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ReStructuredText
271 lines
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ReStructuredText
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:local:
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OpenMP Support
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==============
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Clang fully supports OpenMP 4.5. Clang supports offloading to X86_64, AArch64,
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PPC64[LE] and has `basic support for Cuda devices`_.
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* #pragma omp declare simd: :part:`Partial`. We support parsing/semantic
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analysis + generation of special attributes for X86 target, but still
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missing the LLVM pass for vectorization.
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In addition, the LLVM OpenMP runtime `libomp` supports the OpenMP Tools
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Interface (OMPT) on x86, x86_64, AArch64, and PPC64 on Linux, Windows, and macOS.
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For the list of supported features from OpenMP 5.0 see `OpenMP implementation details`_.
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General improvements
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====================
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- New collapse clause scheme to avoid expensive remainder operations.
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Compute loop index variables after collapsing a loop nest via the
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collapse clause by replacing the expensive remainder operation with
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multiplications and additions.
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- The default schedules for the `distribute` and `for` constructs in a
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parallel region and in SPMD mode have changed to ensure coalesced
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accesses. For the `distribute` construct, a static schedule is used
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with a chunk size equal to the number of threads per team (default
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value of threads or as specified by the `thread_limit` clause if
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present). For the `for` construct, the schedule is static with chunk
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size of one.
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- Simplified SPMD code generation for `distribute parallel for` when
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the new default schedules are applicable.
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- When using the collapse clause on a loop nest the default behavior
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is to automatically extend the representation of the loop counter to
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64 bits for the cases where the sizes of the collapsed loops are not
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known at compile time. To prevent this conservative choice and use
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at most 32 bits, compile your program with the
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`-fopenmp-optimistic-collapse`.
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.. _basic support for Cuda devices:
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Cuda devices support
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====================
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Directives execution modes
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--------------------------
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Clang code generation for target regions supports two modes: the SPMD and
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non-SPMD modes. Clang chooses one of these two modes automatically based on the
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way directives and clauses on those directives are used. The SPMD mode uses a
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simplified set of runtime functions thus increasing performance at the cost of
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supporting some OpenMP features. The non-SPMD mode is the most generic mode and
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supports all currently available OpenMP features. The compiler will always
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attempt to use the SPMD mode wherever possible. SPMD mode will not be used if:
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- The target region contains user code (other than OpenMP-specific
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directives) in between the `target` and the `parallel` directives.
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Data-sharing modes
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------------------
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Clang supports two data-sharing models for Cuda devices: `Generic` and `Cuda`
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modes. The default mode is `Generic`. `Cuda` mode can give an additional
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performance and can be activated using the `-fopenmp-cuda-mode` flag. In
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`Generic` mode all local variables that can be shared in the parallel regions
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are stored in the global memory. In `Cuda` mode local variables are not shared
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between the threads and it is user responsibility to share the required data
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between the threads in the parallel regions.
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Features not supported or with limited support for Cuda devices
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---------------------------------------------------------------
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- Cancellation constructs are not supported.
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- Doacross loop nest is not supported.
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- User-defined reductions are supported only for trivial types.
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- Nested parallelism: inner parallel regions are executed sequentially.
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- Static linking of libraries containing device code is not supported yet.
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- Automatic translation of math functions in target regions to device-specific
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math functions is not implemented yet.
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- Debug information for OpenMP target regions is supported, but sometimes it may
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be required to manually specify the address class of the inspected variables.
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In some cases the local variables are actually allocated in the global memory,
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but the debug info may be not aware of it.
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.. _OpenMP implementation details:
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OpenMP 5.0 Implementation Details
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=================================
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The following table provides a quick overview over various OpenMP 5.0 features
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and their implementation status. Please contact *openmp-dev* at
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*lists.llvm.org* for more information or if you want to help with the
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implementation.
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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|Category | Feature | Status | Reviews |
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+==============================+==============================================================+==========================+=======================================================================+
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| loop extension | support != in the canonical loop form | :good:`done` | D54441 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| loop extension | #pragma omp loop (directive) | :part:`worked on` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| loop extension | collapse imperfectly nested loop | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| loop extension | collapse non-rectangular nested loop | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| loop extension | C++ range-base for loop | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| loop extension | clause: if for SIMD directives | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| loop extension | inclusive scan extension (matching C++17 PSTL) | :none:`claimed` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| memory mangagement | memory allocators | :good:`done` | r341687,r357929 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| memory mangagement | allocate directive and allocate clause | :good:`done` | r355614,r335952 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| OMPD | OMPD interfaces | :part:`not upstream` | https://github.com/OpenMPToolsInterface/LLVM-openmp/tree/ompd-tests |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| OMPT | OMPT interfaces | :part:`mostly done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| thread affinity extension | thread affinity extension | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| task extension | taskloop reduction | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| task extension | task affinity | :part:`not upstream` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| task extension | clause: depend on the taskwait construct | :part:`worked on` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| task extension | depend objects and detachable tasks | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| task extension | mutexinoutset dependence-type for tasks | :good:`done` | D53380,D57576 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| task extension | combined taskloop constructs | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| task extension | master taskloop | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| task extension | parallel master taskloop | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| task extension | master taskloop simd | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| task extension | parallel master taskloop simd | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| SIMD extension | atomic and simd constructs inside SIMD code | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| SIMD extension | SIMD nontemporal | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | infer target functions from initializers | :part:`worked on` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | infer target variables from initializers | :part:`worked on` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | OMP_TARGET_OFFLOAD environment variable | :good:`done` | D50522 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | support full 'defaultmap' functionality | :good:`done` | D69204 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | device specific functions | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | clause: device_type | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | clause: extended device | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | clause: uses_allocators clause | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | clause: in_reduction | :part:`worked on` | r308768 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | omp_get_device_num() | :part:`worked on` | D54342 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | structure mapping of references | :none:`unclaimed` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | nested target declare | :good:`done` | D51378 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | implicitly map 'this' (this[:1]) | :good:`done` | D55982 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | allow access to the reference count (omp_target_is_present) | :part:`worked on` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | requires directive | :part:`partial` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | clause: unified_shared_memory | :good:`done` | D52625,D52359 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | clause: unified_address | :part:`partial` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | clause: reverse_offload | :none:`unclaimed parts` | D52780 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | clause: atomic_default_mem_order | :good:`done` | D53513 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | clause: dynamic_allocators | :part:`unclaimed parts` | D53079 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | user-defined mappers | :part:`worked on` | D56326,D58638,D58523,D58074,D60972,D59474 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | mapping lambda expression | :good:`done` | D51107 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | clause: use_device_addr for target data | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | support close modifier on map clause | :good:`done` | D55719,D55892 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | teams construct on the host device | :part:`worked on` | Clang part is done, r371553. |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | support non-contiguous array sections for target update | :part:`worked on` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| device extension | pointer attachment | :none:`unclaimed` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| atomic extension | hints for the atomic construct | :good:`done` | D51233 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| base language | C11 support | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| base language | C++11/14/17 support | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| base language | lambda support | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| misc extension | array shaping | :good:`done` | D74144 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| misc extension | library shutdown (omp_pause_resource[_all]) | :none:`unclaimed parts` | D55078 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| misc extension | metadirectives | :part:`worked on` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| misc extension | conditional modifier for lastprivate clause | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| misc extension | iterator and multidependences | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| misc extension | depobj directive and depobj dependency kind | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| misc extension | user-defined function variants | :part:`worked on` | D67294, D64095, D71847, D71830 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| misc extension | pointer/reference to pointer based array reductions | :none:`unclaimed` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| misc extension | prevent new type definitions in clauses | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| memory model extension | memory model update (seq_cst, acq_rel, release, acquire,...) | :good:`done` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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OpenMP 5.1 Implementation Details
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=================================
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The following table provides a quick overview over various OpenMP 5.1 features
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and their implementation status, as defined in the technical report 8 (TR8).
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Please contact *openmp-dev* at *lists.llvm.org* for more information or if you
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want to help with the implementation.
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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|Category | Feature | Status | Reviews |
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+==============================+==============================================================+==========================+=======================================================================+
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| misc extension | user-defined function variants with #ifdef protection | :part:`worked on` | D71179 |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| misc extension | default(firstprivate) & default(private) | :part:`worked on` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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| loop extension | Loop tiling transformation | :part:`claimed` | |
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+------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
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