Reduce program ROM ram, GCC -ffunction sections -fdata sections -wl, – detailed explanation of GC sections parameters

In this paper, from p_fly The blog of ：https://blog.csdn.net/pengfei240/article/details/55228228
author ： Pit digging specialist

background

Sometimes our program will define some functions and functions that are temporarily unavailable , Although we don't use these functions and functions , But they tend to waste our ROM and RAM Space . This is when using static libraries , More serious . Sometimes , We only use a few functions of the static library , However, the system will automatically link the whole static library to the executable program by default , The size of the executable program is greatly increased .

Parameters,

In order to solve the problems analyzed above , We introduced several parameters in the title .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

Example

main.c The documents are as follows ：

#include <stdio.h>

int fun_0(void)
{
    printf("%s: %d\n", __FUNCTION__, __LINE__);
    return 0;
}

int fun_1(void)
{
    printf("%s: %d\n", __FUNCTION__, __LINE__);
    return 0;
}

int fun_2(void)
{
    printf("%s: %d\n", __FUNCTION__, __LINE__);
    return 0;
}

int fun_3(void)
{
    printf("%s: %d\n", __FUNCTION__, __LINE__);
    return 0;
}

void main(void)
{
        fun_0();
        fun_3();
}

Makefile The documents are as follows ：

main_sections:
        gcc -ffunction-sections -fdata-sections -c main.c
        gcc -Wl,--gc-sections -o [email protected] main.o 

main_normal:
        gcc -c main.c
        gcc -o [email protected] main.o

clean:
        rm -rf *.o main_sections main_normal

verification

function

$ make main_sections
gcc -ffunction-sections -fdata-sections -c main.c
gcc -Wl,--gc-sections -o main_sections main.o 
$ make main_normal
gcc -c main.c
gcc -o main_normal main.o

Compare the size

$ ll main_*
-rwxrwxr-x 1 8896 2 month   16 00:42 main_normal*
-rwxrwxr-x 1 8504 2 month   16 00:42 main_sections*

To see that the binary file using this function is smaller than the binary file not using this function

analysis sections

$ make clean
rm -rf *.o main_sections main_normal
$ make main_sections
gcc -ffunction-sections -fdata-sections -c main.c
gcc -Wl,--gc-sections -o main_sections main.o 
$ readelf -t main.o
...
  [ 5] .text.fun_0
       PROGBITS               PROGBITS         0000000000000000  0000000000000048  0
       0000000000000024 0000000000000000  0                 1
       [0000000000000006]: ALLOC, EXEC
  [ 6] .rela.text.fun_0
       RELA                   RELA             0000000000000000  0000000000000508  24
       0000000000000048 0000000000000018  5                 8
       [0000000000000040]: INFO LINK
  [ 7] .text.fun_1
       PROGBITS               PROGBITS         0000000000000000  000000000000006c  0
       0000000000000024 0000000000000000  0                 1
       [0000000000000006]: ALLOC, EXEC
  [ 8] .rela.text.fun_1
       RELA                   RELA             0000000000000000  0000000000000550  24
       0000000000000048 0000000000000018  7                 8
       [0000000000000040]: INFO LINK
  [ 9] .text.fun_2
       PROGBITS               PROGBITS         0000000000000000  0000000000000090  0
       0000000000000024 0000000000000000  0                 1
       [0000000000000006]: ALLOC, EXEC
  [10] .rela.text.fun_2
       RELA                   RELA             0000000000000000  0000000000000598  24
       0000000000000048 0000000000000018  9                 8
       [0000000000000040]: INFO LINK
  [11] .text.fun_3
       PROGBITS               PROGBITS         0000000000000000  00000000000000b4  0
       0000000000000024 0000000000000000  0                 1
       [0000000000000006]: ALLOC, EXEC
  [12] .rela.text.fun_3
       RELA                   RELA             0000000000000000  00000000000005e0  24
       0000000000000048 0000000000000018  11                8
       [0000000000000040]: INFO LINK

from object You can find ,fun_0 ~ fun_3 Each function is an independent section.
And if you use make main_normal Generated object file , Then share a default sections（.text）.

analysis elf file

$ readelf -a main_normal | grep fun_
    52: 0000000000400526    36 FUNC    GLOBAL DEFAULT   14 fun_0
    55: 000000000040056e    36 FUNC    GLOBAL DEFAULT   14 fun_2
    65: 0000000000400592    36 FUNC    GLOBAL DEFAULT   14 fun_3
    66: 000000000040054a    36 FUNC    GLOBAL DEFAULT   14 fun_1
$ readelf -a main_sections | grep fun_
    49: 0000000000400526    36 FUNC    GLOBAL DEFAULT   14 fun_0
    57: 000000000040054a    36 FUNC    GLOBAL DEFAULT   14 fun_3

Can see , In the final target document , Unused functions are not linked to the final object file .