GCC compilation options

• -fno-builtin Do not use built-in functions

• -ffunction-sections

• -fdata-sections
  demand ： Some functions or variables do not need to be programmed when they are not used
  GCC The link operation is based on section As the smallest processing unit , As long as a section A symbol in is referenced , The section Will be added to the executable program . therefore ,GCC You can use -ffunction-sections and -fdata-sections Create each function or symbol as a sections, Each of them sections Name and function or data Keep the names the same . And in the link phase , -Wl,–gc-sections Instructs the linker to remove unused section（ among -Wl, Represents the following parameter -gc-sections Pass it to the linker ）, This reduces the size of the final executable .
  We often use the following configuration to enable this function ：
  CFLAGS += -ffunction-sections -fdata-sections
  LDFLAGS += -Wl,–gc-sections

• -Wall Discover a series of common error warnings in the program

• -D Defining macro
  -DMACRO In string “1” Definition MACRO macro .
  -DMACRO=DEFN In string “DEFN” Definition MACRO macro .

• -O Optimize registration
  O0 -->> O1 -->> O2 -->>Os–>> O3
  -O0 No optimization ,-O1 Is the default value ,-O3 The highest level of optimization
  https://blog.csdn.net/zzhongcy/article/details/91445924

• mabi Specify the data model and floating-point parameter transfer rules
  https://blog.csdn.net/zoomdy/article/details/79353313

• -nostartfiles When connecting, do not use the startup file of the standard system
  https://my.oschina.net/saly/blog/130920

• -T,–script=: Use scriptfile As linker scripts . This script will replace ld Default linker script for （ Instead of adding to it ）, So the script has to specify everything that is required for the output file . If there is no script file in the current directory ,“ld” Will be in -L Search in the directory specified by the

• [email protected] Target file ,$^ All dependent files ,$< The first dependency file
  give an example gcc -o main main.o
  Equivalent gcc -o [email protected] $^

• -lm Option tells the compiler , The mathematical functions used in our program should reach libm.so Look in the library file
  （ for example printf） be located libc.so Library file , Use libc.so The library functions in do not need to be added at compile time -lc Options , Because this option is gcc Default options