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