In this pap er we explain how to tune Lisp co de for p erformance by intro ducing the prop er typ e declarations using the appropriate data structures and compiler information We also explain how eciency is achieved by the compilers These techniques ID: 11022
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suchasstructs,arraysandhashtables.Arraysal-lowyoutostoreandaccessLispobjectsaccordingtoarectilinearcoordinatesystem,andthuscanbecon-sideredastheequivalentofmalloc'edorcalloc'edmemoryareasinC.TheLispfunctionforcreat-ingarraysismake-array.InLisp,arrayscanbemultidimensional.Onlisting2,youcanseeacalltoarray-dimensionwhichretrievesthearray's rstranksize(noneedtostorethisinformationinadatastructureasinC).Rememberthatweareusing1Dlineararraystorepresentourimages.Thefunctionforaccessingarrayelementsisarefandassignationisperformedviasetf.Unsurprisingly,dotimesisamacroperformingaloopinamannersimilartotheforlanguageconstructinC.RunningthisfunctioninaLispinterpretershowsthatitisapproximately2300timesslowerthantheCversion(systemtimeandgarbagecollectionex-cluded).Thecompiledversionhoweverisonly60timesslowerthantheequivalentCcode.Finally,evenwithoptimizationturnedon(seesection5.5),wearestill20timesslowerthanC.Tounderstandwhywearegettingthispoorperfor-mance,onehastorealizethatourLispcodeisun-typed:contrarytoC,thevariablesandfunctionar-gumentsweusedcouldholdanyLispobject.Forinstance,weusearray-dimensiononthefunctionparameterto,butnothingpreventsusfrompassingsomethingelsethananarray;weperformarithmeticoperationsonthefunctionparameterval,butnoth-ingpreventsusfrompassingsomethingelsethananumber.Asaconsequence,thecompiledLispcodehastocheckdynamicallythatthevariablesweuseareofthepropertypewithrespecttotheoperationswewanttoapplytothem.OurnextstepshouldthenbetoprovidetypeinformationtotheLispcompiler,justaswedoinC.5TypingLispCode5.1TypingmechanismsTheCommon-Lispstandardprovidesmeanstode-claretheexpectedtypesofLispobjectsatcompiletime.Itshouldbenotedhoweverthatoneisneverforcedtodeclarethetypeofavariable:typescanbedeclaredwhentheyareknown,orleftunspeci edotherwise.TheLispcompilersareexpectedtodothebesttheycanaccordingtotheinformationtheyhave. Inaway,sincethestandardizationofCommon-Lisp,itisnotcorrectanymoretosaythatLispisadynam-icallytypedlanguage:itcanbeeitherdynamicallyorstaticallytypedattheprogrammer'swill.TherearedierentwaystospecifytypesinCommon-Lisp.The rstoneisbypassingspeci cargumentstofunctions.Forinstance,ifyouwanttocreateanarrayandyouknowthatthisarraywillonlycontainsingleprecision oatingpointnumbers,youcanpassthe:element-typekeywordparametertothefunc-tionmake-arraylikethis: (make-arraysize:element-type'single-float)Thenextwaytospecifytypesisbymeansofdecla-rations:thismechanismisusedtoprecisethetypeofafunctionparameterorafreshlyboundvariable.Atypedeclarationshouldappearnearthe rstoc-currenceofthevariableitrefersto.Listing3showsthenextversionofouradditionalgorithm,withtypedeclarationsissued. (defunadd(tofromval)(declare(type(simple�arraysingle�float())tofrom))(declare(typesingle�floatval))(let((size(array�dimensionto0)))(dotimes(isize)(setf(areftoi)(+(areffromi)val))))) Listing3:PixelAddition,TypedLispVersion Asyoucansee,wehavedeclaredthe(expected)typeofthe3functionparameters:twoarraysofsingle-floatvalues(inCommon-Lisp,simplear-raydenotestheequivalentofCarrays;thereareotherkindsofarraysthatwewillnotdescribehere),andasingle-floatparameter.The(*)speci ca-tionindicatesthatthearraysare1D,buttheactualsizeisunknown(atcompile-time).2Darrayswouldhavebeendescribedwith(**)forinstance.Athirdwaytoprovidetypedeclarationswillbede-scribedinsection6.2.5.2ObjectsRepresentationTounderstandwhytypedeclarationsmatterforop-timizingLispperformance,onehastorealizetheim-plicationsofdynamictypingabit.SinceLispobjectscanbeofanytype(worse:ofanysize),thevariablesinLispdon'tcarrytypeinformation.Rather,theobjectsthemselvesareresponsibleforprovidingtheirowntype.Thismeansthatmostofthetime,Lisp