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 ¶

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	assumes directives	worked on
misc	assume directive	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