Arm Toolchain for Embedded (ATfE)

Arm Toolchain for Embedded (ATfE) is a C and C++ toolchain from Arm based: on the free and open-source LLVM Compiler Infrastructure and the Picolib C library for baremetal targets.
ATfE is fined-tuned with a particular focus on performance for newer: ARM products (post 2024) like 64-bit Arm Architectures (AArch64), or the M-Profile Vector Extension (MVE, a 32-bit Armv8.1-M extension).

Installation

Download and install a Arm toolchain for embedded build for your operating system and extract it on your file system.
Set these environment variables:
- Set ZEPHYR_TOOLCHAIN_VARIANT to llvm.
- Set LLVM_TOOLCHAIN_PATH to the toolchain installation directory.

To check that you have set these variables correctly in your current environment, follow these example shell sessions (the LLVM_TOOLCHAIN_PATH values may be different on your system):

echo $ZEPHYR_TOOLCHAIN_VARIANT
llvm
echo $LLVM_TOOLCHAIN_PATH
/home/you/Downloads/ATfE

echo $ZEPHYR_TOOLCHAIN_VARIANT
llvm
echo $LLVM_TOOLCHAIN_PATH
/home/you/Downloads/ATfE

> echo %ZEPHYR_TOOLCHAIN_VARIANT%
llvm
> echo %LLVM_TOOLCHAIN_PATH%
C:\ATfE

You can also set ZEPHYR_TOOLCHAIN_VARIANT and LLVM_TOOLCHAIN_PATH as CMake variables when generating a build system for a Zephyr application, like so:
```
west build ... -- -DZEPHYR_TOOLCHAIN_VARIANT=llvm -DLLVM_TOOLCHAIN_PATH=...
```

Toolchain settings

Because LLVM is widely compatible with GNU tools, When builiding with any LLVM toolchain, you have to specify some settings to let the compiler know what tools to use:

Linker

Set CONFIG_LLVM_USE_LLD=y to use LLVM linker.
Set CONFIG_LLVM_USE_LD=y to use the GNU LD linker.

Runtime library

Set CONFIG_COMPILER_RT_RTLIB=y to use LLVM runtime library.
Set CONFIG_LIBGCC_RTLIB=y to use LibGCC runtime library.

west build ... -- -DZEPHYR_TOOLCHAIN_VARIANT=llvm -DLLVM_TOOLCHAIN_PATH=... -DCONFIG_LLVM_USE_LLD=y -DCONFIG_COMPILER_RT_RTLIB=y