understanding_strict_aliasing

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1、UnderstandingStrictAliasingALIASINGOnepointerissaidtoaliasanotherpointerwhenbothrefertothesamelocationorobject.Inthisexample,0uint32_t1swap_words(uint32_targ)2{3uint16_t*constsp=(uint16_t*)&arg;4uint16_thi=sp[0];5uint16_tlo=sp[1];67sp[1]=hi;8sp[0]=lo;910return(arg);11}UsingGCC3.4.1andabove

2、,theabovecodewillgeneratewarning:dereferencingtype-punnedpointerwillbreakstrict-aliasingrulesonline3.Thememoryreferredtobyspisanaliasofargbecausetheyrefertothesameaddressinmemory.InC99,itisillegaltocreateanaliasofadifferenttypethantheoriginal.Thisisoftenreferedtoasthestrictaliasingrule.The

3、ruleisenabledbydefaultinGCCatoptimizationlevelsatoraboveO2.Althoughtheaboveexamplewouldcompile,theresultsareundefined.Morethanlikely（很有可能）,argwouldbereturnedunchangedbecauseapointertouint16_tcannotbeanaliastoapointertouint32_twhenapplyingthestrictaliasingrule.Dereferencingacastofavariablef

4、romonetypeofpointertoadifferenttypeisusuallyinviolation（违反）ofthestrictaliasingrule.However,havingmultiplerepresentationsofthesamelocationinmemoryisoftenbeneficial.Properlybalancingthecompiler'smemoryoptimizationsandtheprogrammer'soptimizationsbasedonreal-worldcontextanddataisabitofablackar

5、t.Itrequiresanunderstandingofthetradeoffsamongwhat'spermittedbythestandard,what'stherealityofcompilersandthevalueofaparticulartransformationbasedonthearchitectureandthedata.It'sworthitintheendthoughwhentheresultsspeakforthemselves.Alloftheexamplesinthisarticlehavebeentestedwithvariousversi

6、onsofGCC.Althoughyoucanexpectmostoftheexamplestogeneratesimilarresultsacrossthemajorcompilers,programmers'expectationsshouldalwaysbevalidatedforthecompilersandcompilerrevisionsrequired.Readon（继续读下去）fordetailsonthestrictaliasingruleandsomecommonpitfalls.WHATISSTRICTALIASING?Strictaliasingis

7、anassumption,madebytheC(orC++)compiler,thatdereferencingpointerstoobjectsofdifferenttypeswillneverrefertothesamememorylocation(i.e.aliaseachother.)Herearesomebasicexamplesofassumptionsthatmaybemadebythecompilerwhenstrictaliasingisenabled:Pointerstodifferentbui