Installation

In the future, std::offload should become available in nightly builds for users. For now, everyone still needs to build rustc from source.

Build instructions

First you need to clone and configure the Rust repository:

git clone --depth=1 git@github.com:rust-lang/rust.git
cd rust
./configure --enable-llvm-link-shared --release-channel=nightly --enable-llvm-assertions --enable-offload --enable-enzyme --enable-clang --enable-lld --enable-option-checking --enable-ninja --disable-docs

Afterwards you can build rustc using:

./x build --stage 1 library

Afterwards rustc toolchain link will allow you to use it through cargo:

rustup toolchain link offload build/host/stage1
rustup toolchain install nightly # enables -Z unstable-options

Build instruction for LLVM itself

git clone --depth=1 git@github.com:llvm/llvm-project.git 
cd llvm-project
mkdir build
cd build
cmake -G Ninja ../llvm -DLLVM_TARGETS_TO_BUILD="host,AMDGPU,NVPTX" -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_ENABLE_PROJECTS="clang;lld" -DLLVM_ENABLE_RUNTIMES="offload,openmp" -DLLVM_ENABLE_PLUGINS=ON -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=.
ninja
ninja install

This gives you a working LLVM build.

Testing

run

./x test --stage 1 tests/codegen/gpu_offload

Usage

It is important to use a clang compiler build on the same llvm as rustc. Just calling clang without the full path will likely use your system clang, which probably will be incompatible.

/absolute/path/to/rust/build/x86_64-unknown-linux-gnu/stage1/bin/rustc --edition=2024 --crate-type cdylib src/main.rs --emit=llvm-ir  -O -C lto=fat -Cpanic=abort -Zoffload=Enable
/absolute/path/to/rust/build/x86_64-unknown-linux-gnu/llvm/bin/clang++ -fopenmp --offload-arch=native -g  -O3 main.ll -o main -save-temps
LIBOMPTARGET_INFO=-1  ./main

The first step will generate a main.ll file, which has enough instructions to cause the offload runtime to move data to and from a gpu. The second step will use clang as the compilation driver to compile our IR file down to a working binary. Only a very small Rust subset will work out of the box here, unless you use features like build-std, which are not covered by this guide. Look at the codegen test to get a feeling for how to write a working example. In the last step you can run your binary, if all went well you will see a data transfer being reported:

omptarget device 0 info: Entering OpenMP data region with being_mapper at unknown:0:0 with 1 arguments:
omptarget device 0 info: tofrom(unknown)[1024]
omptarget device 0 info: Creating new map entry with HstPtrBase=0x00007fffffff9540, HstPtrBegin=0x00007fffffff9540, TgtAllocBegin=0x0000155547200000, TgtPtrBegin=0x0000155547200000, Size=1024, DynRefCount=1, HoldRefCount=0, Name=unknown
omptarget device 0 info: Copying data from host to device, HstPtr=0x00007fffffff9540, TgtPtr=0x0000155547200000, Size=1024, Name=unknown
omptarget device 0 info: OpenMP Host-Device pointer mappings after block at unknown:0:0:
omptarget device 0 info: Host Ptr           Target Ptr         Size (B) DynRefCount HoldRefCount Declaration
omptarget device 0 info: 0x00007fffffff9540 0x0000155547200000 1024     1           0            unknown at unknown:0:0
// some other output
omptarget device 0 info: Exiting OpenMP data region with end_mapper at unknown:0:0 with 1 arguments:
omptarget device 0 info: tofrom(unknown)[1024]
omptarget device 0 info: Mapping exists with HstPtrBegin=0x00007fffffff9540, TgtPtrBegin=0x0000155547200000, Size=1024, DynRefCount=0 (decremented, delayed deletion), HoldRefCount=0
omptarget device 0 info: Copying data from device to host, TgtPtr=0x0000155547200000, HstPtr=0x00007fffffff9540, Size=1024, Name=unknown
omptarget device 0 info: Removing map entry with HstPtrBegin=0x00007fffffff9540, TgtPtrBegin=0x0000155547200000, Size=1024, Name=unknown