LLVM Embedded Toolchain for Arm

This repository contains build scripts and auxiliary material for building a bare-metal LLVM based toolchain targeting Arm based on:

• clang + llvm
• lld
• libc++abi
• libc++
• compiler-rt
• picolibc (newlib in versions 14 and earlier)

Goal

The goal is to provide an LLVM based bare-metal toolchain that can target the Arm architecture family from Armv6-M and newer. The toolchain follows the ABI for the Arm Architecture and attempts to provide typical features needed for embedded and realtime operating systems.

Supported architectures

• Armv6-M
• Armv7-M
• Armv7E-M
• Armv8-M Mainline
• Armv8.1-M Mainline
• Armv4T (experimental)
• Armv5TE (experimental)
• Armv6 (experimental, using the Armv5TE library variant)
• AArch64 armv8.0 (experimental)

C++ support

C++ is partially supported with the use of libc++ and libc++abi from LLVM. Features that are not supported include:

• Exceptions
• RTTI
• Multithreading

Components

The LLVM Embedded Toolchain for Arm relies on the following upstream components

License

Content of this repository is licensed under Apache-2.0. See LICENSE.txt.

The resulting binaries are covered under their respective open source licenses, see component links above.

Host platforms

LLVM Embedded Toolchain for Arm is built and tested on Ubuntu 18.04 LTS.

The Windows version is built on Windows Server 2019 and lightly tested on Windows 10.

Building on macOS is functional for x86_64 and Apple Silicon.

Binary packages are provided for major LLVM releases for Linux and Windows.

Getting started

Download a release of the toolchain for your platform from Github releases and extract the archive into an arbitrary directory.

On Ubuntu 20.04 and later libtinfo5 is required: apt install libtinfo5. (This requirement will be removed in LLVM Embedded Toolchain for Arm 17.)

On macOS the toolchain binaries are quarantined by com.apple.quarantine. To run the executables change directory to bin and run the following command to remove the com.apple.quarantine:

find . -type f -perm +0111 | xargs xattr -d com.apple.quarantine

Using the toolchain

To use the toolchain, on the command line you need to provide:

• A configuration file specified with --config, or a suitable set of command line options including the crt0 library to use - see using the toolchain without config files.
• A linker script specified with -T. Default picolibcpp.ld & picolibc.ld scripts are provided and can be used directly or included from a custom linker script.

For example:

$ PATH=<install-dir>/LLVMEmbeddedToolchainForArm-<revision>/bin:$PATH
$ clang --config armv6m_soft_nofp_semihost.cfg -T picolibc.ld -o example example.c

The available configuration files can be listed using:

$ ls <install-dir>/LLVMEmbeddedToolchainForArm-<revision>/bin/*.cfg

Note: If you are using the toolchain in a shared environment with untrusted input, make sure it is sufficiently sandboxed.

Using the toolchain without config files

Instead of using a config file you can directly specify options. The following options are required:

• The target triple
• Disabling exceptions and RTTI
• The crt0 library - either crt0 or crt0-semihost
• The semihosting library, if using crt0-semihost.
• A linker script, the same as when using a config file.

For example:

$ clang \
--target=armv6m-none-eabi \
-fno-exceptions \
-fno-rtti \
-lcrt0-semihost \
-lsemihost \
-T picolibc.ld \
-o example example.c

clang's multilib system will automatically select an appropriate set of libraries based on your compile flags. clang will emit a warning if no appropriate set of libraries can be found.

To display the directory selected by the multilib system, add the flag -print-multi-directory to your clang command line options.

To display all available multilibs run clang with the flag -print-multi-lib and a target triple like --target=aarch64-none-elf or --target=arm-none-eabi.

It's possible that clang will choose a set of libraries that are not the ones you want to use. In this case you can bypass the multilib system by providing a --sysroot option specifying the directory containing the include and lib directories of the libraries you want to use. For example:

$ clang \
--sysroot=<install-dir>/LLVMEmbeddedToolchainForArm-<revision>/lib/clang-runtimes/arm-none-eabi/armv6m_soft_nofp \
--target=armv6m-none-eabi \
-fno-exceptions \
-fno-rtti \
-lcrt0-semihost \
-lsemihost \
-T picolibc.ld \
-o example example.c

Building from source

LLVM Embedded Toolchain for Arm is an open source project and thus can be built from source. Please see the Building from source guide for detailed instructions.

Providing feedback and reporting issues

Please raise an issue via Github issues.

Contributions and Pull Requests

Please see the Contribution Guide for details.