OpenMP Support

Clang fully supports OpenMP 4.5. Clang supports offloading to X86_64, AArch64, PPC64[LE] and has basic support for Cuda devices.

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.

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

General improvements

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 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.

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.
  • 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 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) worked on  
loop #pragma omp loop bind worked on  
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 not upstream
OMPT OMPT interfaces mostly done  
thread affinity thread affinity done  
task taskloop reduction done  
task task affinity not upstream
task clause: depend on the taskwait construct mostly 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 worked on  
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() worked on D54342
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 partial  
device clause: unified_shared_memory done D52625,D52359
device clause: unified_address partial  
device clause: reverse_offload unclaimed parts D52780
device clause: atomic_default_mem_order done D53513
device clause: dynamic_allocators unclaimed parts D53079
device user-defined mappers worked on 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 unclaimed  
device map clause reordering based on map types unclaimed  
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]) unclaimed parts D55078
misc metadirectives worked on 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 worked on D67294, D64095, D71847, D71830, D109635
misc pointer/reference to pointer based array reductions unclaimed  
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, as defined in the technical report 8 (TR8). 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  
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 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  
device new async target memory copy routines unclaimed  
device thread_limit clause on target construct worked on  
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 unclaimed  
device interop construct partial parsing/sema done: D98558, D98834, D98815
device assorted routines for querying interoperable properties unclaimed  
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 worked on  
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 unclaimed  
misc scope construct unclaimed  
misc routines for controlling and querying team regions unclaimed  
misc changes to ompt_scope_endpoint_t enum unclaimed  
misc omp_display_env routine unclaimed  
misc extended OMP_PLACES syntax unclaimed  
misc OMP_NUM_TEAMS and OMP_TEAMS_THREAD_LIMIT env vars unclaimed  
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 worked on  
misc masked construct and related combined constructs worked on D99995, D100514
misc default(firstprivate) & default(private) partial firstprivate done: D75591
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 unclaimed  
task ‘strict’ modifier for taskloop construct unclaimed  
task inoutset in depend clause unclaimed  
task nowait clause on taskwait worked on  

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