Doc. No.: N 1665
Date: 2012-11-01
Reply to: Clark Nelson

Intel® Cilk™ Plus Language Extension Specification


This specification encompasses C++ as well as C. As a result, many things are said that are not relevant to C, and for simplicity, the terminology and conventions of C++ are used where they differ from those of C.

Any use of pragma syntax in this document may be interpreted as a placeholder for a syntax more suitable for standardization.

Cilk is a trademark of Intel Corporation in the U.S. and/or other countries.

More information about Intel® Cilk™ Plus can be found at



This document is part of the Intel® Cilk™ Plus Language Specification. The language specification comprises a set of technical specifications describing the language and the run-time support for the language. Together, these documents provide the detail needed to implement a compliant compiler. At this time the language specification contains these parts:

This document defines the Intel® Cilk™ Plus extension to C and C++. The language extension is supported by a run time user mode work stealing task scheduler which is not directly exposed to the application programmer. However, some of the semantics of the language and some of the guarantees provided require specific behavior of the task scheduler. The programmer visible parts of the language include the following constructs:

  1. Three keywords (_Cilk_spawn, _Cilk_sync and _Cilk_for) to express tasking
  2. Hyperobjects, which provide local views to shared objects
  3. Array notations
  4. Elemental functions
  5. SIMD loops

An implementation of the language may take advantage of all parallelism resources available in the hardware. On a typical CPU, these include at least multiple cores and vector units. Some of the language constructs, e.g. _Cilk_spawn, utilize only core parallelism; some, e.g. SIMD loops, utilize only vector parallelism, and some, e.g. elemental functions, utilize both. The defined behavior of every deterministic Cilk program is the same as the behavior of a similar C or C++ program known as the serialization. While execution of a C or C++ program may be considered as a linear sequence of statements, execution of a tasking program is in general a directed acyclic graph. Parallel control flow may yield a new kind of undefined behavior, a data race, whereby parts of the program that may execute in parallel access the same memory location in an indeterminate order, with at least one of the accesses being a write access. In addition, if an exception is thrown, code code may still be executed that would not have been executed in a serial execution.

Keywords for Tasking

Cilk Plus adds the following new keywords:

A program that uses these keywords other than as defined in the grammar extension below is ill-formed.

Keyword Aliases

The header <cilk/cilk.h> defines the following aliases for the Cilk keywords:

#define cilk_spawn _Cilk_spawn
#define cilk_sync  _Cilk_sync
#define cilk_for   _Cilk_for


The three keywords are used in the following new productions:

_Cilk_sync ;

The call production of the grammar is modified to permit the keyword _Cilk_spawn before the expression denoting the function to be called:

_Cilk_spawnopt postfix-expression ( expression-listopt )

Consecutive _Cilk_spawn tokens are not permitted. The postfix-expression following _Cilk_spawn is called a spawned function. The spawned function may be a normal function call, a member-function call, or the function-call (parentheses) operator of a function object (functor) or a call to a lambda expression. Overloaded operators other than the parentheses operator may be spawned only by using the function-call notation (e.g., operator+(arg1,arg2)). There shall be no more than one _Cilk_spawn within a full expression. A function that contains a spawn statement is called a spawning function.

A program is ill formed if the _Cilk_spawn form of this expression appears other than in one of the following contexts:

(A _Cilk_spawn expression may be permitted in more contexts in the future.)

A statement with a _Cilk_spawn on the right hand side of an assignment or declaration is called an assignment spawn or initializer spawn, respectively and the object assigned or initialized by the spawn is called the receiver.

The iteration-statement is extended by adding another form of for loop:

# pragma cilk grainsize = expression new-line
grainsize-pragmaopt _Cilk_for ( for-init-decl ; condition ; expression ) statement
grainsize-pragmaopt _Cilk_for ( assignment-expression ; condition ; expression ) statement


Tasking Execution Model

A strand is a serially-executed sequence of instructions that does not contain a spawn point or sync point (as defined below). At a spawn point, one strand (the initial strand) ends and two strands (the new strands) begin. The initial strand runs in series with each of the new strands but the new strands may run in parallel with each other. At a sync point, one or more strands (the initial strands) end and one strand (the new strand) begins. The initial strands may run in parallel with one another but each of the initial strands runs in series with the new strand. A single strand can be subdivided into a sequence of shorter strands in any manner that is convenient for modeling the computation. A maximal strand is one that cannot be included in a longer strand.

The strands in an execution of a program form a directed acyclic graph (DAG) in which spawn points and sync points comprise the vertices and the strands comprise the directed edges, with time defining the direction of each edge. (In an alternative DAG representation, sometimes seen in the literature, the strands comprise the vertices and the dependencies between the strands comprise the edges.)

Serialization rule

The behavior of a deterministic Intel® Cilk™ Plus program is defined in terms of its serialization, as defined in this section. If the serialization has undefined behavior, the Intel® Cilk™ Plus program also has undefined behavior.

The strands in an execution of a program are ordered according to the order of execution of the equivalent code in the program's serialization. Given two strands, the earlier strand is defined as the strand that would execute first in the serial execution of the same program with the same inputs, even though the two strands may execute in either order or concurrently in the actual parallel execution. Similarly, the terms earliest, latest, and later are used to designate strands according to their serial ordering. The terms left, leftmost, right, and rightmost are equivalent to earlier, earliest, later, and latest, respectively.

The serialization of a pure C or C++ program is itself.

If a C or C++ program has defined behavior and does not use the tasking keywords or library functions, it is a Cilk Plus program with the same defined behavior.

The serializations of _Cilk_spawn and _Cilk_sync are empty.

If a Cilk Plus program has defined deterministic behavior, then that behavior is the same as the behavior of the C or C++ program derived from the original by removing all instances of the keywords _Cilk_spawn, and _Cilk_sync.

The serialization of _Cilk_for is for.

If a Cilk Plus program has defined deterministic behavior, then that behavior is the same as the behavior of the C or C++ program derived from the original by replacing each instance of the _Cilk_for keyword with for.

Spawning blocks

A spawning block is a region of the program subject to special rules. Spawning blocks may be nested. The body of a nested spawning block is not part of the outer spawning block. Spawning blocks never partially overlap. The body of a spawning function is a spawning block. A _Cilk_for statement is a spawning block and the body of the _Cilk_for loop is a (nested) spawning block.

Every spawning block includes an implicit _Cilk_sync executed on exit from the block, including abnormal exit due to an exception. Destructors for automatic objects with scope ending at the end of the spawning block are invoked before the implicit _Cilk_sync. The receiver is assigned or initialized to the return value before executing the implicit _Cilk_sync at the end of a function. An implicit or explicit _Cilk_sync within a nested spawning block will synchronize with _Cilk_spawn statements only within that spawning block, and not with _Cilk_spawn statements in the surrounding spawning block.

_Cilk_for Loops

Syntactic constraints

To simplify the grammar, some restrictions on _Cilk_for loops are stated here in text form. Where a constraint on an expression is expressed grammatically, parentheses around a required expression or sub-expression are allowed.

The three items inside parentheses in the grammar, separated by semicolons, are the initialization, condition, and increment. (Cilk Plus imposes no constraints on the form of the initialization, beyond those of the base language.) We also need to define range-based Cilk-for loops.

The condition shall have one of the following forms:

expression < expression
expression > expression
expression <= expression
expression >= expression
expression != expression

Exactly one of the operands of the comparison operator shall be just the name of the loop's control variable. The operand that is not the control variable is called the limit.

The loop increment shall have the following form:

increment , single-increment
++ identifier
identifier ++
-- identifier
identifier --
identifier += expression
identifier = identifier + expression
identifier = expression + identifier
identifier -= expression
identifier = identifier - expression

Each comma in the grammar of increment shall represent a use of a built-in comma operator. Where identifier occurs more than once in the grammar of a single-increment, the same variable shall be named as each occurrence. The variable modified by the top-level operation of a single-increment is an induction variable. Each induction variable shall have integral, pointer or class type, shall have automatic storage duration, and shall be declared either in the initialization clause of the loop or local to the smallest spawning block containing the loop. Any expression operand of a single-increment shall have integral or enumeration type.

In exactly one single-increment, the identifier shall name the loop's control variable. A loop shall have exactly one control variable. (The control variable of a loop is determined by considering the loop's condition and increment together.)

The table indicates the stride corresponding to the syntactic form.

Syntax Stride
++identifier +1
--identifier -1
identifier += expression expression
identifier = identifier + expression
identifier = expression + identifier
identifier -= expression -(expression)
identifier = identifier - expression

The notion of stride exists for exposition only and does not need to be computed. In particular, for the case of identifier -= expression, a program may be well formed even if expression is unsigned.

A program that contains a return, break, or goto statement that would transfer control into or out of a _Cilk_for loop is ill-formed. Moved from above.

Within each iteration of the loop body, the name of each induction variable refers to a new object, as if the name were declared as an object within the body of the loop, with automatic storage duration and with the const-qualified version of the type of the original object. If an induction variable is declared before the loop initialization, then the final value of the induction variable is the same as for the serialization of the program. Moved from above. This actually describes semantics, not constraints.

Requirements on types and operators

The type of an induction variable shall be copy constructible. (For the purpose of specification, all C types are considered copy constructible.) For a single-increment that modifies variable V, if it uses operator +, ++ or +=, the expression:

V += ( difference_type)(incr)

shall be well-formed; if it uses operator -, -- or -=, the expression

V -= ( difference_type )(incr)

shall be well-formed. The loop is a use of the required operator function.

The initialization, condition, and increment parts of a _Cilk_for shall meet all of the semantic requirements of the corresponding serial for statement. In addition, depending on the syntactic form of the condition, a _Cilk_for adds the following requirements on the types of the control variable (var), limit, and stride, and the loop count is computed as follows, evaluated in infinite integer precision. (In the following table, first is the value of var immediately after initialization, if any.)

Condition syntax Requirements Loop count
var < limit
limit > var
(limit) - (first) shall be well-formed and shall yield an integral difference_type;
stride shall be > 0
(( limit ) - ( first )) / stride
var > limit
limit < var
(first) - (limit) shall be well-formed and shall yield an integral difference_type;
stride shall be < 0
(( first ) - ( limit )) / -stride
var <= limit
limit >= var
(limit) - (first) shall be well-formed and shall yield an integral difference_type;
stride shall be > 0
(( limit ) - ( first ) + 1) / stride
var >= limit
limit <= var
(first) - (limit) shall be well-formed and shall yield an integral difference_type;
stride shall be < 0
(( first ) - ( limit ) + 1) / -stride
var != limit
limit != var
(limit) - (first) and (first) - (limit) shall be well-formed and yield the same integral difference_type;
stride shall be != 0
if stride is positive
then ((limit) - (first)) / stride
else ((first) - (limit)) / -stride

Dynamic constraints

If the stride does not meet the requirements in the table above, the behavior is undefined. If this condition can be determined statically, the compiler is encouraged (but not required) to issue a warning. (Note that the incorrect loop might occur in an unexecuted branch, e.g., of a function template, and thus should not cause a compilation failure in all cases.)

If an induction variable is modified other than as a side effect of evaluating the loop increment expression, the behavior of the program is undefined.

If X and Y are values of an induction variable that occur in consecutive evaluations of the loop condition in the serialization, then

((limit) - X) - ((limit) - Y)

evaluated in infinite integer precision, shall equal the stride associated with the induction variable. If the condition expression is true on entry to the loop, then the loop count shall be non-negative.

Programmer note: Unsigned wraparound is not allowed.

The increment and limit expressions may be evaluated fewer times than in the serialization. If different evaluations of the same expression yield different values, the behavior of the program is undefined.

The copy constructor for the control variable may be executed more times than in the serialization.

If evaluation of the increment or limit expression, or a required operator+= or operator-= throws an exception, the behavior of the program is undefined.

If the loop body throws an exception that is not caught within the same iteration of the loop, it is unspecified which other loop iterations execute. If multiple loop iterations throw exceptions that are not caught in the loop body, the _Cilk_for statement throws the exception that would have occurred first in the serialization of the program.

Grainsize pragma

A _Cilk_for iteration-statement may optionally be preceded by a grainsize-pragma. The grainsize pragma shall immediately precede a _Cilk_for loop and may not appear anywhere else in a program, except that other pragmas that appertain to the _Cilk_for loop may appear between the grainsize-pragma and the _Cilk_for loop. The expression in the grainsize pragma shall evaluate to a type convertible to signed long. The presence of the pragma provides a hint to the runtime specifying the number of serial iterations desired in each chunk of the parallel loop. The grainsize expression is evaluated at runtime. If there is no grainsize pragma, or if the grainsize evaluates to 0, then the runtime will pick a grainsize using its own internal heuristics. If the grainsize evaluates to a negative value, the behavior is unspecified. (The meaning of negative grainsizes is reserved for future extensions.) The grainsize pragma applies only to the _Cilk_for statement that immediately follows it — the grain sizes for other _Cilk_for statements are not affected.


The _Cilk_spawn keyword suggests to the implementation that an executed statement or part of a statement may be run in parallel with following statements. A consequence of this parallelism is that the program may exhibit undefined behavior not present in the serialization. Execution of a _Cilk_spawn keyword is called a spawn. Execution of a _Cilk_sync statement is called a sync. A statement that contains a spawn is called a spawning statement.

The following sync of a _Cilk_spawn refers to the next _Cilk_sync executed (dynamically, not lexically) in the same spawning block. Which spawn the sync follows is implied from context. The following sync may be the implicit _Cilk_sync at the end of a spawning block.

A spawn point is a C sequence point at which a control flow fork is considered to have taken place. Any operations within the spawning expression that are not required by the C/C++ standards to be sequenced after the spawn point shall be executed before the spawn point. The strand that begins at the statement immediately following the spawning statement (in execution order) is called the continuation of the spawn. The sequence of operations within the spawning statement that are sequenced after the spawn point comprise the child of the spawn. The scheduler may execute the child and the continuation in parallel. Informally, the parent is the spawning block containing the initial strand, the spawning statements, and their continuations but excluding the children of all of the spawns. The children of the spawns within a single spawning block are siblings of one another.

The spawn points associated with different spawning statements are as follows:

For example, in the following two statements:

x[g()] = _Cilk_spawn f(a + b);

The call to function f is the spawn point and the statement a++; is the continuation. The expression a + b and the initialization of the temporary variable holding that value, and the evaluation of x[g()] take place before the spawn point. The execution of f, the assignment to x[g()], and the destruction of the temporary variable holding a + b take place in the child.

If a statement is followed by an implicit sync, that sync is the spawn continuation.

Programmer note: The sequencing may be more clear if

x = _Cilk_spawn f(a + b);

is considered to mean

	// Evaluate arguments and receiver address before spawn point
	T tmp = a + b; // T is the type of a + b
	U &r = x[g()]; // U is the type of x[0]
	_Cilk_spawn { r = f(tmp); tmp.~T(); }

A setjmp/longjmp call pair within the same spawning block has undefined behavior if a spawn or sync is executed between the setjmp and the longjmp. A setjmp/longjmp call pair that crosses a spawning block boundary has undefined behavior. A goto statement is not permitted to enter or exit a spawning block.


A sync statement indicates that all children of the current spawning block must finish executing before execution may continue within the spawning block. The new strand coming out of the _Cilk_sync is not running in parallel with any child strands, but may still be running in parallel with parent and sibling strands (other children of the calling function).

There is an implicit sync at the end of every spawning block. If a spawning statement appears within a try block, a sync is implicitly executed at the end of that try block, as if the body of the try were a spawning block. If a spawning block has no children at the time of a sync, then the sync has no observable effect. (The compiler may elide an explicit or implicit sync if it can statically determine that the sync will have no observable effect.)

Programmer note: Because implicit syncs follow destructors, writing _Cilk_sync at the end of a function may produce a different effect than the implicit sync. In particular, if an assignment spawn or initializer spawn is used to modify a local variable, the function will generally need an explicit _Cilk_sync to avoid a race between assignment to the local variable by the spawned function and destruction of the local variable by the parent function.


There is an implicit _Cilk_sync before a try-block, and before a throw, after the exception object has been constructed.

If a spawned function terminates with an exception, the exception propagates from the point of the corresponding sync.

When several exceptions are pending and not yet caught, later exception objects (in the serial execution order of the program) are destructed in an unspecified order before the earliest exception is caught.



Cilk defines a category of objects called hyperobjects. Hyperobjects allow thread-safe access to shared objects by giving each parallel strand a separate instance of the object.

Parallel code uses a hyperobject by performing a hyperobject lookup operation. The hyperobject lookup returns a reference to an object, called a view, that is guaranteed not to be shared with any other active strands in the program. The sequencing of a hyperobject lookup within an expression is not specified. The runtime system creates a view when needed, using callback functions provided by the hyperobject type. When strands synchronize, the hyperobject views are merged into a single view, using another callback function provided by the hyperobject type.

The view of a hyperobject visible to a program may change at any spawn or sync (including the implicit spawns and syncs within a _Cilk_for loop). The identity (address) of the view does not change within a single strand. The view of a given hyperobject visible within a given strand is said to be associated with that view. A hyperobject has the same view before the first spawn within a spawning block as after a sync within the same spawning block, even though the thread ID may not be the same (i.e., hyperobject views are not tied to threads). A hyperobject has the same view upon entering and leaving a _Cilk_for loop and within the first iteration (at least) of the _Cilk_for loop. A special view is associated with a hyperobject when the hyperobject is initially created. This special view is called the leftmost view or earliest view because it is always visible to the leftmost (earliest) descendent in the depth-first, left-to-right traversal of the program's spawn tree. The leftmost view is given an initial value when the hyperobject is created.

Programmer note: If two expressions compute the same address for a view, then they have not been scheduled in parallel. This property yields one of the simplest ways by which a program can observe the runtime behavior of the scheduler.

Implementation note: An implementation can optimize hyperobject lookups by performing them only when a view has (or might have) changed. This optimization can be facilitated by attaching implementation-specific attributes to the hyperobject creation, lookup, and/or destruction operations.


The vast majority of hyperobjects belong to a category known as reducers. Each reducer type provides a reduce callback operation that merges two views in a manner specific to the reducer. For a pair of views V1 and V2, the result of calling reduce(V1, V2) is notated as V1⊗V2. Each reducer also provides an identity callback operation that initializes a new view.

The reduce callback for a classical reducer implements an operation ⊗ such that (a⊗b)⊗c==a⊗(b⊗c) (i.e., ⊗ is associative). The view-initialization callback for such a reducer sets the view to an identity value I such that I⊗v==v and v⊗I==v for any value v of value_type. Given an associative ⊗ and an identity I, the triplet (value_type, ⊗, I) describes a mathematical monoid. For example, (int, +, 0) is a monoid, as is (list, concatenate, empty). If each individual view, R, of a classical reducer is modified using only expressions that are equivalent to RRv (where v is of value_type), then the reducer computes the same value in the parallel program as would be computed in the serialization of the program. (In actuality, the in the expression RRv can represent a set of mutually-associative operations. For example, += and -= are mutually associative.) For example, a spawned function or _Cilk_for body can append items onto the view of a list reducer with monoid (list, concatenate, empty). At the end of the parallel section of code, the reducer's view contains the same list items in the same order as would be generated in a serial execution of the same code.

Given a set of strands entering a sync, S1,S2,S3,…Sn, associated with views V1,V2,V3,…Vn, respectively such that Si is earlier in the serial ordering than Si+1, a single view, W, emerges from the sync with value W←V1⊗V2⊗V3⊗…⊗Vn, such that the left-to-right order is maintained but the grouping (associativity) of the operations is unspecified. The timing of this reduction is unspecified — in particular, subsequences typically will be computed asynchronously as child tasks complete. Every view except the one emerging from the sync is destroyed after the merge. If any of the strands does not have an associated view, then the invocation of the reduce callback function can be elided (i.e., the missing view is treated as an identity).

A strand is never associated with more than one view for a given reducer, but multiple strands can be associated with the same view if those strands are not scheduled in parallel (at run time). Specifically, for a given reducer, the association of a strand to a view of the reducer obeys the following rules:

Even before the final reduction, the leftmost view of a reducer will contain the same value as in the serial execution. Other views, however, will contain partial values that are different from the serial execution.

If ⊗ is not associative or if identity does not yield a true identity value then the result of a set of reductions will be non-deterministic (i.e., it will vary based on runtime scheduling). Such non-classical reducers are nevertheless occasionally useful. Note that, for a classical reducer, the ⊗ operator needs to be associative, but does not need to be commutative.

Hyperobjects in C++

C++ hyperobject syntax

Note: The syntax described here is the syntax used in the Intel products. Intel is considering a different syntax for future, either in addition to or instead of the syntax described below.

At present, reducers are the only kind of hyperobject supported. In C++, every reducer hyperobject has a hyperobject type, which is an instantiation of the cilk::reducer class template. The cilk::reducer class template has a single template type parameter, Monoid, which shall be a class type.

To define a reducer, a program defines a monoid class with public members representing the monoid, (T, ⊗, identity) as follows:

value_type typedef for T
reduce(value_type* left,
	value_type* right)
evaluate *left = *left*right
identity(value_type* p) construct identity object at *p
destroy(value_type* p) call the destructor on the object *p
allocate(size_t size) return a pointer to size bytes of raw memory
deallocate(value_type* p) deallocate the raw memory at *p

If any of the above functions do not modify the state of the monoid (most monoids carry no state), then those functions may be declared static or const. The monoid type may derive from an instantiation of cilk::monoid_base<T>, which defines value_type and provides default implementations for identity, destroy, allocate, and deallocate. The derived class needs to define reduce and override only those functions for which the default is incorrect.

For a given monoid, M, the type cilk::reducer<M> defines a hyperobject type. The cilk::reducer class template provides constructors, a destructor, and (const and non-const versions of) value_type& operator() and value_type& view(), both of which return a reference to the current view.

A hyperobject is created by defining an instance of cilk::reducer<M>:

cilk::reducer<M> hv(args);

Where args is a list of M::value_type constructor arguments used to initialize the leftmost view of hv. A hyperobject lookup is performed by invoking the member function, view or member operator() on the hyperobject, as in the following examples:


In these examples, append is an operation to be applied to the current view of hv, and is presumably consistent with the associative operation defined in the monoid, M.

Modifying a hyperobject view in a way that is not consistent with the associative operation in the monoid can lead to subtle bugs. For example, addition is not associative with multiplication, so performing a multiplication on the view of a summing reducer will almost certainly produce incorrect results. To prevent this kind of error, it is common to wrap reducers in proxy classes that expose only the valid associative operations. All of the reducers included in the standard reducer library have such wrappers.

C++ hyperobject behavior

An object of type M::value_type is constructed by the reducer constructor. This object is called the initial view or leftmost view of the hyperobject. When a hyperobject goes out of scope, the destructor is called on the leftmost view. It is unspecified whether M::allocate and M::deallocate are called to allocate and deallocate the leftmost view (they are not called in the current Intel implementation).

The implementation may create a view at any spawn that has been scheduled in parallel, or may lazily defer creation until the first access within a strand. The implementation creates a view by calling M::allocate followed by M::identity. (This is in addition to the initial view created by construction of the hyperobject.) The calls to M::allocate and M::identity are part of the strand for the purpose of establishing the absence of a data race.

At any sync or at the end of any spawned (child) function, the runtime may merge two views by calling M::reduce(left, right), where right is the earliest remaining view that is later than left. The M::reduce function is expected to store the merged result in the left view. After the merge, the runtime destroys the right view by calling M::destroy followed by M::deallocate. Every view except the leftmost view is passed exactly once as the second argument to reduce. The calls to M::reduce, M::destroy and M::deallocate happen after completion of both of the strands that formerly owned the left and right views.

If a monoid member function executes a hyperobject lookup (directly or through a function call), the behavior of the program is undefined.

For purposes of establishing the absence of a data race, a hyperobject view is considered a distinct object in each parallel strand. A hyperobject lookup is considered a read of the hyperobject.

Hyperobjects in C

C hyperobject syntax

Note: The syntax described here is the syntax used in the Intel products. Intel is considering a different syntax for future, either in addition to or instead of the syntax described below.

The C mechanism for defining and using hyperobjects depends on a small number of typedefs and preprocessor macros provided in the Cilk library. C does not have the template capabilities of C++ and thus has a less abstract hyperobject syntax. Unlike C++, each C hyperobject variable is unique — there is no named type that unites similar hyperobjects. There is, however, an implicit hyperobject type defined by the operations that comprise the hyperobjects' monoid. The provided macros facilitate creating reducer variables, which are the only type of hyperobject currently supported. The terms reducer and hyperobject are used interchangeably in this section.

To define a C reducer, the program defines three functions representing operations on a monoid (T, ⊗, identity):

void T_reduce(void* r, void* left, void* right);
void T_identity(void* r, void* view);
void T_destroy(void* r, void* view);

The names of these functions are for illustration purposes only and must be chosen, as usual, to avoid conflicts with other identifiers. The purposes of these functions are as follows:

T_reduce Evaluate *(T*)left = *(T*) left*(T*) right
T_identity Initialize a T value to identity
T_destroy Clean up (destroy) a T value

The r argument to each of these functions is a pointer to the actual reducer variable and is usually ignored. Since most C types do not require cleanup on destruction, the T_destroy function often does nothing. As a convenience, the Cilk library makes this common implementation available as a library function, __cilkrts_hyperobject_noop_destroy.

A reducer, hv, is defined and given an initial value, init, using the CILK_C_DECLARE_REDUCER and CILK_C_INIT_REDUCER macros as follows:

	CILK_C_INIT_REDUCER(T_identity, T_reduce, T_destroy,

The init expression is used to initialize the leftmost reducer view. The CILK_C_DECLARE_REDUCER macro defines a struct and can be used in a typedef or extern declaration as well:


The CILK_C_INIT_REDUCER macro expands to a static initializer for a hyperobject of any type. After initialization, the leftmost view of the reducer is available as hv.value.

If a reducer is local to a function, it shall be registered before first use using the CILK_C_REGISTER_REDUCER macro and unregistered after its last use using the CILK_C_UNREGISTER_REDUCER macro:

/* use hv here */

For the purpose of registration and unregistration, first use and last use are defined with respect to the serialization of the program. The reducer view immediately before unregistration shall be the same (have the same address) as the reducer view immediately after registration. In practice, this means that any spawns after the registration have been synced before the unregistration and that no spawns before the registration have been synced before the unregistration. Registration and unregistration are optional for reducers declared in global scope. The value member of the reducer continues to be available after unregistration, but a hyperobject lookup on an unregistered reducer results in undefined behavior unless the reducer is registered again.

A hyperobject lookup is performed using the REDUCER_VIEW macro:

REDUCER_VIEW(hv) += expr;

As in the case of a C++ reducer, modifying a reducer other than through the correct associative operations can cause bugs. Unfortunately, C does not have sufficient abstraction mechanisms to prevent this kind of error. Nevertheless, the Cilk library provides wrapper macros to simplify the declaration and initialization, though not the safety, of library-provided reducers in C. For example, you can define and initialize a summing reducer this way:


A C reducer can be declared, defined, and accessed within C++ code, but a C++ reducer cannot be used within C code.

C hyperobject behavior

The macro CILK_C_DECLARE_REDUCER(T) defines a struct with a data member of type T, named value. The macro CILK_C_INIT_REDUCER(I,R,D,V) expands to a braced-init-list appropriate for initializing a variable, hv, of structure type declared with CILK_C_DECLARE_REDUCER(T) such that hv, can be recognized by the runtime system as a C reducer with value type T, identity function I, reduction function R, destroy function D, and initial value V.

Invoking CILK_C_REGISTER_REDUCER(hv) makes a call into the runtime system that registers hv.value as the initial, or leftmost, view of the C hyperobject hv. The macro CILK_C_UNREGISTER_REDUCER(hv) makes a call into the runtime system that removes hyperobject hv from the runtime system's internal map. Attempting to access hv after it has been unregistered will result in undefined behavior. If a hyperobject is never registered, the leftmost view will be associated with the program strand before the very first spawn in the program and will follow the leftmost branch of the execution DAG. This association is typically useful only for hyperobjects in global scope.

The implementation may create a view at any spawn that has been scheduled in parallel, or may lazily defer creation until the first access within a strand. The implementation creates a view by allocating it with malloc, then calling the identity function specified in the reducer initialization. (This is in addition to the initial view created by construction of the reducer.) The call to the identity function is part of the strand for the purpose of establishing the absence of a data race.

At any sync or at the end of any spawned (child) function, the runtime may merge two views by calling the reduction function (specified in the reducer initialization) on the values left and right, where right is the earliest remaining view that is later than left. The reduction function is expected to store the merged result in the left view. After the merge, the runtime destroys the right view by calling the destroy function for the hyperobject, then deallocates it using free. Every view except the leftmost view is passed exactly once as the second argument the reduction function. The calls to reduction and destroy functions happen after completion of both of the strands that formerly owned the left and right views.

If a monoid function executes a hyperobject lookup, the behavior of the program is undefined.

For purposes of establishing the absence of a data race, a hyperobject view is considered a distinct object in each parallel strand. A hyperobject lookup is considered a read of the hyperobject.

Array notation


This section provides a specification for the array notation portion of the Intel® Cilk™ Plus language extension. Array notation is intended to allow users to directly express high level parallel vector array operations in their programs. This assists the compiler in performing vectorization and auto-parallelization. From the users' point of view, they will see more predictable vectorization, improved performance and better hardware resource utilization. Array notation is an extension of the standard C/C++ languages, including features that are designed for easy expression of array operations and simplified parallel function invocation.

The Section Expression

The section expression selects multiple array elements for a data-parallel operation.

The syntax of a section expression is as follows:

postfix-expression [ section-triplet ]
expression : expression : expression
expression : expression

Each of the expressions in a section triplet shall have integer type. The postfix expression in a section expression shall have type pointer to complete object type; the type of the section expression is type (i.e. the same type as the corresponding simple subscript expression; there is no section type).

The sequence of expressions delimited by the brackets in a section expression is termed a section triplet (even when there are fewer than three expressions). The expressions in a triplet are interpreted, respectively as: begin, length, and stride. Each section triplet represents a sequence of subscript values, starting at begin, with length elements, where each element increases by stride:

begin, begin + stride, begin + stride * 2, …, begin
		+ stride * (length - 1)

When no stride expression is present, the value of stride is 1. When the triplet contains no expressions (i.e. consists entirely of a single colon), the value of begin is 0, and the value of length is the number of elements in the array being subscripted. If this shorthand is used, the type of the array being subscripted shall have a declared size (which can be non-constant in the case of a VLA).

For example, A[0:3:2] refers to elements 0, 2, and 4 of the array A. A[:] refers to the entire array A, assuming A is a one-dimensional array with known upper bound.

The expressions in a triplet are converted to ptrdiff_t.

If stride is negative, then begin identifies the uppermost index. If length is less than or equal to zero, the sequence of subscript values is empty.

Every expression has a rank, determined as follows.

In an assignment expression, if the right operand has nonzero rank, the left operand shall have the same rank as the right operand.


Expression Rank
A[3:4][0:10] 2
A[3][0:10] 1
A[3:4][0] 1
A[:][:] 2
A[3][0] 0

An array section is an lvalue postfix expression with rank greater than zero.

For example, A[0:3][0:4] refers to 12 elements in the two-dimensional array A, starting at row 0, column 0, and ending at row 2, column 3.

Examples of section expressions:

int *p;
int A[n][m];
p[:] = ... // not valid. p has no declared size.
A[:][:] = ... // The entire 2D array A.
p[1:5] = ... // p[1],p[2], ... p[5].

Every section triplet of an array section has a relative rank, defined as its ordinal number among the other triplets, from left to right, starting at 0.


Expression Relative rank of 0:10
A[:][0:10][:] 1
A[0:10][:][:] 0
A[:][:][0:10] 2
A[i[:]][j[0:10]][k[:]] 1

The shape of an array section is defined as a vector: (length0,length1,...,lengthn-1), where n is the rank of the array section, and lengthi is the length of the triplet with relative rank i. The size of an array section is the product of the length values of its shape.

A full expression shall have rank zero, unless it appears in an expression statement or as the controlling expression of an if statement.

In general, a statement containing a section expression is executed once for each element of the section; some operations of these executions may be performed in parallel. However, in such a statement, a sub-expression with rank zero is evaluated only once. For example:

a[:] = f1(b[:]) + f2(c) + d;

f1 is called for each element of b. f2(c) is evaluated once; that value is used to compute the new value of each element of a. If f2 changes the value of d, the value used for d in this example is unspecified. If f1 changes the value of d, the behavior is likely to be undefined.

When two array sections are required to have common rank, if they do not have the same shape, the behavior is undefined.

When two expressions are required to have common rank, and one of them has zero rank, the expression with zero rank is evaluated only once; the resulting value is used as many times as necessary to fully execute the containing statement.

Operations on Array Sections


An array section assignment is a parallel operation that modifies every element of the array section on the left-hand side.

When the left operand of assignment has non-zero rank, the assignment of each element is unsequenced with respect to the assignment of every other element, and the value computation for each element is unsequenced with respect to the value computation for every other element.

For example, if any section on the right-hand side of an assignment overlaps with the left-hand side, and the overlap is not complete, the behavior is undefined.

a[0:10] = a[10:10];	// no overlap; well-defined
a[0:10:2] = a[1:10:2];	// no overlap; well-defined
a[0:10] = a[0:10] + 1;	// complete overlap; well-defined
a[0:10] = a[1:10];	// incomplete overlap; undefined


// Copy elements 10->19 in A to elements 0->9 in B.
B[0:10] = A[10:10];
// Undefined behavior. Triplets 0:10 and 0:100 are not the same size.
B[0:10] = A[0:100];
// Transpose row 0, columns 0-9 of A, into column 0, rows 0-9 of B.
B[0:10][0] = A[0][0:10];
// Copy the specified array section in the 2nd and 3rd dimensions of A into
// the 1st and 4th dimensions of B.
B[0:10][0][0][0:5] = A[3][0:10][0:5][5]
// Undefined behavior. The corresponding triplets with the same relative rank
// (0:9, 0:5) and (0:5, 0:9) do not have the same number of elements.
B[0:9][0:5] = A[0:5][0:9];
// OK since the triplets on both sides have the same number of elements.
// The 5 elements in A are scattered to the even-index elements in B
// (0,2,4...8). The values of the odd-index elements (1,3,5...7) are not
// changed.
B[0:5:2] = A[0:5];

Arithmetic Operations

When applied to an array section or sections, the C/C++ arithmetic operators are applied to each element of the array section(s). Note that multiplication is performed element-wise and does not correspond to traditional vector/matrix multiplication.


// Set all elements of A to 1.0.
A[:] = 1.0;
// Set every element of 3x3 shape in A to the value of B[0].
A[0:3][0:3] = B[0];
// Error: the number of dimensions (rank) must match,
// or be equal to 0.
A[0:2][0:2] = B[0:2];
// Element-wise addition of all elements in A and B, resulting in C.
C[:] = A[:] + B[:];
// Element-wise multiplication of all elements in A and B,
// resulting in C.
C[:] = A[:] * B[:];
// Add elements 10->19 from A with elements 0->9 from B and place in
// elements 20->29 in C.
C[20:10] = A[10:10] + B[0:10];
// Element-wise addition of the first 10 elements in column 2 of A and
// column 3 of B, resulting in column 0 of C.
C[0:10][0] = A[0:10][2] + B[0:10][3];
// Matrix addition of the 2x2 matrices in A and B starting at A[3][3]
// and B[5][5], placed in C starting at C[0][0].
C[0:2][0:2] = A[3:2][3:2] + B[5:2][5:2];
// Add the array section along the 1st and 2nd dimensions of B to the
// elements in the array section along the 2nd and 3rd dimensions of A,
// placing them in an array section in C.
C[0:9][0][0:9] = A[0][0:9][0:9] + B[0:9][0:9][4];
// Element-wise addition of first 10 elements in A and B,
// resulting in A.
A[0:10] = A[0:10] + B[0:10];
// Element-wise negation of first 10 elements in A, resulting in A.
A[0:10] = -A[0:10];
// Multiply every element in A by 2.
A[:] *= 2;
// Add one to each element in A.
// Element-wise equality test of B and C, resulting in an array of
// Boolean values, which are placed in A.
A[:] = B[:] == C[:];

Function calls

If a function is called with an array section argument, the function is mapped, or called with successive elements in the array. For example:

type fn(type arg1, type2 arg2);
type in[N], out[N];
type2 in2[N];
out[x:y:z] = fn(in[x:y:z], in2[x:y:z]);

The function fn is mapped over array sections of arrays in and in2. The function fn will be called with arguments (in[x], in2[x]), with arguments (in[x+z], in2[x+z]), etc. The results of the function calls are collected into a section of array out.

The executions of function calls mapped from a given statement are unsequenced with respect to one another.

Reduction Operations

The reduction operations accumulate a result over all the values in an array section. There are specialized reductions for specific operations, and two general reductions which perform a user-specified operation. A reduction operation resembles a generic function, which can take a variety of argument types; in some cases, the return type of the operation matches the type of the section argument.

The section argument of a reduction operation shall have rank greater than zero. The rank of a reduction operation is zero.

Each of the specialized reduction operations resembles a function that takes a single section argument, whose (element) type shall be an arithmetic type. The specialized reduction operations are listed here:

Reduction operation Result value Result type Result value when the argument is an empty section
__sec_reduce_add The sum of the argument values The argument type Zero
__sec_reduce_mul The product of the argument values The argument type One
__sec_reduce_max The maximum argument value The argument type The minimum value representable in the result type
__sec_reduce_min The minimum argument value The argument type The maximum value representable in the result type
__sec_reduce_max_ind The index of the maximum argument value intptr_t Unspecified
__sec_reduce_min_ind The index of the minimum argument value intptr_t Unspecified
__sec_reduce_all_zero One if all argument values are zero; zero otherwise int One
__sec_reduce_all_nonzero One if all argument values are nonzero; zero otherwise int One
__sec_reduce_any_zero One if any argument value is zero; zero otherwise int Zero
__sec_reduce_any_nonzero One if any argument value is nonzero; zero otherwise int Zero

For __sec_reduce_max_ind and __sec_reduce_min_ind, the rank of the argument shall be exactly one.

The general reduction operations are described by these pseudo-declarations, where T represents a type:

T __sec_reduce(T initial, T section, T (*operation)(T, T));
void __sec_reduce_mutating(T &result, T section, T2 (*operation)(T *, T));

The second argument is the section argument; the result type of the reduction is the element type of the section. The result type shall be copy constructible.

The first argument is the initial value for the operation. Its type shall be the result type of the reduction. For __sec_reduce_mutating, it is also the object into which the result is computed, and it shall be an lvalue.

The third argument identifies the abstract operation for which to perform the reduction. It shall designate a function that takes two arguments having the same type as the result of the reduction. For __sec_reduce, it shall return its result as a value of the result type of the reduction. For __sec_reduce_mutating, its first argument will refer to an object of the result type of the reduction, into which it shall compute its result.

In C, the operation argument shall have pointer-to-function type. For __sec_reduce, its first argument will also be a value of the result type of the reduction; for __sec_reduce_mutating, its first argument will be a pointer to an object of the result type of the reduction.

In C++, if the result type of the reduction is a class type, then names in the operation argument are looked up as if used in a member function of that class. If the operation argument is an id-expression referring to a set of overloaded functions, overload resolution is performed as for a binary operator; i.e. overload resolution is used to determine whether to invoke the operation as a member function (e.g. (a.f(b)) or as a non-member function (e.g. (f(a, b)). Otherwise, the operation argument shall be a callable object; if it has pointer-to-member type, then the operation is invoked as a member function call, otherwise it is invoked with two arguments.

Invocations of the function designated by the operation argument are unsequenced with respect to one another. It is unspecified how the elements of the section, the initial/result object, and any introduced temporary objects, are paired by calls to the operation function, except that if the rank of the section is one and the operation function is associative, then the result is the same as for left-to-right reduction, where the initial value is taken as leftmost, and the element with index begin + stride * (length - 1) is taken as rightmost.


type fn(type in1, type in2);
type in[N], out;
out = __sec_reduce(initial, in[x:y:z], fn);

The reduction will be computed analogously to:

tmp = initial;
for each element X of in[x:y:z]
	tmp = fn(tmp, X);

The result of the reduction will be the final value of tmp.

For example, the two reduction operations given here compute the same result:

double add(double in1, double in2) { return in1+in2; }
out = __sec_reduce(0, in[x:y:z], add); // accumulate using add()
out = __sec_reduce_add(in[x:y:z]); // accumulate using built-in +

The compiler may produce more optimized code when the specialized reduction operations are used.

Array Implicit Index

In writing code that uses array sections, it is sometimes useful to explicitly reference the indices of the individual elements in a section. For example, the user may wish to fill an array with a function of the element index, rather than a single value.

Conceptually, an array section operation can be thought of as expanding into a loop with an implicit index variable for each relative rank of the section. For each relative rank, the value of the implicit index variable ranges between zero and one less than the length of the triplet with that relative rank. The __sec_implicit_index operation returns the value of the implicit index variable for a specified relative rank. It behaves as a function with the following declaration:

intptr_t __sec_implicit_index(int relative_rank);

The argument shall be an integer constant expression. For purposes of rank checking, the rank of an implicit index operation is zero, although it is reevaluated for each element, like an expression of rank one.


int A[10], B[10][10];
// A[0] = 0, A[1] = 1, A[2] = 2,...
A[:] = __sec_implicit_index(0);
// B[i][j] = i+j;
B[:][:] = __sec_implicit_index(0) + __sec_implicit_index(1);
// The length of each dimension is 2. The value of __sec_implicit_index
// is either 0 or 1, regardless of the subscripts of the affected elements.
A[i:2:s][j:2:t] = __sec_implicit_index(0) ^ __sec_implicit_index(1);

Sections in if statements

If the rank of the controlling expression of an if clause is nonzero, the rank of every full expression in every substatement of that if statement (including substatements of those substatements, recursively) shall equal the rank of the controlling expression of the if clause. If the shape of a full expression of a substatement does not match the shape of the controlling expression, the behavior is undefined.

When the controlling expression of an if statement has nonzero rank, the entire if statement, including its dependent statements, is executed for each element of the shape of the controlling expression.

Example: Sections as Array Parameters

The array notation enables a vector kernel style of programming, where vector code is encapsulated within a function, with a parameterized vector length. In this case, concurrency happens within a single function invocation, unlike when mapping function calls over an array section, where concurrency happens between function invocations.

The following example illustrates how to combine array section vectorization inside a function body with threading for parallel function calls. The vector length m is 256 in this example.

void saxpy_vec(int m, float a, float x[restrict m], float y[m])
	y[:] += a * x[:];

int main(void)
	float a[2048], b[2048];
	_Cilk_for (int i = 0; i < 2048; i += 256)
		saxpy_vec(256, 2.0, (a + i), (b + i));

By writing the function explicitly with array arguments, the programmer can write code with easily customizable vector lengths and runtime model choices.

Please note that functions cannot return array section values. It may be helpful to return a pointer to the array as in standard C/C++ and take sections of the return value in the callee.

Elemental Functions


This section describes the elemental functions portion of the Cilk Plus language. Elemental functions are a data parallel programming construct. The use of elemental functions consists of the following three steps:

  1. The programmer writes a function that uses scalar values in standard C/C++ to describe the operation to be performed on a single element.
  2. The programmer adds a vector attribute with optional clauses to be described below, so that the compiler generates a vector variant of the function, which operates on a vector of elements instead of a single one.
  3. Lastly, the programmer writes the invocation of the function to operate on arrays of arguments instead of single arguments.

The function is invoked with vectors of arguments iteratively until the whole array of arguments is processed. Each such invocation is of a single vector variant of the function.


The semantics of an elemental function are that the execution order among its invocations is unsequenced.

The execution of an elemental function depends on the language construct used at the invocation site, as follows.

  1. If the function is called from a C/C++ for loop, then the compiler may invoke a vector variant instead of the original function, and iterate in the loop fewer times. While in this context the replacement is always correct, whether the replacement will actually be done is implementation dependent and is subject to performance heuristics.
  2. Adding a #pragma simd to the C/C++ for loop will ensure that a vector variant of the function is called. Instances of the function will be invoked iteratively in a single execution strand until all elements of the array arguments have been processed.
  3. If the array notation syntax is used, then the execution is the same as in case #2 above. (However, Intel is considering augmenting the implementation of this syntax and allowing concurrent execution of invocations.)
  4. If the function is invoked from a _Cilk_for loop, then a vector variant is used, and the execution of the _Cilk_for loop is as described in the _Cilk_for section. This may result in multiple invocations of the function executing concurrently.

Implementation note: The compiler will generate three translations of the source code into machine code. One is for the compliant version, which operates on a single element per invocation. The second the short vector variant, that operates on multiple elements at a time. The number of elements to be used in by the short vector variant of the function is determined by the types of arguments and return value of the function, and the width of the vector registers in the target CPU. The programmer can use a vectorlength clause (described below) to override the compiler's choice. The third version is designed to receive an additional argument which is a mask argument. This argument is implicit and the application programmer does not explicitly provide it. The mask argument is required for the cases in which the function is called from a loop under a conditional statement. The application programmer can use a mask clause to suppress the generation of either the second or the 3rd version.

The vector attribute

The vector attribute is a general-purpose attribute, applicable to functions. For compatibility with the Microsoft compiler, it can appear in a __declspec; for compatibility with the GNU compiler, it can appear in an __attribute__; in C++0X, it can appear in an attribute-specifier.

vector ( elemental-clausesopt )
elemental-clauses , elemental-clause

For each vector attribute, one vector variant of the vectorized function is created. The vector attribute and its associated clauses are considered part of the function signature. If the function declaration is inconsistent with the function prototype with respect to these clauses then the behavior is undefined.

An elemental function shall not have an exception specification.

The processor clause

processor ( processor-name )

Directs the compiler to create a vector version of the function for the given target processor. The default processor is taken from the implicit or explicit processor- or architecture- specific flag in the compiler command line. The target processor defines both the vector ISA that the compiler is allowed to use, and the width of the vectors.

The vectorlength clauses

vectorlengthfor ( type-name )
vectorlength ( constant-expression-list )
constant-expression-list , constant-expression

A constant-expression in a vectorlength clause shall be a valid integer constant expression. In the context of a vector attribute, the constant-expression-list in a vectorlength clause shall have only one constant-expression.

Every vector variant of an elemental function has a vector length (VL). If the vectorlength clause is used, the VL is the value of the argument of that clause. Otherwise, the function's characteristic type is found. If the vectorlengthfor clause is used, the characteristic type is the type specified as its argument; otherwise, if the return type is not void, then the characteristic type is the return type; otherwise, if the function has any non-uniform, non-linear parameters, then the characteristic type is the type of the first such parameter; otherwise, the characteristic type is int. The VL is then determined from the characteristic type and the processor's vector register. For example:

The uniform clause

uniform ( uniform-param-list )
uniform-param-list , parameter-name

The identifier in a parameter-name shall match the name of a parameter of the function to which the containing clause applies; the parameter shall have integral or pointer type.

If the value of an argument corresponding to a uniform parameter differs between invocations from a single execution of a loop, the behavior is undefined. (The values of these parameters can be broadcasted to all iterations as a performance optimization.)

The linear clause

linear ( linear-param-list )
elemental-linear-param-list , elemental-linear-param
parameter-name : elemental-linear-step

A constant-expression in an elemental-linear-step shall be an integer constant expression. A parameter referenced as an elemental-linear-step shall be the subject of a uniform clause.

No parameter of an elemental function shall be the subject of more than one uniform or linear clause.

For a linear parameter, if the corresponding argument values in consecutive iterations (in the serial version of the program) do not differ by the value of the elemental-linear-step, the behavior is undefined.

The mask clauses


By default, for every vector variant declared, two implementations are provided: one especially suitable for conditional invocation (i.e. masked), and another especially suitable for unconditional invocation (i.e. unmasked). If all invocations are conditional, generation of the unmasked implementation can be suppressed using the mask clause. Similarly, if all invocations are unconditional, generation of the masked implementation can be suppressed using the nomask clause. (Using both clauses together has no effect: both implementations are provided. This is also the default.)

SIMD loops


This section describes the SIMD loop portion of the Cilk Plus language. The SIMD pragma can be applied to a loop, to indicate that the iterations of the loop can be divided into contiguous chunks of a specific length, and that within each chunk, multiple iterations can be executed concurrently.

SIMD loop definition

# pragma simd simd-clausesopt new-line iteration-statement
simd-clauses simd-clause
data-privatization-clause [ private clause]
data-privatization-in-clause [ firstprivate clause]
data-privatization-out-clause [ lastprivate clause]
data-reduction-clause [ reduction clause]

The iteration-statement following #pragma simd shall be a for loop. The loop's control clause and body are subject to the same restrictions as in a _Cilk_for loop. The loop control variable shall have integer or pointer type.

The syntax and semantics of the various simd-openmp-data-clauses are detailed in the OpenMP specification. (, Section 2.9.3).

The vectorlength clauses

If the vectorlengthfor clause is used, the length of the chunk (vector length, or VL) is computed as for an elemental function, using the vectorlengthfor argument type as the characteristic type. If not, a VL is selected to try to generate the most efficient vector code. If the vectorlength clause is used, the VL is selected from among the values of its arguments.

The linear clause

linear ( simd-linear-variable-list )
simd-linear-variable-list , simd-linear-variable
id-expression : simd-linear-step

The id-expression in a simd-linear-variable shall designate a variable with scalar type. The conditional-expression in a simd-linear-step shall either satisfy the requirements of an integer constant expression, or be a reference to a variable with integer type. In any SIMD loop, no variable shall be the subject of more than one linear clause, and no variable shall be the subject of a linear clause and also an OpenMP data clause.

If a simd-linear-variable has no associated simd-linear-step, the constant value 1 is used for the simd-linear-step. If the simd-linear-step refers to a variable, and the variable is modified during the execution of the loop, or if the value of the object designated by the id-expression in a simd-linear-variable does not increase by the value of the expression in the associated simd-linear-step in each iteration of the loop, the behavior is undefined.

Restrictions on elemental functions and SIMD loops

The following language constructs shall not appear within the body of an elemental function, nor in a SIMD loop:

If execution of a function called from an elemental function or SIMD loop terminates with an exception or a call to longjmp, the behavior is undefined.

Intel is considering removal of some of these restrictions in the future.

Note: Because invocations of an elemental function, and iterations of a SIMD loop, are implicitly allowed to be concurrent, modifying any non-atomic non-local object carries the potential for a data race, and therefore undefined behavior.