Getting Started with oneAPI DPC++

The DPC++ Compiler compiles C++ and SYCL* source files with code for both CPU and a wide range of compute accelerators such as GPU and FPGA.

Table of contents

Prerequisites

Alternatively, you can use a Docker image that has everything you need for building pre-installed:

docker run --name sycl_build -it -v /local/workspace/dir/:/src ghcr.io/intel/llvm/ubuntu2204_build /bin/bash

This command will start a terminal session, from which you can proceed with the instructions below. See Docker BKMs for more info on Docker commands.

Create DPC++ workspace

Throughout this document DPCPP_HOME denotes the path to the local directory created as DPC++ workspace. It might be useful to create an environment variable with the same name.

Linux:

export DPCPP_HOME=~/sycl_workspace
mkdir $DPCPP_HOME
cd $DPCPP_HOME

git clone https://github.com/intel/llvm -b sycl

Windows (64-bit):

Open a developer command prompt using one of two methods:

  • Click start menu and search for “x64 Native Tools Command Prompt for VS XXXX”, where XXXX is a version of installed Visual Studio.

  • Ctrl-R, write “cmd”, click enter, then run "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvarsall.bat" x64

set DPCPP_HOME=%USERPROFILE%\sycl_workspace
mkdir %DPCPP_HOME%
cd %DPCPP_HOME%

git clone --config core.autocrlf=false https://github.com/intel/llvm -b sycl

Build DPC++ toolchain

The easiest way to get started is to use the buildbot configure and compile scripts.

In case you want to configure CMake manually the up-to-date reference for variables is in these files.

Linux:

python $DPCPP_HOME/llvm/buildbot/configure.py
python $DPCPP_HOME/llvm/buildbot/compile.py

Windows (64-bit):

python %DPCPP_HOME%\llvm\buildbot\configure.py
python %DPCPP_HOME%\llvm\buildbot\compile.py

You can use the following flags with configure.py (full list of available flags can be found by launching the script with --help):

  • --werror -> treat warnings as errors when compiling LLVM

  • --cuda -> use the cuda backend (see Nvidia CUDA)

  • --hip -> use the HIP backend (see HIP)

  • --hip-platform -> select the platform used by the hip backend, AMD or NVIDIA (see HIP AMD or see HIP NVIDIA)

  • --enable-all-llvm-targets -> build compiler (but not a runtime) with all supported targets

  • --shared-libs -> Build shared libraries

  • -t -> Build type (Debug or Release)

  • -o -> Path to build directory

  • --cmake-gen -> Set build system type (e.g. --cmake-gen "Unix Makefiles")

You can use the following flags with compile.py (full list of available flags can be found by launching the script with --help):

  • -o -> Path to build directory

  • -t, --build-target -> Build target (e.g., clang or llvm-spirv). Default is deploy-sycl-toolchain

  • -j, --build-parallelism -> Number of threads to use for compilation

Please note that no data about flags is being shared between configure.py and compile.py scripts, which means that if you configured your build to be placed in non-default directory using -o flag, you must also specify this flag and the same path in compile.py options. This allows you, for example, to configure several different builds and then build just one of them which is needed at the moment.

Build DPC++ toolchain with libc++ library

There is experimental support for building and linking DPC++ runtime with libc++ library instead of libstdc++. To enable it the following CMake options should be used.

Linux:

-DSYCL_USE_LIBCXX=ON \
-DSYCL_LIBCXX_INCLUDE_PATH=<path to libc++ headers> \
-DSYCL_LIBCXX_LIBRARY_PATH=<path to libc++ and libc++abi libraries>

You can also use configure script to enable:

python %DPCPP_HOME%\llvm\buildbot\configure.py --use-libcxx \
--libcxx-include <path to libc++ headers> \
--libcxx-library <path to libc++ and libc++ abi libraries>
python %DPCPP_HOME%\llvm\buildbot\compile.py

Build DPC++ toolchain with support for NVIDIA CUDA

To enable support for CUDA devices, follow the instructions for the Linux or Windows DPC++ toolchain, but add the --cuda flag to configure.py. Note, the CUDA backend has Windows support; Windows Subsystem for Linux (WSL) is not needed to build and run the CUDA backend.

Refer to NVIDIA CUDA Installation Guide for Linux or NVIDIA CUDA Installation Guide for Windows for CUDA toolkit installation instructions.

Errors may occur if DPC++ is built with a toolkit version which is higher than the CUDA driver version. In order to check that the CUDA driver and toolkits match, use the CUDA executable deviceQuery which is usually found in $CUDA_INSTALL_DIR/cuda/extras/demo_suite/deviceQuery.

NOTE: An installation of at least CUDA 11.6 is recommended because there is a known issue with some math built-ins when using -O1/O2/O3 Optimization options for CUDA toolkits prior to 11.6 (This is due to a bug in earlier versions of the CUDA toolkit: see this issue).

NOTE: CUDA toolkit versions earlier than 11.0 are not regularly tested, but should work for appropriate devices. Note that for example some oneapi extensions that require sm_80 and later architectures also require at least CUDA 11.0.

The CUDA backend should work on Windows or Linux operating systems with any GPU with compute capability (SM version) sm_50 or above. The default SM version for the NVIDIA CUDA backend is sm_50. Users of sm_3X devices can attempt to specify the target architecture ahead of time, provided that they use a 11.X or earlier CUDA toolkit version, but some features may not be supported. The CUDA backend has been tested with different Ubuntu Linux distributions and a selection of supported CUDA toolkit versions and GPUs. The backend is tested by a relevant device/toolkit prior to a ONEAPI plugin release. Go to the plugin release pages for further details.

Non-standard CUDA location:

If the CUDA toolkit is installed in a non-default location on your system, two considerations must be made.

Firstly, do not add the toolkit to your standard environment variables (PATH, LD_LIBRARY_PATH), as to do so will create conflicts with OpenCL headers.

Secondly, set the CUDA_LIB_PATH environment variable and pass the CMake variable CUDA_TOOLKIT_ROOT_DIR as follows:

CUDA_LIB_PATH=/path/to/cuda/toolkit/lib64/stubs CC=gcc CXX=g++ python $DPCPP_HOME/llvm/buildbot/configure.py --cuda --cmake-opt="-DCUDA_TOOLKIT_ROOT_DIR=/path/to/cuda/toolkit"

CUDA_LIB_PATH=/path/to/cuda/toolkit/lib64/stubs CC=gcc CXX=g++ python $DPCPP_HOME/llvm/buildbot/compile.py

$DPCPP_HOME/llvm/build/bin/clang++ -std=c++17 -O3 -fsycl -fsycl-targets=nvptx64-nvidia-cuda --cuda-path=/path/to/cuda/toolkit *.cpp -o a.out

LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$DPCPP_HOME/llvm/build/lib ./a.out

Build DPC++ toolchain with support for HIP AMD

There is beta support for oneAPI DPC++ for HIP on AMD devices. It is not feature complete and it still contains known and unknown bugs. Currently it has only been tried on Linux, with ROCm 4.2.0, 4.3.0, 4.5.2, 5.3.0, and 5.4.3, using the AMD Radeon Pro W6800 (gtx1030), MI50 (gfx906), MI100 (gfx908) and MI250x (gfx90a) devices. The backend is tested by a relevant device/toolkit prior to a oneAPI plugin release. Go to the plugin release pages for further details.

To enable support for HIP devices, follow the instructions for the Linux DPC++ toolchain, but add the --hip flag to configure.py.

Enabling this flag requires an installation of ROCm on the system, for instruction on how to install this refer to AMD ROCm Installation Guide for Linux.

The DPC++ build assumes that ROCm is installed in /opt/rocm, if it is installed somewhere else, the directory must be provided through the CMake variable UR_HIP_ROCM_DIR which can be passed using the --cmake-opt option of configure.py as follows:

python $DPCPP_HOME/llvm/buildbot/configure.py --hip \
  --cmake-opt=-DUR_HIP_ROCM_DIR=/usr/local/rocm

If further customization is required — for instance when the layout of individual directories can not be inferred from UR_HIP_ROCM_DIR — it is possible to specify the location of HIP include, HSA include and HIP library directories, using the following CMake variables:

  • UR_HIP_INCLUDE_DIR,

  • UR_HIP_HSA_INCLUDE_DIR,

  • UR_HIP_LIB_DIR. These options are all passed through to Unified Runtime, more detail about them can be found here.

LLD is necessary for the AMDGPU compilation chain. The AMDGPU backend generates a standard ELF relocatable code object that can be linked by lld to produce a standard ELF shared code object which can be loaded and executed on an AMDGPU target. The LLD project is enabled by default when configuring for HIP. For more details on building LLD refer to LLD Build Guide.

Build DPC++ toolchain with support for HIP NVIDIA

There is experimental support for oneAPI DPC++ for HIP on Nvidia devices. There is no continuous integration for this and there are no guarantees for supported platforms or configurations.

This is a compatibility feature and the CUDA backend should be preferred to run on NVIDIA GPUs.

To enable support for HIP NVIDIA devices, follow the instructions for the Linux DPC++ toolchain, but add the --hip and --hip-platform NVIDIA flags to configure.py.

Enabling this flag requires HIP to be installed, more specifically HIP NVCC, as well as the CUDA Runtime API to be installed, see NVIDIA CUDA Installation Guide for Linux.

Currently, this has only been tried on Linux, with ROCm 4.2.0 or 4.3.0, with CUDA 11, and using a GeForce 1060 device.

Build DPC++ toolchain with support for ARM processors

There is no continuous integration for this, and there are no guarantees for supported platforms or configurations.

To enable support for ARM processors, follow the instructions for the linux toolchain, but add the --host-target "AArch64;ARM;X86" flag to configure.py.

For CUDA support also add the --cuda flag.

Currently, this has only been tried on Linux, with CUDA 12.3, and using a Grace Hopper platform with a ARM64 processor and an H100 GPU.

Build DPC++ toolchain with support for runtime kernel fusion and JIT compilation

Support for the experimental SYCL extension for user-driven kernel fusion at runtime is enabled by default. The same mechanism is used to allow JIT compilation of AMD and Nvidia kernels.

To disable support for these features, follow the instructions for the Linux DPC++ toolchain, but add the --disable-jit flag.

Both kernel fusion and JIT compilation of AMD and Nvidia kernels are currently not yet supported on the Windows platform.

Build Doxygen documentation

Building Doxygen documentation is similar to building the product itself. First, the following tools need to be installed:

  • doxygen

  • graphviz

  • sphinx

Then you’ll need to add the following options to your CMake configuration command:

-DLLVM_ENABLE_DOXYGEN=ON

After CMake cache is generated, build the documentation with doxygen-sycl target. It will be put to $DPCPP_HOME/llvm/build/tools/sycl/doc/html directory.

Build DPC++ toolchain with a custom Unified Runtime

DPC++ uses the Unified Runtime under the hood to provide implementations of various SYCL backends. By default the source code for the Unified Runtime will be acquired using CMake’s FetchCotent. The specific repository URL and revision tag used can be found in the file sycl/cmake/modules/FetchUnifiedRuntime.cmake searching for the variables UNIFIED_RUNTIME_REPO and UNIFIED_RUNTIME_TAG.

In order to enable developers, a number of CMake variables are available to control which revision of Unified Runtime should be used when building DPC++:

  • SYCL_UR_OVERRIDE_FETCH_CONTENT_REPO is a variable which can be used to override the UNIFIED_RUNTIME_REPO variable used by FetchContent to attain the Unified Runtime source code.

  • SYCL_UR_OVERRIDE_FETCH_CONTENT_TAG is a variable which can be used to override the UNIFIED_RUNTIME_TAG variable used by FetchContent to attain the Unified Runtime source code.

  • SYCL_UR_USE_FETCH_CONTENT is an option to control if CMake should use FetchContent to pull in the Unified Runtime repository, it defaults to ON. When set to OFF, FetchContent will not be used, instead:

    • The path specified by variable SYCL_UR_SOURCE_DIR will be used with add_directory(). This can be used to point at an adjacent directory containing a clone of the Unified Runtime repository.

    • The path sycl/unified-runtime will be used, if it exists. This can be used as-if an in-tree build.

  • SYCL_UR_SOURCE_DIR is a variable used to specify the path to the Unified Runtime repository when SYCL_UR_USE_FETCH_CONTENT is set of OFF.

Build DPC++ libclc with a custom toolchain

libclc is an implementation of the OpenCL required libraries, as described in the OpenCL C specification, additionally providing definitions of SPIR-V builtins. It is built to target-specific bitcode, that is linked against SYCL binaries. By default, the built system uses the SYCL toolchain currently being built to create libclc bitcode. This can be suboptimal in case of debug builds, in which case debug tools are used to build non-debug libclc bitcode (the notion of debug builds doesn’t really apply to libclc), resulting in very long compilation time. In order to specify a directory containing custom toolchain users can set: LIBCLC_CUSTOM_LLVM_TOOLS_BINARY_DIR variable. Care is required, as the changes to the local SYCL tree might not be reflected in the custom location during the build time.

Deployment

TODO: add instructions how to deploy built DPC++ toolchain.

Use DPC++ toolchain

Install low level runtime

To run DPC++ applications on OpenCL devices, OpenCL implementation(s) must be present in the system.

To run DPC++ applications on Level Zero devices, Level Zero implementation(s) must be present in the system. You can find the link to the Level Zero spec in the following section Find More.

The Level Zero RT for GPU, OpenCL RT for GPU, OpenCL RT for CPU, FPGA emulation RT and TBB runtime which are needed to run DPC++ application on Intel GPU or Intel CPU devices can be downloaded using links in the dependency configuration file and installed following the instructions below. The same versions are used in PR testing.

Linux:

  1. Extract the archive. For example, for the archives oclcpuexp_<cpu_version>.tar.gz and fpgaemu_<fpga_version>.tar.gz you would run the following commands

    # Extract OpenCL FPGA emulation RT
mkdir -p /opt/intel/oclfpgaemu_<fpga_version>
cd /opt/intel/oclfpgaemu_<fpga_version>
tar zxvf fpgaemu_<fpga_version>.tar.gz
# Extract OpenCL CPU RT
mkdir -p /opt/intel/oclcpuexp_<cpu_version>
cd /opt/intel/oclcpuexp_<cpu_version>
tar -zxvf oclcpuexp_<cpu_version>.tar.gz

  2. Create ICD file pointing to the new runtime (requires sudo access)

    # OpenCL FPGA emulation RT
echo  /opt/intel/oclfpgaemu_<fpga_version>/x64/libintelocl_emu.so | sudo tee
  /etc/OpenCL/vendors/intel_fpgaemu.icd
# OpenCL CPU RT
echo /opt/intel/oclcpuexp_<cpu_version>/x64/libintelocl.so | sudo tee
  /etc/OpenCL/vendors/intel_expcpu.icd

  3. Extract or build TBB libraries using links in the dependency configuration file. For example, for the archive oneapi-tbb-<tbb_version>-lin.tgz:

    mkdir -p /opt/intel
cd /opt/intel
tar -zxvf oneapi-tbb*lin.tgz

  4. Copy files from or create symbolic links to TBB libraries in OpenCL RT folder:

    # OpenCL FPGA emulation RT
ln -s /opt/intel/oneapi-tbb-<tbb_version>/lib/intel64/gcc4.8/libtbb.so
  /opt/intel/oclfpgaemu_<fpga_version>/x64/libtbb.so
ln -s /opt/intel/oneapi-tbb-<tbb_version>/lib/intel64/gcc4.8/libtbbmalloc.so
  /opt/intel/oclfpgaemu_<fpga_version>/x64/libtbbmalloc.so
ln -s /opt/intel/oneapi-tbb-<tbb_version>/lib/intel64/gcc4.8/libtbb.so.12
  /opt/intel/oclfpgaemu_<fpga_version>/x64/libtbb.so.12
ln -s /opt/intel/oneapi-tbb-<tbb_version>/lib/intel64/gcc4.8/libtbbmalloc.so.2
  /opt/intel/oclfpgaemu_<fpga_version>/x64/libtbbmalloc.so.2
# OpenCL CPU RT
ln -s /opt/intel/oneapi-tbb-<tbb_version>/lib/intel64/gcc4.8/libtbb.so
  /opt/intel/oclcpuexp_<cpu_version>/x64/libtbb.so
ln -s /opt/intel/oneapi-tbb-<tbb_version>/lib/intel64/gcc4.8/libtbbmalloc.so
  /opt/intel/oclcpuexp_<cpu_version>/x64/libtbbmalloc.so
ln -s /opt/intel/oneapi-tbb-<tbb_version>/lib/intel64/gcc4.8/libtbb.so.12
  /opt/intel/oclcpuexp_<cpu_version>/x64/libtbb.so.12
ln -s /opt/intel/oneapi-tbb-<tbb_version>/lib/intel64/gcc4.8/libtbbmalloc.so.2
  /opt/intel/oclcpuexp_<cpu_version>/x64/libtbbmalloc.so.2

  5. Configure library paths (requires sudo access)

    echo /opt/intel/oclfpgaemu_<fpga_version>/x64 | sudo tee
  /etc/ld.so.conf.d/libintelopenclexp.conf
echo /opt/intel/oclcpuexp_<cpu_version>/x64 | sudo tee -a
  /etc/ld.so.conf.d/libintelopenclexp.conf
sudo ldconfig -f /etc/ld.so.conf.d/libintelopenclexp.conf

Windows (64-bit):

  1. If you need OpenCL runtime for Intel GPU as well, then update/install it first. Do it before installing OpenCL runtime for Intel CPU runtime as OpenCL runtime for Intel GPU installer may re-write some important files or settings and make existing OpenCL runtime for Intel CPU runtime not working properly.

  2. Extract the archive with OpenCL runtime for Intel CPU and/or for Intel FPGA emulation using links in the dependency configuration file. For example, to c:\oclcpu_rt_<cpu_version>.

  3. Extract the archive with TBB runtime or build it from sources using links in the dependency configuration file. For example, to c:\oneapi-tbb-<tbb_version>.

  4. Run Command Prompt as Administrator. To do that click Start button, type Command Prompt, click the Right mouse button on it, then click Run As Administrator, then click Yes to confirm.

  5. In the opened windows run install.bat provided with the extracted files to install runtime to the system and setup environment variables. So, if the extracted files are in c:\oclcpu_rt_<cpu_version>\ folder, then type the command:

    # Install OpenCL FPGA emulation RT
# Answer Y to clean previous OCL_ICD_FILENAMES configuration and ICD records cleanup
c:\oclfpga_rt_<fpga_version>\install.bat c:\oneapi-tbb-<tbb_version>\redist\intel64\vc14
# Install OpenCL CPU RT
# Answer N for ICD records cleanup
c:\oclcpu_rt_<cpu_version>\install.bat c:\oneapi-tbb-<tbb_version>\redist\intel64\vc14

Obtain prerequisites for ahead of time (AOT) compilation

Ahead of time compilation requires ahead of time compiler available in PATH. There is AOT compiler for each device type:

  • GPU, Level Zero and OpenCL runtimes are supported,

  • CPU, OpenCL runtime is supported,

  • Accelerator (FPGA or FPGA emulation), OpenCL runtime is supported.

GPU

  • Linux

    There are two ways how to obtain GPU AOT compiler ocloc:

  • Windows

    • GPU AOT compiler ocloc is a part of Intel® oneAPI Base Toolkit (Intel® oneAPI DPC++/C++ Compiler component).
      Make sure that the following path to ocloc binary is available in PATH environment variable:

      • <oneAPI installation location>/compiler/<version>/windows/lib/ocloc

CPU

Accelerator

  • Accelerator AOT compiler aoc is a part of Intel® oneAPI Base Toolkit (Intel® oneAPI DPC++/C++ Compiler component).
    Make sure that these binaries are available in PATH environment variable:

    • aoc from <oneAPI installation location>/compiler/<version>/<OS>/lib/oclfpga/bin

    • aocl-ioc64 from <oneAPI installation location>/compiler/<version>/<OS>/bin

Test DPC++ toolchain

Run in-tree LIT tests

To verify that built DPC++ toolchain is working correctly, run:

Linux:

python $DPCPP_HOME/llvm/buildbot/check.py

Windows (64-bit):

python %DPCPP_HOME%\llvm\buildbot\check.py

Make sure that psutil package is installed. If no OpenCL GPU/CPU runtimes are available, the corresponding tests are skipped.

If CUDA support has been built, it is tested only if there are CUDA devices available.

If testing with HIP for AMD, the lit tests will use gfx906 as the default architecture. It is possible to change it by adding -Xsycl-target-backend=amdgcn-amd-amdhsa --offload-arch=<target> to the CMake variable SYCL_CLANG_EXTRA_FLAGS.

Run DPC++ E2E tests

Follow instructions from the link below to build and run tests: README

Run Khronos* SYCL* conformance test suite (optional)

Khronos* SYCL* conformance test suite (CTS) is intended to validate implementation conformance to Khronos* SYCL* specification. DPC++ compiler is expected to pass significant number of tests, and it keeps improving.

Follow Khronos* SYCL* CTS instructions from README file to obtain test sources and instructions how build and execute the tests.

Run simple DPC++ application

A simple DPC++ or SYCL* program consists of following parts:

  1. Header section

  2. Allocating buffer for data

  3. Creating SYCL queue

  4. Submitting command group to SYCL queue which includes the kernel

  5. Wait for the queue to complete the work

  6. Use buffer accessor to retrieve the result on the device and verify the data

  7. The end

Creating a file simple-sycl-app.cpp with the following C++/SYCL code:

#include <sycl/sycl.hpp>

int main() {
  // Creating buffer of 4 elements to be used inside the kernel code
  sycl::buffer<size_t, 1> Buffer(4);

  // Creating SYCL queue
  sycl::queue Queue;

  // Size of index space for kernel
  sycl::range<1> NumOfWorkItems{Buffer.size()};

  // Submitting command group(work) to queue
  Queue.submit([&](sycl::handler &cgh) {
    // Getting write only access to the buffer on a device.
    sycl::accessor Accessor{Buffer, cgh, sycl::write_only};
    // Executing kernel
    cgh.parallel_for<class FillBuffer>(
        NumOfWorkItems, [=](sycl::id<1> WIid) {
          // Fill buffer with indexes.
          Accessor[WIid] = WIid.get(0);
        });
  });

  // Getting read only access to the buffer on the host.
  // Implicit barrier waiting for queue to complete the work.
  sycl::host_accessor HostAccessor{Buffer, sycl::read_only};

  // Check the results
  bool MismatchFound = false;
  for (size_t I = 0; I < Buffer.size(); ++I) {
    if (HostAccessor[I] != I) {
      std::cout << "The result is incorrect for element: " << I
                << " , expected: " << I << " , got: " << HostAccessor[I]
                << std::endl;
      MismatchFound = true;
    }
  }

  if (!MismatchFound) {
    std::cout << "The results are correct!" << std::endl;
  }

  return MismatchFound;
}

To build simple-sycl-app put bin and lib to PATHs:

Linux:

export PATH=$DPCPP_HOME/llvm/build/bin:$PATH
export LD_LIBRARY_PATH=$DPCPP_HOME/llvm/build/lib:$LD_LIBRARY_PATH

Windows (64-bit):

set PATH=%DPCPP_HOME%\llvm\build\bin;%PATH%
set LIB=%DPCPP_HOME%\llvm\build\lib;%LIB%

and run following command:

clang++ -fsycl simple-sycl-app.cpp -o simple-sycl-app.exe

When building for CUDA or HIP NVIDIA, use the CUDA target triple as follows:

clang++ -fsycl -fsycl-targets=nvptx64-nvidia-cuda \
  simple-sycl-app.cpp -o simple-sycl-app-cuda.exe

Linux & Windows (64-bit):

./simple-sycl-app.exe
The results are correct!

NOTE: Currently, when the application has been built with the CUDA target, the CUDA backend must be selected at runtime using the ONEAPI_DEVICE_SELECTOR environment variable.

ONEAPI_DEVICE_SELECTOR=cuda:* ./simple-sycl-app-cuda.exe

NOTE: oneAPI DPC++/SYCL developers can specify SYCL device for execution using device selectors (e.g. sycl::cpu_selector_v, sycl::gpu_selector_v, Intel FPGA selector(s)) as explained in following section Code the program for a specific GPU.

AOT Target architectures

NOTE: When building for HIP AMD, you MUST use the AMD target triple and specify the target architecture with -Xsycl-target-backend --offload-arch=<arch> as follows:

clang++ -fsycl -fsycl-targets=amdgcn-amd-amdhsa \
  -Xsycl-target-backend --offload-arch=gfx906              \
  simple-sycl-app.cpp -o simple-sycl-app-amd.exe

The target architecture may also be specified for the CUDA backend, with -Xsycl-target-backend --cuda-gpu-arch=<arch>. Specifying the architecture is necessary if an application aims to use newer hardware features, such as native atomic operations or the joint_matrix extension. Moreover, it is possible to pass specific options to CUDA ptxas (such as --maxrregcount=<n> for limiting the register usage or --verbose for printing generation statistics) using the -Xcuda-ptxas flag.

clang++ -fsycl -fsycl-targets=nvptx64-nvidia-cuda \
  simple-sycl-app.cpp -o simple-sycl-app-cuda.exe \
  -Xcuda-ptxas --maxrregcount=128 -Xcuda-ptxas --verbose \
  -Xsycl-target-backend --cuda-gpu-arch=sm_80

To build simple-sycl-app ahead of time for GPU, CPU or Accelerator devices, specify the target architecture. The examples provided use a supported alias for the target, representing a full triple. Additional details can be found in the Users Manual.

-fsycl-targets=spir64_gen for GPU, -fsycl-targets=spir64_x86_64 for CPU, -fsycl-targets=spir64_fpga for Accelerator.

Multiple target architectures are supported.

E.g., this command builds simple-sycl-app for GPU and CPU devices in ahead of time mode:

clang++ -fsycl -fsycl-targets=spir64_gen,spir64_x86_64 simple-sycl-app.cpp -o simple-sycl-app-aot.exe

Additionally, user can pass specific options of AOT compiler to the DPC++ compiler using -Xsycl-target-backend option, see Device code formats for more. To find available options, execute:

ocloc compile --help for GPU, opencl-aot --help for CPU, aoc -help -sycl for Accelerator.

The simple-sycl-app.exe application doesn’t specify SYCL device for execution, so SYCL runtime will use default_selector logic to select one of accelerators available in the system. In this case, the behavior of the default_selector can be altered using the ONEAPI_DEVICE_SELECTOR environment variable, setting cuda:* forces the usage of the CUDA backend (if available), hip:* forces the usage of the HIP backend (if available), opencl:* will force the usage of the OpenCL backend.

ONEAPI_DEVICE_SELECTOR=cuda:* ./simple-sycl-app-cuda.exe

The default is the OpenCL backend if available.

NOTE: nvptx64-nvidia-cuda is usable with -fsycl-targets if clang was built with the cmake option SYCL_ENABLE_BACKENDS=cuda.

Build DPC++ application with CMake

DPC++ applications can be built with CMake by simply using DPC++ as the C++ compiler and by adding the SYCL specific flags. For example assuming clang++ is on the PATH, a minimal CMakeLists.txt file for the sample above would be:

# Modifying the compiler should be done before the project line
set(CMAKE_CXX_COMPILER "clang++")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsycl")

project(simple-sycl-app LANGUAGES CXX)

add_executable(simple-sycl-app simple-sycl-app.cpp)

NOTE: compiling SYCL programs requires passing the SYCL flags to clang++ for both the compilation and linking stages, so using add_compile_options to pass the SYCL flags is not enough on its own, they should also be passed to add_link_options, or more simply the SYCL flags can just be added to CMAKE_CXX_FLAGS.

NOTE: When linking a SYCL application, clang++ will implicitly link it against libsycl.so, so there is no need to add -lsycl to target_link_libraries in the CMake.

Code the program for a specific GPU

To assist in finding a specific SYCL compatible device out of all that may be available, a “device selector” may be used. A “device selector” is a ranking function (C++ Callable) that will give an integer ranking value to all the devices on the system. It can be passed to sycl::queue, sycl::device and sycl::platform constructors. The highest ranking device is then selected. SYCL has built-in device selectors for selecting a generic GPU, CPU, or accelerator device, as well as one for a default device. Additionally, a user can define their own as function, lambda, or functor class. Device selectors returning negative values will “reject” a device ensuring it is not selected, but values 0 or higher will be selected by the highest score with ties resolved by an internal algorithm (see Section 4.6.1 of the SYCL 2020 specification)

The example below illustrates how to use a device selector to create device and queue objects bound to Intel GPU device:

#include <sycl/sycl.hpp>

int main() {

  auto NEOGPUDeviceSelector = [](const sycl::device &Device){
    using namespace sycl::info;

    const std::string DeviceName = Device.get_info<device::name>();
    bool match = Device.is_gpu() && (DeviceName.find("HD Graphics NEO") != std::string::npos);
    return match ? 1 : -1;
  };

  try {
    sycl::queue Queue(NEOGPUDeviceSelector);
    sycl::device Device(NEOGPUDeviceSelector);
  } catch (sycl::exception &E) {
    std::cout << E.what() << std::endl;
  }
}

The device selector below selects an NVIDIA device only, and won’t execute if there is none.

int CUDASelector(const sycl::device &Device) {
  using namespace sycl::info;
  const std::string DriverVersion = Device.get_info<device::driver_version>();

  if (Device.is_gpu() && (DriverVersion.find("CUDA") != std::string::npos)) {
    std::cout << " CUDA device found " << std::endl;
    return 1;
  };
  return -1;
}

C++ standard

  • DPC++ runtime and headers require C++17 at least.

  • DPC++ compiler builds apps as C++17 apps by default. Higher versions of standard are supported as well.

Known Issues and Limitations

  • SYCL 2020 support work is in progress.

  • 32-bit host/target is not supported.

  • DPC++ works only with OpenCL low level runtimes which support out-of-order queues.

  • On Windows linking DPC++ applications with /MTd flag is known to cause crashes.

CUDA back-end limitations

  • Windows support is currently experimental and not regularly tested.

  • sycl::sqrt is not correctly rounded by default as the SYCL specification allows lower precision, when porting from CUDA it may be helpful to use -fsycl-fp32-prec-sqrt to use the correctly rounded square root, this is significantly slower but matches the default precision used by nvcc, and this clang++ flag is equivalent to the nvcc -prec-sqrt flag, except that it defaults to false.

  • No Opt (O0) uses the IPSCCP compiler pass by default, although the IPSCCP pass can be switched off at O0 using the -mllvm -use-ipsccp-nvptx-O0=false flag at the user’s discretion. The reason that the IPSCCP pass is used by default even at O0 is that there is currently an unresolved issue with the nvvm-reflect compiler pass: This pass is used to pick the correct branches depending on the SM version which can be optionally specified by the --cuda-gpu-arch flag. If the arch flag is not specified by the user, the default value, SM 50, is used. Without the execution of the IPSCCP pass at -O0 when using a low SM version, dead instructions which require a higher SM version can remain. Since corresponding issues occur in other backends future work will aim for a universal solution to these issues.

HIP back-end limitations

  • Requires a ROCm compatible operating system, for full details of supported Operating System for ROCm, please refer to the ROCm Supported Operating Systems.

  • Support is still in a beta state, but the backend is being actively developed.

  • Global offsets are currently not supported.

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