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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 {ref}OpenMP implementation details <openmp-implementation-details> and {ref}OpenMP 51 implementation details <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-implementation-details)=

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	{good}done	D54441
loop	#pragma omp loop (directive)	{part}partial	D145823 (combined forms)
loop	#pragma omp loop bind	{part}worked on	D144634 (needs review)
loop	collapse imperfectly nested loop	{good}done
loop	collapse non-rectangular nested loop	{good}done
loop	C++ range-base for loop	{good}done
loop	clause: if for SIMD directives	{good}done
loop	inclusive scan (matching C++17 PSTL)	{good}done
memory management	memory allocators	{good}done	r341687,r357929
memory management	allocate directive and allocate clause	{good}done	r355614,r335952
OMPD	OMPD interfaces	{good}done	D99914 (Supports only HOST(CPU) and Linux
OMPT	OMPT interfaces (callback support)	{good}done
thread affinity	thread affinity	{good}done
task	taskloop reduction	{good}done
task	task affinity	{part}not upstream	https://github.com/jklinkenberg/openmp/tree/task-affinity
task	clause: depend on the taskwait construct	{good}done	D113540 (regular codegen only)
task	depend objects and detachable tasks	{good}done
task	mutexinoutset dependence-type for tasks	{good}done	D53380,D57576
task	combined taskloop constructs	{good}done
task	master taskloop	{good}done
task	parallel master taskloop	{good}done
task	master taskloop simd	{good}done
task	parallel master taskloop simd	{good}done
SIMD	atomic and simd constructs inside SIMD code	{good}done
SIMD	SIMD nontemporal	{good}done
device	infer target functions from initializers	{part}worked on
device	infer target variables from initializers	{good}done	D146418
device	OMP_TARGET_OFFLOAD environment variable	{good}done	D50522
device	support full ‘defaultmap’ functionality	{good}done	D69204
device	device specific functions	{good}done
device	clause: device_type	{good}done
device	clause: extended device	{good}done
device	clause: uses_allocators clause	{good}done	PR157025
device	clause: in_reduction	{part}worked on	r308768
device	omp_get_device_num()	{good}done	D54342,D128347
device	structure mapping of references	{none}unclaimed
device	nested target declare	{good}done	D51378
device	implicitly map ‘this’ (this[:1])	{good}done	D55982
device	allow access to the reference count (omp_target_is_present)	{good}done
device	requires directive	{good}done
device	clause: unified_shared_memory	{good}done	D52625,D52359
device	clause: unified_address	{part}partial
device	clause: reverse_offload	{part}partial	D52780,D155003
device	clause: atomic_default_mem_order	{good}done	D53513
device	clause: dynamic_allocators	{part}unclaimed parts	D53079
device	user-defined mappers	{good}done	D56326,D58638,D58523,D58074,D60972,D59474
device	map array-section with implicit mapper	{good}done	PR101101
device	mapping lambda expression	{good}done	D51107
device	clause: use_device_addr for target data	{good}done
device	support close modifier on map clause	{good}done	D55719,D55892
device	teams construct on the host device	{good}done	r371553
device	support non-contiguous array sections for target update	{good}done	PR144635
device	pointer attachment	{part}being repaired	@abhinavgaba (PR153683)
atomic	hints for the atomic construct	{good}done	D51233
base language	C11 support	{good}done
base language	C++11/14/17 support	{good}done
base language	lambda support	{good}done
misc	array shaping	{good}done	D74144
misc	library shutdown (omp_pause_resource[_all])	{good}done	D55078
misc	metadirectives	{part}mostly done	D91944, PR128640
misc	conditional modifier for lastprivate clause	{good}done
misc	iterator and multidependences	{good}done
misc	depobj directive and depobj dependency kind	{good}done
misc	user-defined function variants	{good}done.	D67294, D64095, D71847, D71830, D109635
misc	pointer/reference to pointer based array reductions	{good}done
misc	prevent new type definitions in clauses	{good}done
memory model	memory model update (seq_cst, acq_rel, release, acquire,...)	{good}done

(openmp-51-implementation-details)=

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	{good}done	D120290, D120007, D118632, D120200, D116261, D118547, D116637
atomic	‘fail’ clause on atomic construct	{part}worked on	D123235 (in progress)
base language	C++ attribute specifier syntax	{good}done	D105648
device	‘present’ map type modifier	{good}done	D83061, D83062, D84422
device	‘present’ motion modifier	{good}done	D84711, D84712
device	‘present’ in defaultmap clause	{good}done	D92427
device	map clause reordering based on ‘present’ modifier	{none}unclaimed
device	device-specific environment variables	{none}unclaimed
device	omp_target_is_accessible routine	{good}done	PR138294
device	omp_get_mapped_ptr routine	{good}done	D141545
device	new async target memory copy routines	{good}done	D136103
device	thread_limit clause on target construct	{part}partial	D141540 (offload), D152054 (host, in progress)
device	has_device_addr clause on target construct	{none}unclaimed
device	use_device_ptr/addr preserve host address when lookup fails	{good}done	PR174659
device	iterators in map clause or motion clauses	{good}done	PR159112
device	indirect clause on declare target directive	{part}In Progress
device	allow virtual functions calls for mapped object on device	{part}partial
device	interop construct	{part}partial	parsing/sema done: D98558, D98834, D98815
device	assorted routines for querying interoperable properties	{part}partial	D106674
loop	Loop tiling transformation	{good}done	D76342
loop	Loop unrolling transformation	{good}done	D99459
loop	‘reproducible’/‘unconstrained’ modifiers in ‘order’ clause	{part}partial	D127855
memory management	alignment for allocate directive and clause	{good}done	D115683
memory management	‘allocator’ modifier for allocate clause	{good}done	PR114883
memory management	‘align’ modifier for allocate clause	{good}done	PR121814
memory management	new memory management routines	{none}unclaimed
memory management	changes to omp_alloctrait_key enum	{none}unclaimed
memory model	seq_cst clause on flush construct	{good}done	PR114072
misc	‘omp_all_memory’ keyword and use in ‘depend’ clause	{good}done	D125828, D126321
misc	error directive	{good}done	D139166
misc	scope construct	{good}done	D157933, PR109197
misc	routines for controlling and querying team regions	{part}partial	D95003 (libomp only)
misc	omp_display_env routine	{good}done	D74956
misc	extended OMP_PLACES syntax	{none}unclaimed
misc	OMP_NUM_TEAMS and OMP_TEAMS_THREAD_LIMIT env vars	{good}done	D138769
misc	‘target_device’ selector in context specifier	{part}worked on
misc	begin/end declare variant	{good}done	D71179
misc	dispatch construct and function variant argument adjustment	{part}worked on	D99537, D99679
misc	assumes directives	{part}worked on
misc	assume directive	{good}done
misc	nothing directive	{good}done	D123286
misc	masked construct and related combined constructs	{good}done	D99995, D100514, PR121741 (parallel_masked_taskloop) PR121746 (parallel_masked_task_loop_simd), PR121914 (masked_taskloop) PR121916 (masked_taskloop_simd)
misc	default(firstprivate) & default(private)	{good}done	D75591 (firstprivate), D125912 (private)
other	deprecating master construct	{none}unclaimed
OMPT	changes to ompt_scope_endpoint_t enum	{good}done	D90752
OMPT	new barrier types added to ompt_sync_region_t enum	{good}done	D90752
OMPT	async data transfers added to ompt_target_data_op_t enum	{good}done	D90752
OMPT	new barrier state values added to ompt_state_t enum	{good}done	D90752
OMPT	new ‘emi’ callbacks for external monitoring interfaces	{good}done
OMPT	device tracing interface	{part}in progress	jplehr
task	‘strict’ modifier for taskloop construct	{none}unclaimed
task	inoutset in depend clause	{good}done	D97085, D118383
task	nowait clause on taskwait	{part}partial	parsing/sema done: D131830, D141531

(openmp-5-2-implementation-details)=

OpenMP 5.2 Implementation Details

The following table provides a quick overview of various OpenMP 5.2 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.

Feature	C/C++ Status	Fortran Status	Reviews
omp_in_explicit_task()	{none}unclaimed	{none}unclaimed
semantics of explicit_task_var and implicit_task_var	{none}unclaimed	{none}unclaimed
ompx sentinel for C/C++ directive extensions	{none}unclaimed	{none}unclaimed
ompx prefix for clause extensions	{none}unclaimed	{none}unclaimed
if clause on teams construct	{none}unclaimed	{none}unclaimed
step modifier added	{none}unclaimed	{none}unclaimed
declare mapper: Add iterator modifier on map clause	{none}unclaimed	{none}unclaimed
declare mapper: Add iterator modifier on map clause	{none}unclaimed	{none}unclaimed
memspace and traits modifiers to uses allocator i	{none}unclaimed	{none}unclaimed
Add otherwise clause to metadirectives	{none}unclaimed	{none}unclaimed
doacross clause with support for omp_cur_iteration	{none}unclaimed	{none}unclaimed
position of interop_type in init clause on iterop	{none}unclaimed	{none}unclaimed
implicit map type for target enter/exit data	{none}unclaimed	{none}unclaimed
work OMPT type for work-sharing loop constructs	{good}done	{good}done	PR189347, PR97429
extend ompt_dispatch_t for new enum values	{good}done	{good}done	D122107
allocate and firstprivate on scope directive	{none}unclaimed	{none}unclaimed
Change loop consistency for order clause	{none}unclaimed	{none}unclaimed
Add memspace and traits modifiers to uses_allocators	{none}unclaimed	{none}unclaimed
Keep original base pointer on map w/o matched candidate	{none}unclaimed	{none}unclaimed
Pure procedure support for certain directives	{none}N/A	{none}unclaimed
ALLOCATE statement support for allocators	{none}N/A	{none}unclaimed
dispatch construct extension to support end directive	{none}N/A	{none}unclaimed

(openmp-5-2-deprecations)=

OpenMP 5.2 Deprecations

	C/C++ Status	Fortran Status	Reviews
Linear clause syntax	{none}unclaimed	{none}unclaimed
The minus operator	{none}unclaimed	{none}unclaimed
Map clause modifiers without commas	{none}unclaimed	{none}unclaimed
The use of allocate directives with ALLOCATE statement	{good}N/A	{none}unclaimed
uses_allocators list syntax	{none}unclaimed	{none}unclaimed
The default clause on metadirectives	{none}unclaimed	{none}unclaimed
The delimited form of the declare target directive	{none}unclaimed	{good}N/A
The use of the to clause on the declare target directive	{none}unclaimed	{none}unclaimed
The syntax of the destroy clause on the depobj construct	{none}unclaimed	{none}unclaimed
keyword source and sink as task-dependence modifiers	{none}unclaimed	{none}unclaimed
interop types in any position on init clause of interop	{none}unclaimed	{none}unclaimed
ompd prefix usage for some ICVs	{none}unclaimed	{none}unclaimed

(openmp-6-0-implementation-details)=

OpenMP 6.0 Implementation Details

The following table provides a quick overview of various OpenMP 6.0 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.

Feature	C/C++ Status	Fortran Status	Reviews
free-agent threads	{none}unclaimed	{none}unclaimed
threadset clause	{part}partial	{none}unclaimed	Parse/Sema/Codegen: PR13580
Recording of task graphs	{part}in progress	{part}in progress	clang: jtb20, flang: kparzysz
Parallel inductions	{none}unclaimed	{none}unclaimed
init_complete for scan directive	{none}unclaimed	{none}unclaimed
loop interchange transformation	{good}done	{none}unclaimed	Clang (interchange): PR93022 Clang (permutation): PR92030
loop reverse transformation	{good}done	{none}unclaimed	PR92916
loop stripe transformation	{good}done	{none}unclaimed	PR119891
loop fusion transformation	{part}in progress	{good}done	PR139293 PR161213 PR168898
loop index set splitting transformation with count clause	{part}in progress	{none}unclaimed	@amitamd7
loop transformation apply clause	{none}unclaimed	{none}unclaimed
loop fuse transformation	{good}done	{none}unclaimed
workdistribute construct		{none}in progress	@skc7, @mjklemm
task_iteration	{none}unclaimed	{none}unclaimed
memscope clause for atomic and flush	{none}unclaimed	{none}unclaimed
transparent clause (hull tasks)	{part}partial	{none}unclaimed	Clang parsing/sema PR174646
rule-based compound directives	{part}In Progress	{part}In Progress	kparzysz Testing for Fortran missing
C23, C++23	{none}unclaimed
Fortran 2023		{none}unclaimed
decl attribute for declarative directives	{none}unclaimed	{none}unclaimed
C attribute syntax	{none}unclaimed
pure directives in DO CONCURRENT		{none}unclaimed
Optional argument for all clauses	{part}partial	{part}In Progress	Parse/Sema (nowait): PR159628
Function references for locator list items	{none}unclaimed	{none}unclaimed
All clauses accept directive name modifier	{none}unclaimed	{none}unclaimed
Extensions to depobj construct	{none}unclaimed	{none}unclaimed
Extensions to atomic construct	{none}unclaimed	{none}unclaimed
Private reductions	{part}mostly	{none}unclaimed	Parse/Sema: PR129938 Codegen: PR134709
Self maps	{part}partial	{none}unclaimed	parsing/sema done: PR129888
Release map type for declare mapper	{none}unclaimed	{none}unclaimed
Extensions to interop construct	{none}unclaimed	{none}unclaimed
no_openmp_constructs	{good}done	{none}unclaimed	PR125933
safe_sync and progress with identifier and API	{none}unclaimed	{none}unclaimed
OpenMP directives in concurrent loop regions	{good}done	{none}unclaimed	PR125621
atomics constructs on concurrent loop regions	{good}done	{none}unclaimed	PR125621
Loop construct with DO CONCURRENT		{part}In Progress
device_type clause for target construct	{none}unclaimed	{none}unclaimed
nowait for ancestor target directives	{none}unclaimed	{none}unclaimed
New API for devices' num_teams/thread_limit	{none}unclaimed	{none}unclaimed
Host and device environment variables	{part}in progress	{none}unclaimed	@amitamd7
num_threads ICV and clause accepts list	{none}unclaimed	{none}unclaimed
Numeric names for environment variables	{none}unclaimed	{none}unclaimed
Increment between places for OMP_PLACES	{none}unclaimed	{none}unclaimed
OMP_AVAILABLE_DEVICES envirable	{none}unclaimed	{none}unclaimed	(should wait for “Traits for default device envirable” being done)
Traits for default device envirable	{part}in progress	{none}unclaimed	ro-i
Optionally omit array length expression	{good}done	{none}unclaimed	(Parse) PR148048, (Sema) PR152786
Canonical loop sequences	{part}in progress	{part}in progress	Clang: PR139293
Clarifications to Fortran map semantics	{none}unclaimed	{none}unclaimed
default clause at target construct	{good}done	{none}unclaimed	PR162910
ref count update use_device_{ptr, addr}	{none}unclaimed	{none}unclaimed
Clarifications to implicit reductions	{none}unclaimed	{none}unclaimed
ref modifier for map clauses	{part}In Progress	{none}unclaimed
map-type modifiers in arbitrary position	{good}done	{none}unclaimed	PR90499
Lift nesting restriction on concurrent loop	{good}done	{none}unclaimed	PR125621
priority clause for target constructs	{none}unclaimed	{none}unclaimed
changes to target_data construct	{none}unclaimed	{none}unclaimed
Non-const do_not_sync for nowait/nogroup	{none}unclaimed	{none}unclaimed
need_device_addr modifier for adjust_args clause	{part}partial	{none}unclaimed	Parsing/Sema: PR143442 PR149586
need_device_ptr modifier for adjust_args clause	{none}unclaimed	{none}unclaimed
Prescriptive num_threads	{good}done	{none}unclaimed	PR160659 PR146403 PR146404 PR146405
Message and severity clauses	{good}done	{none}unclaimed	PR146093
Local clause on declare target	{good}done	{none}unclaimed	clang Parse/Sema: PR186281 clang Codegen: PR196431
groupprivate directive	{part}In Progress	{part}partial	Flang: kparzysz, mjklemm Flang parser: PR153807 Flang sema: PR154779 Clang parse/sema: PR158134
variable-category on default clause	{good}done	{none}unclaimed
Changes to omp_target_is_accessible	{part}In Progress	{part}In Progress
defaultmap implicit-behavior ‘storage’	{good}done	{none}unclaimed	PR158336
defaultmap implicit-behavior ‘private’	{good}done	{none}unclaimed	PR158712
OMPT: ompt_get_buffer_limits entry point	{part}partial	{good}N/A	Definition: PR195829
OMPT: ompt_any_record_ompt_t for device tracing	{good}done	{good}N/A	PR195829
OMPT: ompt_target_data_transfer_rect(_async) & subvolume	{part}partial	{good}N/A	Enum: PR195829
OMPT: ompt_target_data_transfer(_async)	{part}partial	{good}N/A	Enum: PR195829
OMPT: ompt_target_data_memset(_async)	{part}partial	{good}N/A	Enum: PR195829 Callbacks: PR194168
OMPT: workdistribute work callback enum	{part}partial	{good}N/A	Enum: PR195829
OMPT: transparent task flag enum (importing/exporting)	{part}partial	{good}N/A	Enum: PR195829
OMPT: dependence type {out, inout}_all_memory	{part}partial	{good}N/A	Enum: PR195829
OMPT: removed master callback	{none}unclaimed	{good}N/A
OMPT: removed sync_region_barrier(_implicit) enum value	{none}unclaimed	{good}N/A

(openmp-6-0-deprecations)=

OpenMP 6.0 Deprecations

	C/C++ Status	Fortran Status	Reviews
OMPT: non-emi target callbacks	{none}unclaimed	{good}N/A
OMPT: transfer_to_device / transfer_from_device enums	{none}unclaimed	{good}N/A

(openmp-6-1-implementation-details)=

OpenMP 6.1 Implementation Details (Experimental)

The following table provides a quick overview over various OpenMP 6.1 features and their implementation status. Since OpenMP 6.1 has not yet been released, the following features are experimental and are subject to change at any time. Please post on the Discourse forums (Runtimes - OpenMP category) for more information or if you want to help with the implementation.

Feature	C/C++ Status	Fortran Status	Reviews
dyn_groupprivate clause	{part}partial	{part}In Progress	C/C++: Host device support missing
loop flatten transformation	{none}unclaimed	{none}unclaimed
loop grid/tile modifiers for sizes clause	{none}unclaimed	{none}unclaimed
attach map-type modifier	{part}In Progress	{none}unclaimed	C/C++: @abhinavgaba; RT: @abhinavgaba (PR149036, PR158370)
need_device_ptr modifier for adjust_args clause	{part}partial	{none}unclaimed	Clang Parsing/Sema: PR168905 PR169558
fallback modifier for use_device_ptr clause	{good}done	{none}unclaimed	Clang: @abhinavgaba (PR170578, PR173931) RT: @abhinavgaba (PR169603)
dims modifier for num_teams, thread_limit, and num_threads clauses	{part}partial	{part}In Progress	C/C++: @kevinsala ([PR206412]); Fortran: @skc7, @kparzysz, @mjklemm

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’	{part}prototyped	D126323
device extension	‘ompx_hold’ map type modifier	{part}prototyped	D106509, D106510
device extension	‘ompx_bare’ clause on ‘target teams’ construct	{part}prototyped	PR66844, PR70612
device extension	Multi-dim ‘num_teams’ and ‘thread_limit’ clause on ‘target teams ompx_bare’ construct	{part}partial	PR99732, PR101407, PR102715