东南大学 编译原理 课件 Cp9 (2).ppt

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1、Chapter9CodeOptimizationZhaiYuqingZhouXiaoyuOverviewPositionofcodeoptimizerFrontendSourceprogramCodeoptimizerCodegeneratorIntermediatecodeIntermediatecodetargetprogramPurposeofCodeOptimizertogetbetterefficiencyRunfasterTakelessspacePlacesforPotentialImprovementsSourcecodeUsercanp

2、rofileprogram,changealgorithmortransformloops.IntermediatecodeCompilercanimproveloops,procedurecallsoraddresscalculationsPlacesforPotentialImprovementsTargetcodeCompilercanuseregisters,selectinstructionsordopeepholetransformationsOrganizationofthecodeOptimizerControl-FlowAnalysis

3、IdentifyblocksandloopsintheflowgraphofaprogramData-flowanalysisCollectinformationabouttheprogramasawholeandtodistributethisinformationtoeachblockintheflowgraph.Control-flowanalysisData-flowanalysistransformationsBasicblocksMaximalsequencesofconsecutivethree-addressinstructionswit

4、hthepropertiesthatTheflowofcontrolcanonlyenterthebasicblockthroughthefirstinstructionintheblock.Controlwillleavetheblockwithouthaltingorbranching,exceptpossiblyatthelastinstructionintheblock.AnExampleT1=a*aT2=a*bT3=2*T2T4=T1+T3T5=b*bT6=T4+T5TheUseofaNameSupposethree-addressstatem

5、entiassignsavaluetox.Ifstatementjhasxasanoperand,andcontrolcanflowfromstatementitojalongapaththathasnointerveningassignmentstox,thenwesaystatementjusesthevalueofxcomputedatstatementi.LivevariableWefurthersaythatxisliveatstatementi.Anameinabasicblockissaidtobeliveatagivenpointifit

6、svalueisusedafterthatpointintheprogram,perhapsinanotherbasicblock.PartitionintoBasicBlocksInput:Asequenceofthree-addressstatementsOutput:Alistofbasicblockswitheachthree-addressstatementinexactlyoneblock.Method:Wefirstdeterminethesetofleaders,thefirststatementsofbasicblocks.Determ

7、inationofLeadersofBasicBlocks(1)Thefirststatementisaleader.(2)Anystatementthatisthetargetofaconditionalorunconditionalgotoisaleader.(3)Anystatementthatimmediatelyfollowsagotoorconditionalgotostatementisaleader.PartitionintoBasicBlocksForeachleader,itsbasicblockconsistsoftheleader

8、andallstatementsuptobutnotincludingthene