Enabling Link-Time Optimization - Keil

You must enable Link-Time Optimization (LTO) in both armclang and armlink . To enable LTO: At compilation time, use the armclang option -flto to produce ELF ... 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ToenableLTO: Atcompilationtime,usethearmclangoption-fltotoproduceELFfilessuitableforLTO.TheseELFfilescontainbitcodeina.llvmbcsection. Note:Thearmclangoption-Omaxautomaticallyenablesthe-fltooption. Atlinktime,usethearmlinkoption--ltotoenableLTOforthespecifiedbitcodefiles. Note:Ifyouusethe-fltooptionwithoutthe-coption,armclangautomatically passesthe--ltooptiontoarmlink. Example1:Optimizingallsourcefiles ThefollowingexampleperformsLTOacrossallsourcefiles: armclang--target=arm-arm-none-eabi-march=armv8-a-fltosrc1.csrc2.csrc3.c-ooutput.axf Thisexampledoesthefollowing: armclangcompilestheCsourcefilessrc1.c,src2.c,andsrc3.ctotheELFfilessrc1.o,src2.o,andsrc3.o.TheseELFfilescontainbitcode,andthereforefromelfcannotdisassemblethem. armclangautomaticallyinvokes armlinkwiththe--ltooption. armlinkpassesthebitcodefilessrc1.o,src2.o,andsrc3.otothelink-timeoptimizertoproduceasingleoptimizedELFobjectfile. armlinkcreatestheexecutable output.axffromtheELFobjectfile. Note:Inthisexample,asarmclangautomaticallycallsarmlink,thelink-timeoptimizerhasthesameoptimizationlevelasarmclang.Asnooptimizationlevelisspecifiedforarmclang,itisthedefaultoptimizationlevel-O0,and--lto_level=O0. Example2:Optimizingasubsetofsourcefiles ThefollowingexampleperformsLTOforasubsetofsourcefiles. armclang--target=arm-arm-none-eabi-march=armv8-a-csrc1.c-osrc1.o armclang--target=arm-arm-none-eabi-march=armv8-a-c-fltosrc2.c-osrc2.o armclang--target=arm-arm-none-eabi-march=armv8-a-c-fltosrc3.c-osrc3.o armlink--ltosrc1.osrc2.osrc3.o-ooutput.axf Thisexampledoesthefollowing: armclangcompilestheCsource filesrc1.ctotheELFobjectfilesrc1.o. armclangcompilestheCsource filessrc2.candsrc3.ctotheELFfilessrc2.o andsrc3.o.TheseELFfilescontain bitcode. armlinkpassesthebitcodefilessrc2.oandsrc3.otothelink-timeoptimizertoproduceasingleoptimizedELFobjectfile. armlinkcombinestheELFobjectfilesrc1.owiththeobjectfilethatthelink-timeoptimizerproducestocreatetheexecutableoutput.axf. Note:Inthisexample,becausearmclangandarmlinkarecalledseparately,theyhaveindependentoptimizationlevels.Asnooptimizationlevelisspecifiedforarmclangorarmlink,armclanghasthedefaultoptimizationlevel-O0andthelink-timeoptimizerhasthedefaultoptimizationlevel--lto_level=O2.Youcancallarmclangandarmlinkwithanycombinationofoptimizationlevels. 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