东南大学编译原理课件cp

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1、Chapter9CodeOptimizationZhaiYuqingZhouXiaoyuOverviewPositionofcodeoptimizerFrontendSourceprogramCodeoptimizerCodegeneratorIntermediatecodeIntermediatecodetargetprogramPurposeofCodeOptimizertogetbetterefficiencyRunfasterTakelessspacePlacesforPotentialImprovementsSour

2、cecodeUsercanprofileprogram,changealgorithmortransformloops.IntermediatecodeCompilercanimproveloops,procedurecallsoraddresscalculationsPlacesforPotentialImprovementsTargetcodeCompilercanuseregisters,selectinstructionsordopeepholetransformationsOrganizationofthecodeO

3、ptimizerControl-FlowAnalysisIdentifyblocksandloopsintheflowgraphofaprogramData-flowanalysisCollectinformationabouttheprogramasawholeandtodistributethisinformationtoeachblockintheflowgraph.Control-flowanalysisData-flowanalysistransformationsBasicblocksMaximalsequence

4、sofconsecutivethree-addressinstructionswiththepropertiesthatTheflowofcontrolcanonlyenterthebasicblockthroughthefirstinstructionintheblock.Controlwillleavetheblockwithouthaltingorbranching,exceptpossiblyatthelastinstructionintheblock.AnExampleT1=a*aT2=a*bT3=2*T2T4=T1

5、+T3T5=b*bT6=T4+T5TheUseofaNameSupposethree-addressstatementiassignsavaluetox.Ifstatementjhasxasanoperand,andcontrolcanflowfromstatementitojalongapaththathasnointerveningassignmentstox,thenwesaystatementjusesthevalueofxcomputedatstatementi.LivevariableWefurthersaytha

6、txisliveatstatementi.Anameinabasicblockissaidtobeliveatagivenpointifitsvalueisusedafterthatpointintheprogram,perhapsinanotherbasicblock.PartitionintoBasicBlocksInput:Asequenceofthree-addressstatementsOutput:Alistofbasicblockswitheachthree-addressstatementinexactlyon

7、eblock.Method:Wefirstdeterminethesetofleaders,thefirststatementsofbasicblocks.DeterminationofLeadersofBasicBlocks(1)Thefirststatementisaleader.(2)Anystatementthatisthetargetofaconditionalorunconditionalgotoisaleader.(3)Anystatementthatimmediatelyfollowsagotoorcondit

8、ionalgotostatementisaleader.PartitionintoBasicBlocksForeachleader,itsbasicblockconsistsoftheleaderandallstatementsuptobutnotincludingthene