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.
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	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	https://github.com/OpenMPToolsInterface/LLVM-openmp/tree/ompd-tests
OMPT	OMPT interfaces	mostly 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	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