMET_LLVMLIBC_SUPPORT_SEMIHOST_H
+
+#include <llvm-libc-types/ssize_t.h>
+
+#if __ARM_64BIT_STATE
+#  define ARG_REG_0 "x0"
+#  define ARG_REG_1 "x1"
+#else
+#  define ARG_REG_0 "r0"
+#  define ARG_REG_1 "r1"
+#endif
+
+#if __ARM_64BIT_STATE // A64
+#  define SEMIHOST_INSTRUCTION "hlt #0xf000"
+#elif defined(__thumb__) // T32
+#  if defined(__ARM_ARCH_PROFILE) && __ARM_ARCH_PROFILE == 'M'
+#    define SEMIHOST_INSTRUCTION "bkpt #0xAB"
+#  elif defined(HLT_SEMIHOSTING)
+#    define SEMIHOST_INSTRUCTION ".inst.n 0xbabc" // hlt #60
+#  else
+#    define SEMIHOST_INSTRUCTION "svc 0xab"
+#  endif
+#else // A32
+#  if defined(HLT_SEMIHOSTING)
+#    define SEMIHOST_INSTRUCTION ".inst 0xe10f0070" // hlt #0xf000
+#  else
+#    define SEMIHOST_INSTRUCTION "svc 0x123456"
+#  endif
+#endif
+
+__attribute__((always_inline))
+static long semihosting_call(long val, const void *ptr) {
+  register long v __asm__(ARG_REG_0) = val;
+  register const void *p __asm__(ARG_REG_1) = ptr;
+  __asm__ __volatile__(SEMIHOST_INSTRUCTION
+                       : "+r"(v), "+r"(p)
+                       :
+                       : "memory", "cc");
+  return v;
+}
+
+#define SYS_CLOCK 0x10
+#define SYS_CLOSE 0x02
+#define SYS_ELAPSED 0x30
+#define SYS_ERRNO 0x13
+#define SYS_EXIT 0x18
+#define SYS_EXIT_EXTENDED 0x20
+#define SYS_FLEN 0x0c
+#define SYS_GET_CMDLINE 0x15
+#define SYS_HEAPINFO 0x16
+#define SYS_ISERROR 0x08
+#define SYS_ISTTY 0x09
+#define SYS_OPEN 0x01
+#define SYS_READ 0x06
+#define SYS_READC 0x07
+#define SYS_REMOVE 0x0e
+#define SYS_RENAME 0x0f
+#define SYS_SEEK 0x0a
+#define SYS_SYSTEM 0x12
+#define SYS_TICKFREQ 0x31
+#define SYS_TIME 0x11
+#define SYS_TMPNAM 0x0d
+#define SYS_WRITE0 0x04
+#define SYS_WRITE 0x05
+#define SYS_WRITEC 0x03
+
+#define ADP_Stopped_BranchThroughZero 0x20000
+#define ADP_Stopped_UndefinedInstr 0x20001
+#define ADP_Stopped_SoftwareInterrupt 0x20002
+#define ADP_Stopped_PrefetchAbort 0x20003
+#define ADP_Stopped_DataAbort 0x20004
+#define ADP_Stopped_AddressException 0x20005
+#define ADP_Stopped_IRQ 0x20006
+#define ADP_Stopped_FIQ 0x20007
+#define ADP_Stopped_BreakPoint 0x20020
+#define ADP_Stopped_WatchPoint 0x20021
+#define ADP_Stopped_StepComplete 0x20022
+#define ADP_Stopped_RunTimeErrorUnknown 0x20023
+#define ADP_Stopped_InternalError 0x20024
+#define ADP_Stopped_UserInterruption 0x20025
+#define ADP_Stopped_ApplicationExit 0x20026
+#define ADP_Stopped_StackOverflow 0x20027
+#define ADP_Stopped_DivisionByZero 0x20028
+#define ADP_Stopped_OSSpecific 0x20029
+
+/* SYS_OPEN modes must be one of R,W,A, plus an optional B and optional PLUS */
+#define OPENMODE_R 0
+#define OPENMODE_W 4
+#define OPENMODE_A 8
+#define OPENMODE_B 1
+#define OPENMODE_PLUS 2
+
+struct __llvm_libc_stdio_cookie { int handle; };
+
+#endif // LLVMET_LLVMLIBC_SUPPORT_SEMIHOST_H
diff --git a/llvmlibc-support/stdio_read.c b/llvmlibc-support/stdio_read.c
new file mode 100644
index 00000000..aa2c00fe
--- /dev/null
+++ b/llvmlibc-support/stdio_read.c
@@ -0,0 +1,33 @@
+//
+// Copyright (c) 2022, Arm Limited and affiliates.
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#include <stddef.h>
+
+#include "semihost.h"
+
+ssize_t __llvm_libc_stdio_read(struct __llvm_libc_stdio_cookie *cookie,
+                               const char *buf, size_t size) {
+  size_t args[4];
+  args[0] = (size_t)cookie->handle;
+  args[1] = (size_t)buf;
+  args[2] = (size_t)size;
+  args[3] = 0;
+  ssize_t retval = semihosting_call(SYS_READ, args);
+  if (retval >= 0)
+    retval = size - retval;
+  return retval;
+}
diff --git a/llvmlibc-support/stdio_write.c b/llvmlibc-support/stdio_write.c
new file mode 100644
index 00000000..9b7741fd
--- /dev/null
+++ b/llvmlibc-support/stdio_write.c
@@ -0,0 +1,32 @@
+//
+// Copyright (c) 2022, Arm Limited and affiliates.
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#include <stddef.h>
+
+#include "semihost.h"
+
+ssize_t __llvm_libc_stdio_write(struct __llvm_libc_stdio_cookie *cookie,
+                                const char *buf, size_t size) {
+  size_t args[4];
+  args[0] = (size_t)cookie->handle;
+  args[1] = (size_t)buf;
+  args[2] = (size_t)size;
+  ssize_t retval = semihosting_call(SYS_WRITE, args);
+  if (retval >= 0)
+    retval = size - retval;
+  return retval;
+}
diff --git a/llvmlibc.cfg b/llvmlibc.cfg
new file mode 100644
index 00000000..14aba3bf
--- /dev/null
+++ b/llvmlibc.cfg
@@ -0,0 +1 @@
+--sysroot <CFGDIR>/../lib/clang-runtimes/llvmlibc