OpenMP Support ¶

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: 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.

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

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.

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 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	done	D54441
loop extension	#pragma omp loop (directive)	worked on
loop extension	collapse imperfectly nested loop	done
loop extension	collapse non-rectangular nested loop	done
loop extension	C++ range-base for loop	done
loop extension	clause: if for SIMD directives	done
loop extension	inclusive scan extension (matching C++17 PSTL)	done
memory mangagement	memory allocators	done	r341687,r357929
memory mangagement	allocate directive and allocate clause	done	r355614,r335952
OMPD	OMPD interfaces	not upstream	https://github.com/OpenMPToolsInterface/LLVM-openmp/tree/ompd-tests
OMPT	OMPT interfaces	mostly done
thread affinity extension	thread affinity extension	done
task extension	taskloop reduction	done
task extension	task affinity	not upstream
task extension	clause: depend on the taskwait construct	worked on
task extension	depend objects and detachable tasks	done
task extension	mutexinoutset dependence-type for tasks	done	D53380,D57576
task extension	combined taskloop constructs	done
task extension	master taskloop	done
task extension	parallel master taskloop	done
task extension	master taskloop simd	done
task extension	parallel master taskloop simd	done
SIMD extension	atomic and simd constructs inside SIMD code	done
SIMD extension	SIMD nontemporal	done
device extension	infer target functions from initializers	worked on
device extension	infer target variables from initializers	worked on
device extension	OMP_TARGET_OFFLOAD environment variable	done	D50522
device extension	support full ‘defaultmap’ functionality	done	D69204
device extension	device specific functions	done
device extension	clause: device_type	done
device extension	clause: extended device	done
device extension	clause: uses_allocators clause	done
device extension	clause: in_reduction	worked on	r308768
device extension	omp_get_device_num()	worked on	D54342
device extension	structure mapping of references	unclaimed
device extension	nested target declare	done	D51378
device extension	implicitly map ‘this’ (this[:1])	done	D55982
device extension	allow access to the reference count (omp_target_is_present)	done
device extension	requires directive	partial
device extension	clause: unified_shared_memory	done	D52625,D52359
device extension	clause: unified_address	partial
device extension	clause: reverse_offload	unclaimed parts	D52780
device extension	clause: atomic_default_mem_order	done	D53513
device extension	clause: dynamic_allocators	unclaimed parts	D53079
device extension	user-defined mappers	worked on	D56326,D58638,D58523,D58074,D60972,D59474
device extension	mapping lambda expression	done	D51107
device extension	clause: use_device_addr for target data	done
device extension	support close modifier on map clause	done	D55719,D55892
device extension	teams construct on the host device	done	r371553
device extension	support non-contiguous array sections for target update	done
device extension	pointer attachment	unclaimed
device extension	map clause reordering based on map types	unclaimed
atomic extension	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 extension	array shaping	done	D74144
misc extension	library shutdown (omp_pause_resource[_all])	unclaimed parts	D55078
misc extension	metadirectives	worked on
misc extension	conditional modifier for lastprivate clause	done
misc extension	iterator and multidependences	done
misc extension	depobj directive and depobj dependency kind	done
misc extension	user-defined function variants	worked on	D67294, D64095, D71847, D71830
misc extension	pointer/reference to pointer based array reductions	unclaimed
misc extension	prevent new type definitions in clauses	done
memory model extension	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 contact openmp-dev at lists.llvm.org for more information or if you want to help with the implementation.

Category	Feature	Status	Reviews
atomic extension	‘compare’ and ‘fail’ clauses on atomic construct	unclaimed
base language	C++ attribute specifier syntax	unclaimed
device extension	‘present’ map type modifier	done	D83061, D83062, D84422
device extension	‘present’ motion modifier	done	D84711, D84712
device extension	‘present’ in defaultmap clause	worked on	D92427
device extension	map clause reordering reordering based on ‘present’ modifier	unclaimed
device extension	device-specific environment variables	unclaimed
device extension	omp_target_is_accessible routine	unclaimed
device extension	omp_get_mapped_ptr routine	unclaimed
device extension	new async target memory copy routines	unclaimed
device extension	thread_limit clause on target construct	unclaimed
device extension	has_device_addr clause on target construct	unclaimed
device extension	iterators in map clause or motion clauses	unclaimed
device extension	indirect clause on declare target directive	unclaimed
device extension	allow virtual functions calls for mapped object on device	unclaimed
device extension	interop construct	unclaimed
device extension	assorted routines for querying interoperable properties	unclaimed
loop extension	Loop tiling transformation	worked on	D76342
loop extension	Loop unrolling transformation	unclaimed
loop extension	‘reproducible’/’unconstrained’ modifiers in ‘order’ clause	unclaimed
memory management	alignment extensions for allocate directive and clause	unclaimed
memory management	new memory management routines	unclaimed
memory management	changes to omp_alloctrait_key enum	unclaimed
memory model extension	seq_cst clause on flush construct	unclaimed
misc extension	‘omp_all_memory’ keyword and use in ‘depend’ clause	unclaimed
misc extension	error directive	unclaimed
misc extension	scope construct	unclaimed
misc extension	routines for controlling and querying team regions	unclaimed
misc extension	changes to ompt_scope_endpoint_t enum	unclaimed
misc extension	omp_display_env routine	unclaimed
misc extension	extended OMP_PLACES syntax	unclaimed
misc extension	OMP_NUM_TEAMS and OMP_TEAMS_THREAD_LIMIT env vars	unclaimed
misc extension	‘target_device’ selector in context specifier	unclaimed
misc extension	begin/end declare variant	done	D71179
misc extension	dispatch construct and function variant argument adjustment	unclaimed
misc extension	assume and assumes directives	worked on
misc extension	nothing directive	unclaimed
misc extension	masked construct and related combined constructs	unclaimed
misc extension	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 extension	‘strict’ modifier for taskloop construct	unclaimed
task extension	inoutset in depend clause	unclaimed
task extension	nowait clause on taskwait	unclaimed