How to enable a LLVM backend? - llvm

Question

I am using Archlinux and installed LLVM using the official package (using pacman -S llvm).
I'd like to use it with the wasm-32 backend (available according to the source code).
However, this backend is not enabled on my computer:
$ llc --version
LLVM (http://llvm.org/):
LLVM version 5.0.0
Optimized build.
Default target: x86_64-unknown-linux-gnu
Host CPU: skylake
Registered Targets:
aarch64 - AArch64 (little endian)
aarch64_be - AArch64 (big endian)
amdgcn - AMD GCN GPUs
arm - ARM
arm64 - ARM64 (little endian)
armeb - ARM (big endian)
bpf - BPF (host endian)
bpfeb - BPF (big endian)
bpfel - BPF (little endian)
hexagon - Hexagon
lanai - Lanai
mips - Mips
mips64 - Mips64 [experimental]
mips64el - Mips64el [experimental]
mipsel - Mipsel
msp430 - MSP430 [experimental]
nvptx - NVIDIA PTX 32-bit
nvptx64 - NVIDIA PTX 64-bit
ppc32 - PowerPC 32
ppc64 - PowerPC 64
ppc64le - PowerPC 64 LE
r600 - AMD GPUs HD2XXX-HD6XXX
sparc - Sparc
sparcel - Sparc LE
sparcv9 - Sparc V9
systemz - SystemZ
thumb - Thumb
thumbeb - Thumb (big endian)
x86 - 32-bit X86: Pentium-Pro and above
x86-64 - 64-bit X86: EM64T and AMD64
xcore - XCore
How can I enable LLVM backends?

Answer 1

EDIT (2021-07-23): This answer was updated to use "Motorola 68000" instead of "WebAssembly" as an example experimental target (since Wasm is now stable).
LLVM is not very configurable once it is built.
If you need a configuration beyond the defaults, you have to compile LLVM yourself.
LLVM has a few articles explaining how to compile it, but it does not describe exactly
how to enable additional targets:
Getting Started
Building LLVM with CMake
The enabled targets are controlled by two variables that you need to define when invoking CMake
to prepare the build directory: LLVM_TARGETS_TO_BUILD and LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.
LLVM_TARGETS_TO_BUILD controls only the stable targets.
You can either use the special value all to enable all the stable targets or provide a semicolon-separated list of targets such as ARM;PowerPC;X86. There is an old request
to rename the special value to stable and use all for all the targets.
Its default value is all (see below for the list of targets).
LLVM_EXPERIMENTAL_TARGETS_TO_BUILD is an undocumented (or well hidden) variable that allows you
to enable any target you want. This is also a semicolon-separated list of targets.
The enabled targets will correspond to the union of both lists.
Now, you need to find out the actual name of your target and if it is a stable or experimental target.
The list of stable targets can be found in the Getting Started article.
The default value includes: AArch64, AMDGPU, ARM, AVR, BPF, Hexagon, Lanai, Mips, MSP430, NVPTX, PowerPC, RISCV, Sparc, SystemZ, WebAssembly, X86, XCore.
This list is defined in the main CMakeFile (permalink).
As you can see, WebAssembly is in the list now (in 2021), so it should already be enabled by default. When the question was first asked, it was still an experimental target.
When the question was first asked, WebAssembly was still an experimental target so the rest of the answer will more generally describe how to enable any target. As an example, we'll use "Motorola 68000" instead of wasm.
"Motorola 68000" is not in the list of stable targets. We'll have to find the name used by LLVM and then use LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.
Unfortunately, since this variable is not documented, I wasn't able to find the list of all the targets on their website.
After some trial and error, it seems that the available targets correspond to the names of the directories in /lib/Target. This directory contains a subdirectory named M68k: this is likely the name of the target.
To use LLVM for "Motorola 68000", you'll need to enable the M68k target using the LLVM_EXPERIMENTAL_TARGETS_TO_BUILD
variable when preparing the build directory with CMake.
Here are the steps to compile LLVM with "Motorola 68000" support (adapt it to your own requirements). I used a Linux machine
but you should be able to adapt it to your environment.
Requirements:
CMake
Git
GCC, CLang or Visual Studio depending on your platform
zlib
Clone the LLVM repo. I'll use the /opt/llvm-project directory for the home directory
of my custom version of LLVM (this is the last argument to the command, replace it by the path you want to use).
git clone https://github.com/llvm/llvm-project.git /opt/llvm-project
Navigate to the LLVM sources:
cd /opt/llvm-project/llvm
Create your build directory and navigate to it.
mkdir build && cd build
Use CMake to prepare your build directory. This is the step where you need take care
of setting the variables. In my case I'll use LLVM_EXPERIMENTAL_TARGETS_TO_BUILD="M68k" and
leave LLVM_TARGETS_TO_BUILD to its default value (all stable targets).
Another important variable that I'll set is CMAKE_BUILD_TYPE=Release to get an optimized build and
CMAKE_INSTALL_PREFIX=/opt/llvm-project/llvm/bin to keep this version of LLVM in its directory and do
not interfere with the version I already have on my system (I'll just add this directory to the $PATH
when I'll need it).
cmake -G "Unix Makefiles" -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD="M68k" -DCMAKE_INSTALL_PREFIX=/opt/llvm-project/llvm/bin -DCMAKE_BUILD_TYPE=Release /opt/llvm-project/llvm
Build LLVM, this may take a while:
cmake --build .
Install LLVM:
cmake --build . --target install

Answer 2

You should compile your backend from source. The only pluggable things in LLVM are passes currently.

Answer 3

Your llc --version command says that you installed LLVM version 5.0.0. WASM wasn't integrated into LLVM until LLVM 8.0.0. It was experimental before that.
Changes to the WebAssembly Target
The WebAssembly target is no longer “experimental”! It’s now built by
default, rather than needing to be enabled with
LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.
The object file format and core C ABI are now considered stable. That
said, the object file format has an ABI versioning capability, and one
anticipated use for it will be to add support for returning small
structs as multiple return values, once the underlying WebAssembly
platform itself supports it. Additionally, multithreading support is
not yet included in the stable ABI.
https://releases.llvm.org/8.0.1/docs/ReleaseNotes.html#changes-to-the-webassembly-target