Cilk


Cilk, Cilk++, Cilk Plus and OpenCilk are general-purpose programming languages designed for multithreaded parallel computing. They are based on the C and C++ programming languages, which they extend with constructs to express parallel loops and the fork–join idiom.
Originally developed in the 1990s at the Massachusetts Institute of Technology in the group of Charles E. Leiserson, Cilk was later commercialized as Cilk++ by a spinoff company, Cilk Arts. That company was subsequently acquired by Intel, which increased compatibility with existing C and C++ code, calling the result Cilk Plus. After Intel stopped supporting Cilk Plus in 2017, MIT is again developing Cilk in the form of OpenCilk.

History

MIT Cilk

The Cilk programming language grew out of three separate projects at the MIT Laboratory for Computer Science:
  • Theoretical work on scheduling multi-threaded applications.
  • StarTech – a parallel chess program built to run on the Thinking Machines Corporation's Connection Machine model CM-5.
  • PCM/Threaded-C – a C-based package for scheduling continuation-passing-style threads on the CM-5
In April 1994 the three projects were combined and christened "Cilk". The name Cilk is not an acronym, but an allusion to "nice threads" and the C programming language. The Cilk-1 compiler was released in September 1994.
The original Cilk language was based on ANSI C, with the addition of Cilk-specific keywords to signal parallelism. When the Cilk keywords are removed from Cilk source code, the result should always be a valid C program, called the serial elision of the full Cilk program, with the same semantics as the Cilk program running on a single processor. Despite several similarities, Cilk is not directly related to AT&T Bell Labs' Concurrent C.
Cilk was implemented as a translator to C, targeting the GNU C Compiler. The last version, Cilk 5.4.6, is available from the MIT Computer Science and Artificial Intelligence Laboratory, but is no longer supported.
A showcase for Cilk's capabilities was the Cilkchess parallel chess-playing program, which won several computer chess prizes in the 1990s, including the 1996 Open Dutch Computer Chess Championship.

Cilk Arts and Cilk++

Prior to, the market for Cilk was restricted to high-performance computing. The emergence of multicore processors in mainstream computing meant that hundreds of millions of new parallel computers were being shipped every year. Cilk Arts was formed to capitalize on that opportunity: in 2006, Leiserson launched Cilk Arts to create and bring to market a modern version of Cilk that supports the commercial needs of an upcoming generation of programmers. The company closed a Series A venture financing round in October 2007, and its product, Cilk++ 1.0, shipped in December, 2008.
Cilk++ differs from Cilk in several ways: support for C++, support for loops, and [|hyperobjects] a new construct designed to solve data race problems created by parallel accesses to global variables. Cilk++ was proprietary software. Like its predecessor, it was implemented as a Cilk-to-C++ compiler. It supported the Microsoft and GNU compilers.

Intel Cilk Plus

On July 31, 2009, Cilk Arts announced on its web site that its products and engineering team were now part of Intel Corp. In early 2010, the Cilk website at www.cilk.com began redirecting to the Intel website. Intel and Cilk Arts integrated and advanced the technology further resulting in a September 2010 release of Intel Cilk Plus. Cilk Plus adopts simplifications, proposed by Cilk Arts in Cilk++, to eliminate the need for several of the original Cilk keywords while adding the ability to spawn functions and to deal with variables involved in reduction operations. Cilk Plus differs from Cilk and Cilk++ by adding array extensions, being incorporated in a commercial compiler, and compatibility with existing debuggers.
Cilk Plus was first implemented in the Intel C++ Compiler with the release of the Intel compiler in Intel Composer XE 2010. An open source implementation was contributed by Intel to the GNU Compiler Collection, which shipped Cilk Plus support in version 4.9, except for the keyword, which was added in GCC 5.0. In February 2013, Intel announced a Clang fork with Cilk Plus support. The Intel Compiler, but not the open source implementations, comes with a race detector and a performance analyzer.
Intel later discontinued it, recommending its users switch to instead using either OpenMP or Intel's own TBB library for their parallel programming needs.

Differences between versions

In the original MIT Cilk implementation, the first Cilk keyword is in fact cilk, which identifies a function which is written in Cilk. Since Cilk procedures can call C procedures directly, but C procedures cannot directly call or spawn Cilk procedures, this keyword is needed to distinguish Cilk code from C code. Cilk Plus removes this restriction, as well as the cilk keyword, so C and C++ functions can call into Cilk Plus code and vice versa.

Deprecation of Cilk Plus

In May, 2017, GCC 7.1 was released and marked Cilk Plus support as deprecated. Intel itself announced in September 2017 that they would deprecate Cilk Plus with the 2018 release of the Intel Software Development Tools. In May 2018, GCC 8.1 was released with Cilk Plus support removed.

OpenCilk

After Cilk Plus support was deprecated by Intel, MIT has taken on the development of Cilk in the OpenCilk implementation, focusing on the LLVM/Clang fork now termed "Tapir". OpenCilk remains largely compatible with Intel Cilk Plus. Its first stable version was released in March 2021.

Language features

The principle behind the design of the Cilk language is that the programmer should be responsible for exposing the parallelism, identifying elements that can safely be executed in parallel; it should then be left to the run-time environment, particularly the scheduler, to decide during execution how to actually divide the work between processors. It is because these responsibilities are separated that a Cilk program can run without rewriting on any number of processors, including one.

Task parallelism: spawn and sync

Cilk's main addition to C are two keywords that together allow writing task-parallel programs.
  • The keyword, when preceding a function call, indicates that the function call can safely run in parallel with the statements following it in the calling function. Note that the scheduler is not obligated to run this procedure in parallel; the keyword merely alerts the scheduler that it can do so.
  • A statement indicates that execution of the current function cannot proceed until all previously spawned function calls have completed. This is an example of a barrier method.
Below is a recursive implementation of the Fibonacci function in Cilk, with parallel recursive calls, which demonstrates the, and keywords. The original Cilk required any function using these to be annotated with the keyword, which is gone as of Cilk Plus.

cilk int fib

If this code was executed by a single processor to determine the value of, that processor would create a frame for, and execute lines 1 through 5. On line 6, it would create spaces in the frame to hold the values of and. On line 8, the processor would have to suspend the current frame, create a new frame to execute the procedure, execute the code of that frame until reaching a return statement, and then resume the frame with the value of fib placed into 's variable. On the next line, it would need to suspend again to execute and place the result in 's variable.
When the code is executed on a multiprocessor machine, however, execution proceeds differently. One processor starts the execution of ; when it reaches line 8, however, the keyword modifying the call to tells the processor that it can safely give the job to a second processor: this second processor can create a frame for, execute its code, and store its result in 's frame when it finishes; the first processor continues executing the code of at the same time. A processor is not obligated to assign a spawned procedure elsewhere; if the machine only has two processors and the second is still busy on when the processor executing gets to the procedure call, the first processor will suspend and execute itself, as it would if it were the only processor. Of course, if another processor is available, then it will be called into service, and all three processors would be executing separate frames simultaneously.
If the processor executing were to execute line 13 before both of the other processors had completed their frames, it would generate an incorrect result or an error; would be trying to add the values stored in and, but one or both of those values would be missing. This is the purpose of the keyword, which we see in line 11: it tells the processor executing a frame that it must suspend its own execution until all the procedure calls it has spawned off have returned. When is allowed to proceed past the statement in line 11, it can only be because and have completed and placed their results in and, making it safe to perform calculations on those results.
The code example above uses the syntax of Cilk-5. The original Cilk used a rather different syntax that required programming in an explicit continuation-passing style, and the Fibonacci examples looks as follows:

thread fib
thread sum

Inside 's recursive case, the keyword indicates the creation of a successor thread, which executes the subroutine after waiting for the continuation variables and to be filled in by the recursive calls. The base case and use a operation to set their continuation variable to the value of, effectively "returning" the value to the successor thread.