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Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

ISO/C++17andBeyond:ParallelismandConcurrency

(breakoutsession)

DOECOEPerformancePortability

August22-24,2017Denver,CO

SAND2017-8950PE

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Agenda

§ DOELabparticipationinISO/C++StandardCommittee§ Whyweareinvolved– HPCperformanceportability!§ Yourlab’spoint-of-contact§ Howthe”sausageismade”

§ OverviewofHPCrelevantC++11&17features

§ Overviewofpotential(in-the-works)C++20features§ Whatin-the-worksfeaturesaremostimportanttoyou?§ Whathigh-priorityfeaturesnotin-the-works?

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ISO/C++Committee:YourLab’sPoint-of-Contact§ Eachlabisanindependentcorporatememberofcommittee

§ Oneprimaryrepresentativeexercisesformalvotingrights§ Annualdues,listedinISOdirectory,participateregularly

§ Alternaterepresentativevotingrightsintheabsenceofprimary§ AlsolistedinISOdirectory

§ Anyonefrommemberorgcanparticipateinmeetings

§ Yourlab’sprimaryrepresentative(somealternates?)§ SNL– CarterEdwards§ LANL– StuartHerring§ LLNL– JimReus§ ANL– HalFinkel§ LBNL– BryceLelbach§ ORNL– GrahamLopez

§ FYI:SNLhostingnextISO/C++CommitteeWorkingMeeting§ Nov6-11@Albuquerque

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ISO/C++Committee:HowSausageisSlowlyMade

Language EvolutionWorkingGroup(EWG)

CoreWorkingGroup(CWG)

ProposalPaper

DraftStandard

LibraryEvolutionWorkingGroup(LEWG)

LibraryWorkingGroup(LWG)

ConcurrencyandParallelismStudyGroup(SG1)

ConcurrencyTS

DomainExpertStudyGroups

SubjectTechnicalSpecifications(TS)

FULLCOMMITTEEOFFICIALVOTING

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ISO/C++Committee:Roles&Responsibilities§ ProposalPapers§ Anyonemaysubmitatracked proposalsfor§ Additionsormodificationsto draftstandardortechnicalspecifications

§ DomainExpertStudyGroups§ Applydomainspecific(beyondlanguage/library)expertise;e.g.,§ Concurrency&Parallelism(SG1),FileSystem(SG3),Networking(SG4),...

§ Library(LEWG)andLanguage(EWG)Evolution§ Prioritizationamongalways-too-manyproposalpapers...§ Broadenoughimpacttobeworthsupporting?§ Well-specifiedscope,semantics,interactions,anddomain-expertreview?

§ Library(LWG)andLanguage(CWG)§ Well-specifiedstandardese (wordingforstandard)?§ “QualityAssurance”– mypersonalview§ Typically theonlygroupstobring“strawpoll”motionstothefullcommittee

§ Badformanddramaensuesattemptingtobypassqualityassurance

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HPC-RelevantC++11&17Features§ Parallelalgorithms(C++17)§ Allow functors tobeexecutedinparallelonunspecifiedresources§ ...importantincrementalprogress,C++20improvementsintheworks

§ AtomicOperationsandMemoryModel(C++11,improvedC++17)§ Efficientinter-threadcommunication/synchronization§ Scalableconcurrentlymodifieddatastructures

§ Threads,Mutexes,andConditionsVariables§ C++11pulledpthreads intothestandard w/namechanges&reducedscope

§ Futures,Promises,andAsync L§ ...infamous,whisperedaboutC++11“Konacompromise”§ ...avoidusingthesefornow;atleastbeverycautious

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Productivity-RelevantC++11&17Features

§ LambdaExpressions– inlinefunctors§ C++11:Introduced,dramaticimprovementtoproductivity§ C++17:Languageflawfixedfor[*this]§ NVIDIACUDA8:OffloadlambdaexpressionstoGPU§ Essentialforease-of-useinKokkos,RAJA,...

§ Templatemeta-programmingimprovements§ C++11:Variadic templatearguments§ C++11:<type_traits>§ C++17:constexpr conditionalstatementsJJJJ

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HPC-RelevantPotentialC++20Features§ AtomicOperationsEnhancements§ Floatingpointfetch_add andatomicoperationsonnon-atomictypesØ Buildingblocksenablingscalableparallelscatter-addalgorithms

§ LatchesandBarriers§ Atomic-likethreadsynchronizationmechanisms

§ ExecutorsandExecutionContext§ Executor– specifyhow concurrent/parallelworkisdispatched§ Context– specifywhere concurrent/parallelworkisdispatched§ Fixfuturesandasync

§ Wavefront extensionstoparallelalgorithms§ ”Staggered”or“pipelined”parallelexecutionofloops

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HPC-RelevantPotentialC++20Features§ Coroutines (TS)§ Functionsdesignedtobecallediteratively/concurrently§ Well-definedsuspension/resumption

§ SIMDTypes– PortableVectorIntrinsics§ Guarantee arithmeticoperationsmaptointrinsics forvectorhardware§ Addressvectorwidth,intra-laneoperations,conditionalcontrolflow,...

§ MultidimensionalArrays(finally!)with PolymorphicLayout§ MotivatedbyKokkosmultidimensionalarrays§ Arraytypeincludesrowmajor,columnmajor,...layoutspecification

§ Modules(TS)– improvecompilationperformance§ Concepts“light”– improvetemplatemeta-programming§ Many,manyyearsinthemaking,withsomereputationsontheline...§ ...questionhasbeenraisedif,givenC++17features,thisisstilluseful😲

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OpenDialogue/DeepDiveasRequested

§ C++11spec,C++17draft,proposalpapers§ cppreference.com forC++11,C++14,C++17,andTStowardC++20§ alsohavesomein-hand

§ Prioritiesforpotentialfeaturesmentioned?§ Mayneedtobechampionedbyyou/yourDOELabreps

§ Importantneededfeaturesnotmentioned?§ Mayhavefailedtomention§ Mayneedtobeaddedandchampionedbyyou/yourDOELabreps

§ Deeperlookatexisting/proposedfeatures?§ Sufficientnumberpresentwanttodeep-dive?doitnow§ Otherwiseanoff-linesmallgroupactivity