arm-none-eabi-ar: ../src/n1_main.o plugin needed to handle lto object

Dear all,

I use arm-none-eabi-gcc 7.2.1 in my project, when I want to use -flto option to optimize bin size, error shows:

../../../tools/gcc/linux/gcc-arm-none-eabi/bin/arm-none-eabi-ar: creating /proj/lib_m.a
../../../tools/gcc/linux/gcc-arm-none-eabi/bin/arm-none-eabi-ar: /proj/src/n1.o: plugin needed to handle lto object
../../../tools/gcc/linux/gcc-arm-none-eabi/bin/arm-none-eabi-ar: /projsrc/nb_data.o: plugin needed to handle lto object

what's wrong with my code? 

Thank you for your replay.