OpenMP Support — Clang 18.1.8 documentation

# Clang 18.1.8 documentation

## OpenMP Support

Clang fully supports OpenMP 4.5, almost all of 5.0 and most of 5.1/2. Clang supports offloading to X86_64, AArch64, PPC64[LE], NVIDIA GPUs (all models) and AMD GPUs (all models).

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. OMPT is also supported for NVIDIA and AMD GPUs.

For the list of supported features from OpenMP 5.0 and 5.1 see OpenMP implementation details and OpenMP 51 implementation details.

## 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.

- 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.

## GPU devices support

### 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. Often, the optimizer is able to reduce the cost of Generic mode to the level of Cuda mode, but the flag, as well as other assumption flags, can be used for tuning.

### 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.

- 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 5.0 Implementation Details

The following table provides a quick overview over various OpenMP 5.0 features and their implementation status. Please post on the Discourse forums (Runtimes - OpenMP category) for more information or if you want to help with the implementation.

Category | Feature | Status | Reviews
--- | --- | --- | ---
loop | support != in the canonical loop form | done | D54441
loop | #pragma omp loop (directive) | partial | D145823 (combined forms)
loop | #pragma omp loop bind | worked on | D144634 (needs review)
loop | collapse imperfectly nested loop | done |
loop | collapse non-rectangular nested loop | done |
loop | C++ range-base for loop | done |
loop | clause: if for SIMD directives | done |
loop | inclusive scan (matching C++17 PSTL) | done |
memory management | memory allocators | done | r341687,r357929
memory management | allocate directive and allocate clause | done | r355614,r335952
OMPD | OMPD interfaces | done | https://reviews.llvm.org/D99914 (Supports only HOST(CPU) and Linux
OMPT | OMPT interfaces (callback support) | done |
thread affinity | thread affinity | done |
task | taskloop reduction | done |
task | task affinity | not upstream | https://github.com/jklinkenberg/openmp/tree/task-affinity
task | clause: depend on the taskwait construct | done | D113540 (regular codegen only)
task | depend objects and detachable tasks | done |
task | mutexinoutset dependence-type for tasks | done | D53380,D57576
task | combined taskloop constructs | done |
task | master taskloop | done |
task | parallel master taskloop | done |
task | master taskloop simd | done |
task | parallel master taskloop simd | done |
SIMD | atomic and simd constructs inside SIMD code | done |
SIMD | SIMD nontemporal | done |
device | infer target functions from initializers | worked on |
device | infer target variables from initializers | done | D146418
device | OMP_TARGET_OFFLOAD environment variable | done | D50522
device | support full ‘defaultmap’ functionality | done | D69204
device | device specific functions | done |
device | clause: device_type | done |
device | clause: extended device | done |
device | clause: uses_allocators clause | done |
device | clause: in_reduction | worked on | r308768
device | omp_get_device_num() | done | D54342,D128347
device | structure mapping of references | unclaimed |
device | nested target declare | done | D51378
device | implicitly map ‘this’ (this[:1]) | done | D55982
device | allow access to the reference count (omp_target_is_present) | done |
device | requires directive | done |
device | clause: unified_shared_memory | done | D52625,D52359
device | clause: unified_address | partial |
device | clause: reverse_offload | partial | D52780,D155003
device | clause: atomic_default_mem_order | done | D53513
device | clause: dynamic_allocators | unclaimed parts | D53079
device | user-defined mappers | done | D56326,D58638,D58523,D58074,D60972,D59474
device | mapping lambda expression | done | D51107
device | clause: use_device_addr for target data | done |
device | support close modifier on map clause | done | D55719,D55892
device | teams construct on the host device | done | r371553
device | support non-contiguous array sections for target update | done |
device | pointer attachment | done |
atomic | hints for the atomic construct | done | D51233
base language | C11 support | done |
base language | C++11/14/17 support | done |
base language | lambda support | done |
misc | array shaping | done | D74144
misc | library shutdown (omp_pause_resource[_all]) | done | D55078
misc | metadirectives | mostly done | D91944
misc | conditional modifier for lastprivate clause | done |
misc | iterator and multidependences | done |
misc | depobj directive and depobj dependency kind | done |
misc | user-defined function variants | done. | D67294, D64095, D71847, D71830, D109635
misc | pointer/reference to pointer based array reductions | done |
misc | prevent new type definitions in clauses | done |
memory model | memory model update (seq_cst, acq_rel, release, acquire,…) | done |

## OpenMP 5.1 Implementation Details

The following table provides a quick overview over various OpenMP 5.1 features and their implementation status. Please post on the Discourse forums (Runtimes - OpenMP category) for more information or if you want to help with the implementation.

Category | Feature | Status | Reviews
--- | --- | --- | ---
atomic | ‘compare’ clause on atomic construct | done | D120290, D120007, D118632, D120200, D116261, D118547, D116637
atomic | ‘fail’ clause on atomic construct | worked on | D123235 (in progress)
base language | C++ attribute specifier syntax | done | D105648
device | ‘present’ map type modifier | done | D83061, D83062, D84422
device | ‘present’ motion modifier | done | D84711, D84712
device | ‘present’ in defaultmap clause | done | D92427
device | map clause reordering based on ‘present’ modifier | unclaimed |
device | device-specific environment variables | unclaimed |
device | omp_target_is_accessible routine | unclaimed |
device | omp_get_mapped_ptr routine | done | D141545
device | new async target memory copy routines | done | D136103
device | thread_limit clause on target construct | partial | D141540 (offload), D152054 (host, in progress)
device | has_device_addr clause on target construct | unclaimed |
device | iterators in map clause or motion clauses | unclaimed |
device | indirect clause on declare target directive | unclaimed |
device | allow virtual functions calls for mapped object on device | partial |
device | interop construct | partial | parsing/sema done: D98558, D98834, D98815
device | assorted routines for querying interoperable properties | partial | D106674
loop | Loop tiling transformation | done | D76342
loop | Loop unrolling transformation | done | D99459
loop | ‘reproducible’/’unconstrained’ modifiers in ‘order’ clause | partial | D127855
memory management | alignment for allocate directive and clause | done | D115683
memory management | new memory management routines | unclaimed |
memory management | changes to omp_alloctrait_key enum | unclaimed |
memory model | seq_cst clause on flush construct | unclaimed |
misc | ‘omp_all_memory’ keyword and use in ‘depend’ clause | done | D125828, D126321
misc | error directive | done | D139166
misc | scope construct | worked on | D157933
misc | routines for controlling and querying team regions | partial | D95003 (libomp only)
misc | changes to ompt_scope_endpoint_t enum | unclaimed |
misc | omp_display_env routine | done | D74956
misc | extended OMP_PLACES syntax | unclaimed |
misc | OMP_NUM_TEAMS and OMP_TEAMS_THREAD_LIMIT env vars | done | D138769
misc | ‘target_device’ selector in context specifier | unclaimed |
misc | begin/end declare variant | done | D71179
misc | dispatch construct and function variant argument adjustment | worked on | D99537, D99679
misc | assume and assumes directives | worked on |
misc | nothing directive | done | D123286
misc | masked construct and related combined constructs | worked on | D99995, D100514
misc | default(firstprivate) & default(private) | done | D75591 (firstprivate), D125912 (private)
other | deprecating master construct | unclaimed |
OMPT | new barrier types added to ompt_sync_region_t enum | unclaimed |
OMPT | async data transfers added to ompt_target_data_op_t enum | unclaimed |
OMPT | new barrier state values added to ompt_state_t enum | unclaimed |
OMPT | new ‘emi’ callbacks for external monitoring interfaces | done |
OMPT | device tracing interface | unclaimed |
task | ‘strict’ modifier for taskloop construct | unclaimed |
task | inoutset in depend clause | done | D97085, D118383
task | nowait clause on taskwait | partial | parsing/sema done: D131830, D141531

## OpenMP Extensions

The following table provides a quick overview over various OpenMP extensions and their implementation status. These extensions are not currently defined by any standard, so links to associated LLVM documentation are provided. As these extensions mature, they will be considered for standardization. Please post on the Discourse forums (Runtimes - OpenMP category) to provide feedback.

Category | Feature | Status | Reviews
--- | --- | --- | ---
atomic extension | ‘atomic’ strictly nested within ‘teams’ | prototyped | D126323
device extension | ‘ompx_hold’ map type modifier | prototyped | D106509, D106510