Editing Cilk (section)

===Reducers and hyperobjects===
Cilk++ added a kind of objects called ''hyperobjects'', that allow multiple strands to share state without [[race condition]]s and without using explicit locks. Each strand has a view on the hyperobject that it can use and update; when the strands synchronize, the views are combined in a way specified by the programmer.<ref>{{cite conference |last1=Frigo |first1=Matteo |first2=Pablo |last2=Halpern |first3=Charles E. |last3=Leiserson |first4=Stephen |last4=Lewin-Berlin |title=Reducers and other Cilk++ hyperobjects |conference=Proc. Annual Symposium on Parallelism in Algorithms and Architectures (SPAA) |publisher=ACM |year=2009 |url=http://www.fftw.org/~athena/papers/hyper.pdf}}</ref>

The most common type of hyperobject is a reducer, which corresponds to the reduction clause in [[OpenMP]] or to the algebraic notion of a [[monoid]]. Each reducer has an [[identity element]] and an [[associative operation]] that combines two values. The archetypal reducer is [[summation]] of numbers: the identity element is zero, and the associative ''reduce'' operation computes a sum. This reducer is built into Cilk++ and Cilk Plus:

<syntaxhighlight lang="cpp">
// Compute ∑ foo(i) for i from 0 to N, in parallel.
cilk::reducer_opadd<float> result(0);
cilk_for (int i = 0; i < N; i++)
    result += foo(i);
</syntaxhighlight>

Other reducers can be used to construct [[linked list]]s or strings, and programmers can define custom reducers.

A limitation of hyperobjects is that they provide only limited [[Indeterminacy in concurrent computation|determinacy]]. Burckhardt ''et al.'' point out that even the sum reducer can result in non-deterministic behavior, showing a program that may produce either {{mono|1}} or {{mono|2}} depending on the scheduling order:<ref>{{cite conference |title=Concurrent Programming with Revisions and Isolation Types |first1=Sebastian |last1=Burckhardt |first2=Alexandro |last2=Baldassin |first3=Daan |last3=Leijen |year=2010 |conference=Proc. [[OOPSLA]]/SPLASH |url=http://research.microsoft.com/pubs/132619/revisions-oopsla2010.pdf}}</ref>

<syntaxhighlight lang="cpp">
void add1(cilk::reducer_opadd<int> &r) { r++; }
// ...
cilk::reducer_opadd<int> r(0);
cilk_spawn add1(r);
if (r == 0) { r++; }
cilk_sync;
output(r.get_value());
</syntaxhighlight>