Contents
triSYCL is a pure C++17 header runtime library to provide SYCL support on CPU and accelerators.
To target devices, a device compiler based on Clang/LLVM is required to extract the kernel from the SYCL program, compile it to the kernel and generate on the host side some glue to call the kernel.
The main library is defined in ../include/CL/sycl.hpp as defined by the SYCL standard.
All the headers are mainly called in alphabetic order in ../include/CL/sycl.hpp.
More generally all the header files are included in alphabetic order, but with the standard headers first, then Boost libraries and at last the triSYCL headers.
The coding style is similar to the STL or Boost libraries,
with lines with less than 80 characters, to fit on a standard punch
card. :-)
To avoid ODR conflicts with a pure header library, C++17 inline is
used massively.
Each SYCL concept (for example a cl::sycl::queue) is defined in its
own header file (for example ../include/CL/sycl/queue.hpp) so it is
easy to find the definition of a concept.
If a concept requires more implementation details, these are detailed
inside a related detail directory, for example for the queue,
besides the file ../include/CL/sycl/queue.hpp, there might be also
inside ../include/CL/sycl/queue/detail the files
- ../include/CL/sycl/queue/detail/queue.hpp defines an
implementation detail common to the concept of
queuewith a virtual interface; - ../include/CL/sycl/queue/detail/host_queue.hpp inherits from
the
detail::queueabove to implement the host device on CPU; - ../include/CL/sycl/queue/detail/opencl_queue.hpp inherits from
the
detail::queueabove to implement an OpenCL device.
In ../include/CL/sycl/detail there are also some code used by the implementation but not directly related to some SYCL classes.
The documentation of the source-code itself is based on Doxygen.
Doxygen modules are used to group the code elements according to various themes.
To build the Doxygen documentation, in the top directory, run
makethat will produce tmp/Doxygen/SYCL with the API documentation and
tmp/Doxygen/triSYCL with the documented triSYCL implementation
source code.
To publish the documentation on GitHub:
make publishand finish as explained by the make output.
The implementation for CPU is a pure C++17 templated header library and does not require a specific C++ or SYCL compiler.
The dataflow SYCL infrastructure between kernels related by
buffer/accessors dependencies is implemented in
../include/CL/sycl/command_group/detail/task.hpp with plain C++
std::thread and std::condition_variable. It should be updated
to a more efficient library in the future, such as TBB;
All the kernel code is accelerated with OpenMP, with various options
according to some macros parameters. Only the first dimension of
range is parallelized with OpenMP in
../include/CL/sycl/parallelism/detail/parallelism.hpp
Since in SYCL barriers are available and the CPU triSYCL implementation does not use a compiler to restructure the kernel code, it is implemented in SYCL with CPU threads provided by OpenMP. This is massively inefficient.
Anyway, low-level OpenCL-style barriers should not be used in modern SYCL code. Hierarchical parallelism, which is performance portable between device and CPU, is preferable.
Otherwise, using an OpenCL target on CPU can be used to rely on the CPU OpenCL stack to do CPU-friendly SIMD-ization of the barrier-spaghetti code. But this relies on the triSYCL device compiler...
When targeting an accelerator, even if SYCL is a pure C++ DSEL, a specific compiler is required to extract the kernel code and compile it to some target device and at the same time to compile on the host side some glue code around the extraction boundary to transfer data to and from the device and call the kernel itself.
The device compiler is based on Clang/LLVM 3.9 for now.
Since it is quite more experimental than the CPU path, it is not yet merged into the main branches:
- device branch of triSYCL: https://github.com/triSYCL/triSYCL/tree/device
- Clang supporting triSYCL: https://github.com/triSYCL/clang/tree/sycl/release_39/master
- LLVM supporting triSYCL: https://github.com/triSYCL/llvm/tree/sycl/release_39/master
First download or clone the device compiler repositories, for example with:
git clone --branch sycl/release_39/master git@github.com:triSYCL/llvm.git
cd llvm/tools
git clone --branch sycl/release_39/master git@github.com:triSYCL/clang.git
cd ../..Then compile for example with:
mkdir build
cd build
cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=1 \
-DLLVM_ENABLE_CXX1Y=On \
-DCMAKE_BUILD_TYPE=Release \
-DLLVM_TARGETS_TO_BUILD="X86" \
-DLLVM_BUILD_LLVM_DYLIB:BOOL=ON \
-DLLVM_LINK_LLVM_DYLIB:BOOL=ON \
../llvm
# Use -j8 to speed up compilation if you have 8 cores for example
make -j8You might replace the Release by Debug above if you want to
debug the compiler itself. Look at https://llvm.org/docs/CMake.html
for more information.
Compilation and installation of the triSYCL runtime:
git clone --branch device git@github.com:triSYCL/triSYCL.git
# Compile the triSYCL_tool command
cd triSYCL/src
makeUnfortunately there is no driver yet to generate directly the host and device part and it is up to the end-user for now, since it is still experimental and in development. So using the compiler is... painful. :-(
It is expected to be used as for example with examples from
../tests/device_compiler. Everything is done from
../tests/Makefile when making a target ending with the
.kernel_caller extension such as
tests/device_compiler/single_task_vector_add_drt.kernel_caller.
triSYCL assumes some recent Clang/LLVM installed, independently from the one used by device compiler which might not be new enough.
A recent version of Boost is required. It is available with package
libboost-all-dev on Debian/Ubuntu or with some more modern
specific versions such as libboost1.63-all-dev.
The following assumes that you have an OpenCL ICD
installed on the machine, to allow several OpenCL platforms usable at
the same time on the machine. For example the ocl-icd-libopencl1
package on Debian/Ubuntu.
The device compiler generates the kernels as SPIR-df (de-facto), which is SPIR 2.0 encoded with LLVM IR of a more recent version than LLVM 3.4 expected by the SPIR specification. So a very modern SPIR consumer is required, such as a recent PoCL. It is not the version available in Ubuntu 17.10 for example, so you might compile and install PoCL on your own...
Set up the environment:
# Used by the tests Makefile to find the device compiler
export LLVM_BUILD_DIR=<directory_where_LLVM_is_built>
# Use PoCL OpenCL stack
export BOOST_COMPUTE_DEFAULT_PLATFORM='Portable Computing Language'
# Do not use another OpenCL stack if the one requested is not available
export BOOST_COMPUTE_DEFAULT_ENFORCE=1Compile and execute a small example:
cd tests
make -j2 device_compiler/single_task_vector_add_drt.kernel_caller
device_compiler/single_task_vector_add_drt.kernel_caller
[...]
Queue waiting for kernel completion
**** no errors detectedLet's assume you have installed Xilinx SDx somewhere, and probably a
/etc/OpenCL/vendors/xilinx.icd file containing the string
libxilinxopencl.so to have the OpenCL ICD indirection
working.
Initialize the environment with something like:
export XILINX_SDX=/opt/Xilinx/SDx/2017.2
PATH=$PATH:$XILINX_SDX/bin
export LD_LIBRARY_PATH=$XILINX_SDX/runtime/lib/x86_64:$XILINX_SDX/lib/lnx64.o
# Used by the tests Makefile to find the device compiler
export LLVM_BUILD_DIR=<directory_where_LLVM_is_built>
# Use the Xilinx OpenCL stack
export BOOST_COMPUTE_DEFAULT_PLATFORM=Xilinx
# Do not use another OpenCL stack if the one requested is not available
export BOOST_COMPUTE_DEFAULT_ENFORCE=1Compile and execute a small example:
cd tests
make -j2 device_compiler/single_task_vector_add_drt.kernel_caller
device_compiler/single_task_vector_add_drt.kernel_caller
[...]
Queue waiting for kernel completion
**** no errors detectedNote that since the final code contains the FPGA bit-stream configuration file and not the SPIR representation, it takes quite a lot of time to be generated through SDx...
Figure 1: High-level view of the compilation workflow in triSYCL.
When compiling on CPU, since triSYCL relies on the fact that SYCL is a pure C++ executable DSEL, the C++ SYCL code is just compiled with any host compiler (top of Figure 1) which includes the SYCL runtime (bottom left of Figure 1) which is a plain C++ header file. A CPU executable is generated, using OpenMP for multithreading.
If some OpenCL features are used through the interoperability mode (non-single-source SYCL), then an OpenCL library is required to interact with some OpenCL devices.
When using SYCL in single-source mode on device, the compilation flow is quite more complex because it requires a device compiler to split and compile the code for the final target.
The Clang/LLVM-based device compiler (bottom of Figure 1) compiles the C++ SYCL code as for CPU only, but just keep the kernel part of the code and produce a simple portable intermediate representation (SPIR) of the kernels. For now, since SPIR-V is not yet widely used, triSYCL uses SPIR-df de-facto, a non-conforming SPIR 2.0 encoded in something newer than LLVM 3.4 IR.
Then this SPIR-df output is optionally compiled by some vendor compiler to speed-up the launch time by doing some compilation ahead. With PoCL it is not done (dashed arrow line) but for FPGA it is done ahead-of-time since compilation is very slow.
In single-source mode on device, the source code has also to go through the device compiler, but to do the dual operation: to remove the kernel code and just to keep the host code. This is also where some glue to call the kernels and to do the argument serialization is done.
The kernel binary generated by the other compiler flow is also included in the host code so that the main host executable is self-contained and can start the kernel on the device without having to load the binary from an external file.
Figure 2: Low-level view of the compilation workflow in triSYCL.
The real workflow is currently implemented in ../tests/Makefile
and this is the current source of truth. The path to go for example
from a ex.cpp file to a final ex.kernel_caller is summarized
on Figure 2,
Each intermediate file is characterized by a specific extension:
.cpp- for the single-source SYCL C++ input file;
.bc- some LLVM IR bitcode;
.ll- some LLVM IR in textual assembly syntax;
.kernel_caller- for the final host executable, with the kernel binary internalized so the host can load and launch the kernels on the devices without external files.
Note that the file without any extension is actually the normal CPU-only executable, which does not appear in this picture because it is about compiling for device instead.
All the SYCL LLVM passes are in the lib/SYCL directory of LLVM.
The file extensions used on the host side are:
.pre_kernel_caller.ll- the SYCL C++ code compiled by Clang for the host side, including the call of the kernels;
.kernel_caller.ll- the LLVM IR of the host code after the LLVM triSYCL pass transformations;
To generate the .pre_kernel_caller.ll file, the source code is
compiled with:
clang -O3 -syclwhich is basically clang unchanged, but with loop-idiom
detection pass skipped because otherwise it generates some memory copy
intrinsic functions that prevents some argument flattening to work
later.
The -O3 is important to generate optimized minimal code that can
be massaged later, with a lot of in-lining to have the C++ constructs
to disappear. Otherwise less optimized code breaks a lot of
assumptions in the triSYCL-specific LLVM passes later.
The compilation flow to generate the final .kernel_caller.ll file
is based on LLVM opt to apply a sequence of LLVM passes:
-globalopt -deadargelim- to clean-up the code before SYCL massaging;
-SYCL-args-flattening- is a fundamental SYCL-specific pass that takes the lambda capture (basically a C++ structure passed by address) of a SYCL kernel lambda expression and flattens it as its content. So basically if the capture has several scalar and accessor parameters, the structure address used in the function call is replaced by a function call with all the parameters explicitly passed as arguments. This makes the classical OpenCL-style kernel parameter to show up;
-loop-idiom -deadargelim- then the loop-idiom detection pass which was not applied before to
avoid choking the
SYCL-args-flatteningpass can now be applied to optimize some loops and generate the LLVM intrinsics representing memory copies and initialization for example; -deadargelim- removes some dead code that might be left by previous passes;
-SYCL-serialize-argumentsis another fundamental SYCL-specific pass on host side which replaces a kernel function call by some calls to the runtime to select the kernel and serialize all the kernel arguments.
The input code from the triSYCL headers of the form
cl::sycl::detail::set_kernel_task_marker(t); cl::sycl::detail::instantiate_kernel<KernelName>(/* flatten args */);
is replaced by
cl::sycl::drt::set_kernel(detail::task &task, const char *kernel_name, const char *kernel_short_name); // For each parameter call: // either for a scalar argument cl::sycl::drt::serialize_arg(detail::task &task, std::size_t index, void *arg, std::size_t arg_size); // or for an accessor argument cl::sycl::drt::serialize_accessor_arg(detail::task &task, std::size_t index, void *arg, std::size_t arg_size);
The marking functions generated by triSYCL headers are in ../include/CL/sycl/detail/instantiate_kernel.hpp while the functions used by the transformed code are in ../include/CL/sycl/device_runtime.hpp. The functions from
cl::sycl::drt::are the link to the underlying runtime, such as OpenCL.-deadargelim- again to removes some dead code that might be left by previous pass.
The file extensions used on the kernel side are:
.pre_kernel.ll- the SYCL C++ code compiled by Clang for the host side, including the call of the kernels;
.kernel.bc- the LLVM IR of the host code after the LLVM triSYCL pass transformations;
.kernel.bin- is for the kernel binary to be shipped into the final host executable. This is typically a SPIR LLVM IR bitcode or an FPGA bitstream configuration;
.kernel.internalized.cxxis the kernel binary represented as C++ code so it can just be compiled by a C++ compiler to have it internalized into the final host binary and used by the runtime.
It is constructed from the
.kernel.binfile through the helpertriSYCL_tool --source-in.
To generate the .pre_kernel.ll file, the source code is compiled
with:
clang -O3 -DTRISYCL_DEVICE -sycl -sycl-is-deviceThis is similar to the compilation for the host side and the -O3
is important for the same reasons. -DTRISYCL_DEVICE is used so
the triSYCL headers behave slightly differently on the device code,
mainly enabling some address-space related code used to represent
OpenCL global or local memory for example.
Like for the host side path, the compilation flow to generate the
final .kernel.bc file is based on LLVM opt to apply a sequence
of LLVM passes with:
-globalopt -deadargelim -SYCL-args-flattening -deadargelim- are applied as for the host side. It is important to have globally the same code compiled with the same passes for both host and device side to keep the code synchronized before serialization. Otherwise it would lead to some mismatch and some wrong global code at the end;
-SYCL-kernel-filter- this is one of the most important SYCL-specific pass on the device side, to extract the kernels from the single-source code. Actually it works in 2 passes, in a mark-and-sweep approach. Here is the first pass that marks all the kernel with external linkage (tricking the compiler as it might be useful from outside) and all the non-kernel part with internal linkage;
-globaldce- this is the second stage of kernel selection. It will remove all the dead code of the program. Since only the kernels have been marked as potentially used from the outside, after application of this pass, only what is transitively useful for the kernels are left. So only remains the device code;
-RELGCD- compiling C++ comes with an ABI storing the lists of global static
constructors and destructors. Unfortunately even if at the end these
lists are empty because of SYCL specification, they are not removed
by
-globaldceand it is not supported by SPIR yet. So this SYCL-specific pass removes the empty list of global constructors or destructors (RELGCD); -reqd-workgroup-size-1- in the case the kernel are compiled with only 1 SPIR work-group with 1 work-item (common use case on FPGA), this SYCL-specific pass add a SPIR metadata on the kernels to specify it will be called with only 1 work-item. This way the target compiler can spare some resources on the device;
-inSPIRation- is the SYCL-specific pass generating the SPIR 2.0-style LLVM IR output. Since it generates LLVM IR with the version of the recent LLVM used, it is quite more modern that the official SPIR 2.0 based on LLVM 3.4 IR. So it is a SPIR "de-facto", which is nevertheless accepted by some tools. But by using a down-caster, it could probably make some decent official SPIR 2.0 encoded in LLVM 3.4 IR). Or using a SPIR-V back-end could generate some SPIR-V code.
Look at testing.rst and ../tests/README.rst
Travis CI is used to validate triSYCL with its test suite from tests/ on CPU and OpenCL with interoperability mode, using CMake
ctest.
The device compiler is not tested yet through Travis CI.
Look at ../.travis.yml and ../Dockerfile for the configuration.