OpenCilk language specification

OpenCilk Language Extension Specification
Version 1.0 (2021-02-01)

Copyright © 2020, 2021 Massachusetts Institute of Technology. All rights reserved.

More information about OpenCilk can be found at opencilk.org

Feedback on this specification is encouraged and welcome; please send to contact@opencilk.org

Introduction

This document is one of a set of technical specifications describing the OpenCilk 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 following specifications are available:

  • The OpenCilk Language Specification, this document
  • The OpenCilk Application Binary Interface
  • (XXX Add link)

This document defines the OpenCilk 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

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. SIMD-enabled 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, throwing if an exception may result in is thrown, code being may still be executed that would not have been executed in a serial execution.

The word “shall” is used in this specification to express a diagnosable constraint on a Cilk Plus program.

Related documents

  1. The OpenCilk Application Binary Interface
  2. ISO/IEC 9899:2011, Information Technology – Programming languages – C
  3. ISO/IEC 14882:2011, Information Technology – Programming languages – C++
  4. OpenMP Application Program Interface, Version 4.0 - July 2013

Keywords for Tasking

OpenCilk adds the following new keywords:

  • _Cilk_sync
  • _Cilk_spawn
  • _Cilk_for

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

Grammar

The three keywords are used in the following new productions:

jump-statement:
_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:

postfix-expression:
_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.

Note: The spawned function call may be a normal function call, a member-function call, the function-call (parentheses) operator of a function object (functor), or a call to a lambda expression.

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

  • as the entire body full-expression of an expression statement,
  • as the entire right hand side of an assignment expression that is the entire body full-expression of an expression statement, or
  • as the entire initializer-clause in a simple declaration for an object with automatic storage duration.

(A _Cilk_spawn expression may be permitted in more contexts in the future.) The rank of a spawned function call shall be zero. (See The section expression.)

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:

grainsize-pragma:
# pragma cilk grainsize = expression new-line
iteration-statement:
grainsize-pragmaopt _Cilk_for ( expression ; expression ; expression ) statement
grainsize-pragmaopt _Cilk_for ( declaration expression ; expression ) statement

The three items inside parentheses in the grammar, separated by semicolons, are called the initialization, condition, and increment, respectively. (A semicolon is included in the grammar of declaration.)

Semantics

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 is sequenced before each of the new strands but the new strands are unsequenced with respect to one another (i.e. they 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 are unsequenced with respect to one another but each of the initial strands runs in series with is sequenced before 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 OpenCilk program is defined in terms of its serialization, as defined in this section. If the serialization has undefined behavior, the OpenCilk 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 an OpenCilk with the same defined behavior.

The serializations of _Cilk_spawn and _Cilk_sync are empty.

If an OpenCilk 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 an OpenCilk 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 Task blocks

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

Every spawning task 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 task 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 task block will synchronize with _Cilk_spawn statements only within that task block, and not with _Cilk_spawn statements in the surrounding task block.

The scope of a label defined in a spawning block is limited to that spawning block.

Programmer note: Therefore, goto may not be used to enter or exit a spawning block.

_Cilk_for Loops

The constraints and semantics of a _Cilk_for loop are the same as those of its serialization, unless specified otherwise.

Each iteration of a _Cilk_for loop is a separate strand; they need not be executed serially.

Within each iteration of the loop body, the control variable is considered a unique variable whose address is no longer valid when the iteration completes. the name of the control variable refers to a local object, as if the name were declared as an object within the body of the loop, with automatic storage duration and with the type of the original object. If the control variable is declared before the loop initialization, then the address of the variable at the end of the loop is the same as the address of the variable before the loop initialization and the final value of the control variable is the same as for the serialization of the program.

Syntactic constraints

To simplify the grammar, some restrictions on _Cilk_for loops are stated here in text form. The three items inside parentheses in the grammar, separated by semicolons, are the initialization, condition, and increment. Where a constraint on an expression is expressed grammatically, parentheses around a required expression or sub-expression are allowed.

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.

The initialization shall declare or initialize a single variable, called the control variable. In C only, the control variable may be previously declared, but if so shall be reinitialized, i.e., assigned, in the initialization clause. In C++, the control variable shall be declared and initialized within the initialization clause of the _Cilk_for loop. The variable shall have automatic storage duration. No storage class may be specified for the variable within the initialization clause. The variable shall have integral, pointer, or class type. The variable may not be const or volatile. The variable shall be initialized. Initialization may be explicit, using assignment or constructor syntax, or implicit via a nontrivial default constructor. Within each iteration of the loop body, the control variable is considered a unique variable whose address is no longer valid when the iteration completes. If the control variable is declared before the loop initialization, then the address of the variable at the end of the loop is the same as the address of the variable before the loop initialization and the value of the control variable is the same as for the serialization of the program.

The condition shall have one of the following two forms:

var OP shift-expression
shift-expression OP var

where var is the control variable, optionally enclosed in parentheses. The operator denoted OP shall be one of !=, <=, <, >=, or >. The shift-expression that is not the control variable is called the loop limit.

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 expression. Any implicit conversion applied to that operand is not considered part of the limit expression.

The loop increment shall have one of the following forms: where var is the loop control variable, optionally enclosed in parentheses, and incr is a conditional-expression with integral or enum type. The table indicates the stride corresponding to the syntactic form.

Syntax Stride
++var +1
var++ +1
--var -1
var-- -1
var += incr incr
var -= incr -(incr)

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

++ identifier
identifier ++
-- identifier
identifier --
identifier += expression
identifier -= expression

The variable modified by the increment shall be the control variable.

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

Requirements on types and operators

The type of var shall be copy constructible. (For the purpose of specification, all C types are considered copy constructible.) The control variable shall have unqualified integral, pointer, or copy-constructible class type.

The initialization, condition, and increment parts of a _Cilk_for shall be defined such that the total number of iterations (loop count) can be determined before beginning the loop execution. Specifically, the parts of the _Cilk_for loop 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 var the control variable, limit the limit expression, and stride the stride. (and by extension incr), and

The loop count is computed as follows, evaluated in infinite integer precision when the control variable and limit both have integral or pointer type. ( In the following table, first is the value of var immediately after initialization, var” stands for an expression with the type and value of the control variable, “limit” stands for an expression with the type and value of the limit expression, and “stride” stands for an expression with the type and value of the stride expression. The loop count is computed after the loop initialization is performed, and before the control variable is modified by the loop. The loop count expression shall be well-formed, and shall have integral type. When a stride expression is present, if the divisor of the division is not greater than zero, the behavior is undefined. )

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
Loop count expression and value
Form of condition Form of increment
var++
++var
var--
--var
var += stride var -= stride
var < limit
limit > var
((limit)-(var))
n/a
((limit)-(var)-1)/(stride)+1
((limit)-(var)-1)/-(stride)+1
var > limit
limit < var
n/a
((var)-(limit))
((var)-(limit)-1)/-(stride)+1
((var)-(limit)-1)/(stride)+1
var <= limit
limit >= var
((limit)-(var))+1
n/a
((limit)-(var))/(stride)+1
((limit)-(var))/-(stride)+1
var >= limit
limit <= var
n/a
((var)-(limit))+1
((var)-(limit))/-(stride)+1
((var)-(limit))/(stride)+1
var != limit
limit != var
((limit)-(var))
((var)-(limit))
((stride)<0) ?
((var)-(limit)-1)/-(stride)+1 :
((limit)-(var)-1)/(stride)+1
((stride)<0) ?
((limit)-(var)-1)/-(stride)+1 :
((var)-(limit)-1)/(stride)+1

The incr expression shall have integral or enumeration type. The type of the difference between the limit expression and the control variable is the subtraction type, which shall be integral. When the condition operation is !=, (limit)-(var) and (var)-(limit) shall have the same type. The stride shall be convertible to the subtraction type.

For some expression X with the same type as the subtraction type, if the loop increment uses operator ++ or +=, the expression:

var += (difference_type)(incr) X

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

var -= (difference_type)(incr) X

shall be well-formed. The loop is a use an odr-use of the required operator += or -= function.

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 the control 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 var the control variable that occur in consecutive evaluations of the loop condition in the serialization, then the behavior is undefined if

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

evaluated in infinite integer precision, shall does not equal the stride. If the condition expression is true on entry to the loop, then the behavior is undefined if the computed loop count shall be non-negative is not greater than zero. If the computed loop count is not representable as a value of type unsigned long long, the behavior is undefined.

Programmer note: Unsigned wraparound is not allowed.

If the body of the loop is executed, the increment and limit expressions may be evaluated fewer a different number of 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, but no other iteration is terminated early. 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 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. The grainsize expression need not be evaluated. If it is evaluated, that evaluation is sequenced after the execution of the statement preceding the loop, is sequenced before any execution of the loop body, and is unsequenced with respect to the loop initialization and the evaluation of the limit and stride expressions. 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.

Spawn

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 An expression statement or declaration statement that contains a spawn is called a spawning statement. In a declaration containing a _Cilk_spawn keyword, the initialization of each object declared is treated as a separate statement.

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

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

  • The body of a _Cilk_for loop is a spawning statement with spawn point at the end of the loop condition test.
  • An expression statement containing a single _Cilk_spawn has a spawn point at the sequence point at the call to the spawned function. Any unnamed temporary variables created prior to the spawn point are not destroyed until after the spawn point (i.e., the destructors are invoked in the child).
  • A declaration statement in which an identifier is initialized or assigned with a result of a function call that is being spawned has a spawn point at the sequence point at the call to the spawned function. A declaration statement may consist of multiple comma separated declarations. Each of them may or may not have a spawn, and there can be at most one spawn per expression. The conversion of the function return value, if necessary, and the assignment or initialization of the receiver takes place after the spawn point (i.e., in the child). Any unnamed temporary variables created prior to the spawn point are not destroyed until after the spawn point (i.e., their destructors are invoked in the child).

For example, in the following two statements:

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

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[g()] = _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 task 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 task block boundary has undefined behavior. A goto statement is not permitted to enter or exit a task block.

Sync

A sync statement indicates that all children of the current task block must finish executing before execution may continue within the task 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 task block. If a spawning statement appears within a try block, a sync is implicitly executed at the end of on exit from that try block, as if the body of the try were a task block. If a task 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.

Exceptions

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

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.

Hyperobjects

Description

Cilk Plus defines a category of objects called “hyperobjects”. Hyperobjects allow thread-safe access to shared objects by giving each parallel strand running in parallel 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 task block as after a sync within the same task 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.

Reducers

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:

  1. The strand that initializes the reducer is associated with the leftmost view.
  2. If two strands execute in series (i.e., both strands are part of a larger strand), then both are associated with the same view.
  3. The child strand of a spawn is associated with the same view as the strand that entered the spawn.
  4. If the continuation strand of a spawn is scheduled in parallel with the child, then the continuation strand is associated with a new view, initialized using identity. The implementation may create the new view at any time up until the first hyperobject lookup following the spawn. If the continuation strand does not perform a hyperobject lookup, then the implementation is not required to create a view for that strand.
  5. If the continuation strand of a spawn is not scheduled in parallel with the child strand (i.e., the child and the continuation execute in series), then the continuation strand is associated with the same view as the child strand.
  6. The strand that emerges from a sync is associated with the same view as the leftmost strand entering the sync.

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 and holders are the only kind of hyperobject supported. In C++, every reducer hyperobject has a hyperobject type, which type is an instantiation of the cilk::reducer class template, which is defined in the header <cilk/reducer.h>. The cilk::reducer class template has a single template type parameter, Monoid, which shall be a class type. (See C++ Monoid class requirements, below.)

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() operator*() and value_type& view(), both of which return a an lvalue reference to the current view, and operator->(), which returns the address of the current view.

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

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

Where args is a list of M::valueview_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*() or operator->() on the hyperobject, as in the following examples:

hv.view().append(elem);
(*hv).append(elem);
hv->append(elem); hv().append(elem);

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 possible for the monoid to define a separate view_type class that wraps the value_type and exposes only the valid associative operations. (See Monoid and View class requirements, below.) All of the reducers included in the standard reducer library have such wrappers.

C++ reducer class template

Where the below table indicates that the signature of a function includes the form Args&&..., in an implementation that supports C++ variadic templates, the function shall be defined as a variadic function template. In an implementation that does not support variadic templates, the function shall be defined as a set of templates taking from 0 to N arguments of type const Arg &, where N is at least 4.

Member Purpose
typename Monoid
Template parameter
typedef
typename Monoid::value_type
value_type;
Typedef for the type of the data being reduced.
typedef
typename Monoid::view_type
view_type;
Typedef for the type actually returned by a hyperobject lookup. view_type can be the same as value_type (see below).
template<typename... Args>
reducer(const Args&&... args);
Default-initialize the monoid and construct the leftmost view using constructor arguments, args.
template<typename... Args>
reducer(const Monoid& m,
const Args&&... args);
Initialize the monoid from m and construct the leftmost view using constructor arguments, args. This constructor is useful only for the rare monoid type that contains state. The monoid state is shared by all views of the reducer.
Monoid& monoid();
Monoid const& monoid() const;
Return the monoid instance for this reducer. The same monoid instance is returned for a given reducer regardless of which strand invoked this accessor. This accessor is useful only for the rare monoid type that contains state.
view_type& view();
view_type& view() const;
Return an lvalue reference to the current view (i.e., the view associated with the currently-executing strand).
void move_in(value_type& obj);
Replace the value in the current view with obj. The value of obj after this operation is unspecified. Note that using this operation in parallel with other operations on the same reducer will cause the final reducer value to be indeterminate.
void move_out(value_type& obj);
Replace the value of obj with the value of the current view. The value of the view after this operation is unspecified. Note that using this operation in parallel with other operations on the same reducer will place an indeterminate value in obj and cause the final reducer value to be indeterminate.
void set_value(const value_type& obj);
Replace the value in the current view with obj. Note that using this operation in parallel with other operations on the same reducer will cause the final reducer value to be indeterminate.
type get_value() const;
Return the value of the current view. Note that using this operation in parallel with other operations on the same reducer will return an indeterminate value. The return type is const value_type& if view_type is identical to value_type; otherwise the return value is the same as that returned by view_type::view_get_value().

C++ Monoid class requirements

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

Member name/signature Purpose
value_type
typedef for T, the type of the data being reduced
view_type
typedef for the type actually returned by a hyperobject lookup. view_type can be the same as value_type (see below).
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; return type shall be void*
deallocate(value_type void* p)
deallocate the raw memory at *p, where p is a value returned by a previous call to allocate

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,V>, which defines value_type and view_type as aliases for T and V, respectively (where V defaults to T), 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.

C++ View class requirements

By default, view_type is the same as value_type. Commonly, however, it is a wrapper around value_type that presents a more limited interface in order to achieve a measure of static safety. For example, for a summing reducer, view_type might support += and ++ but not operations like *= that are inconsistent with a summing reduction. Other times, view_type holds a more complex type that allows for more efficient reduction operations.

When view_type is identical to value_type the reducer imposes no further requirements on it beyond those already required by the identity and reduce operations in the monoid.

When view_type differs from value_type, then view_type must provide the following member functions:

Signature Purpose
view_move_in(value_type& v)
Clear the existing contents of the view and replace it with the value v. After calling this function, the new value of v is unspecified (but valid).
view_move_out(value_type& v)
Move the value of the view into v. After calling this function, the new value of the view is unspecified.
view_set_value(const value_type& v)
Set the value of the view to v.
view_get_value() const
Return the value of the view, either as an rvalue or as a const lvalue.

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 header <cilk/reducer.h>. 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:

Function tag Purpose
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_DECLARE_REDUCER(T) hv =
	CILK_C_INIT_REDUCER(T_identity, T_reduce, T_destroy,
		init);

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:

extern CILK_C_DECLARE_REDUCER(T) hv;

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 The behavior is undefined if a reducer is local to a function, it shall be with automatic storage duration is not registered before first use using the CILK_C_REGISTER_REDUCER macro and unregistered after its last use using the CILK_C_UNREGISTER_REDUCER macro:

CILK_C_REGISTER_REDUCER(hv);
/* use hv here */
CILK_C_UNREGISTER_REDUCER(hv);

For the purpose of registration and unregistration, first use and last use are defined with respect to the serialization of the program. If the reducer view immediately before unregistration shall be is not the same (does not have the same address) as the reducer view immediately after registration, the behavior is undefined. 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:

CILK_C_DECLARE_REDUCER(long) hv =
	REDUCER_OPADD_INIT(long, 0);

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(T,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 the start of any strand 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.


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Acknowledgements

We gratefully acknowledge Intel Corporation for generously allowing this OpenCilk document to incorporate material from the following Intel document number 324396-003USr, which may be available here: https://www.cilkplus.org/sites/default/files/open_specifications/Intel_Cilk_plus_lang_spec_1.2.htm

The OpenCilk project was sponsored in part by the United States Air Force Research Laboratory and was accomplished under Cooperative Agreement Number FA8750-19-2-1000. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the United States Air Force or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein.