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, or optionally newlib or LLVM's libc

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 and Baseline
• Armv8.1-M Mainline and Baseline
• Armv4T (experimental)
• Armv5TE (experimental)
• Armv6 (experimental, using the Armv5TE library variant)
• Armv7-A
• Armv7-R
• AArch32 Armv8-A
• AArch32 Armv8-R
• AArch64 Armv8-A

C++ support

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

• Multithreading

LLVM Embedded Toolchain for Arm uses the unstable libc++ ABI version. This ABI uses all the latest libc++ improvements and bugfixes, but may result in link errors when linking against objects compiled against older versions of the ABI. For more information see https://libcxx.llvm.org/DesignDocs/ABIVersioning.html.

Components

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

Component	Link
LLVM	https://github.com/llvm/llvm-project
picolibc	https://github.com/picolibc/picolibc

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.

Testing for some targets uses the freely-available but not open-source Arm FVP models, which have their own licenses. These are not used by default, see Building from source for details.

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.

Pre-requisite for using toolchain on Windows

Install appropriate latest supported Microsoft Visual C++ Redistributable package, such as from Microsoft Visual C++ Redistributable latest supported downloads.

Using the toolchain

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

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

• The target triple.
• The FPU to use.
• Disabling/enabling C++ exceptions and RTTI.
• The C runtime library: either crt0 or crt0-semihost. crt0 will be linked automatically, but this can be suppressed with the -nostartfiles option so that crt0-semihost can be used.
• The semihosting library, if using crt0-semihost.
• A linker script specified with -T. Default picolibcpp.ld and 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 \
--target=armv6m-none-eabi \
-mfpu=none \
-fno-exceptions \
-fno-rtti \
-nostartfiles \
-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 \
-mfpu=none \
-fno-exceptions \
-fno-rtti \
-nostartfiles \
-lcrt0-semihost \
-lsemihost \
-T picolibc.ld \
-o example example.c

The FPU selection can be skipped, but it is not recommended to as the defaults are different to GCC ones.

The builds of the toolchain come packaged with two config files, Omax.cfg and OmaxLTO.cfg. When used, these config files enable several build optimisation flags to achieve highest performance on typical embedded benchmarks. OmaxLTO.cfg enables link-time optimisation (LTO) specific flags. These configs can be optionally passed using the --config flag. For example:

$ clang \
example.c \
...
--config=Omax.cfg \
--config=OmaxLTO.cfg \
-o example

Users should be warned that Omax.cfg enables -ffast-math which breaks IEEE compliance and enables maths optimisations which can affect code correctness. LTOs are kept separately in OmaxLTO.cfg as users may not want LTOs due to potential increase in link time and/or increased memory usage during linking. Some of the options in the config files are undocumented internal LLVM options. For these undocumented options please see the source code of the corresponding optimisation passes in the LLVM project to find out more. Users are also encouraged to create their own configs and tune their own flag parameters. Information on LLVM Embedded Toolchain for Arm specific optimization flags is available in Optimization Flags

Binary releases of the LLVM Embedded Toolchain for Arm are based on release branches of the upstream LLVM Project, thus can safely be used with all tools provided by LLVM releases of matching version.

See Migrating from Arm GNU Toolchain and Experimental newlib support for advice on using LLVM Embedded Toolchain for Arm with existing projects relying on the Arm GNU Toolchain.

Note: picolibc provides excellent support for Arm GNU Toolchain, so projects that require using both Arm GNU Toolchain and LLVM Embedded Toolchain for Arm can choose either picolibc or newlib.

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.