Avoid Dead Code Elimiation

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1、IARApplicationNoteG-003IARApplicationNoteG-003Avoid"DeadCodeElimination"SUMMARYCodethatcompilersconsideras"notuseful"isremoved.Thismightcauseunexpectedeffects.Thisapplicationnoteshowshowtoavoidsomeofthesesituations.KEYWORDSUselesscomputation,Deadcodeelimination,Unreachablecode,Optimizati

2、onTheProblemtobesolvedInordertoproduceefficientcode,amodernoptimizingcompilerperformstransformationsby,forexample,removingredundantcalculations,replacingdivisionbyshiftandremovinguselesscalculations.However,whatseemstobeuselesscodeintheexamplesbelowcausesunwantedeffects.Typicallythefollo

3、wingfunctionalityiswanted:1.delay();/*haltexecutionforawhile*/2.wait();/*waitforanevent*/3.forever();/*dosomethingforever*/1.Example"delay()":SourcecodeOptimizedcodevoiddelay(unsignedinttime)voiddelay(unsignedinttime){{for(i=0;i

4、oidbeep(void)SOUNDER=1;{delay(1000);SOUNDER=1;SOUNDER=0;delay(1000);/*nodelay*/}SOUNDER=0;}2.Example"wait()":SourcecodeOptimizedcodecharkeystroke=0;charkeystroke=0;voidmain(void)voidmain(void){{while(!keystroke);/*"useless"loopmightbefoo();removed*/}foo();}1IARApplicationNoteG-0033.Examp

5、le"forever()":SourcecodeOptimizedcodeVoidmain(void)voidmain(void){{while(1)while(1)/*recognizedby{{thecompiler*/foo();foo();}}}}TheDifficultiesInvolvedNormallyyouneitherwanttolowertheoptimizationlevelevenifthiscouldsolvetheproblem,norrewritethecodetofoolthecompilerthatthecodereallyisused

6、.TheSolution1.Example"delay()"Relyingonthetiminginloopsetcinthecompiledcodeisnotrecommendedsinceitisaveryunstablemethod.Changeswithinsuchafunctionmayaffecttheoptimizationsandregisterallocation,whichchangesthetimingoftheloop.Inordertoimplementatimedelayinacontrolledmanner—useatimer!Belowi

7、sanexampleofatimerimplementationforthe"delay()"problemabove:interrupt[21*4]voidSOUNDER_interrupt(void){P5=0;/*ResetSOUNDER(connectedtoP5)*/TABSR.0=0;/*StoptimerA0*/}voidbeep(void){P5=1;/*SetSOUNDER(connectedtoP5)*/TABSR.0=1;/*StarttimerA0*/}voidmain(void){TA0IC=0x6;/*SetT