gcc参数-Wl,–gc-sections，不链接未用函数，减小可执行文件大小

太过爱你忘了你带给我的痛 2021-07-28 15:55 487阅读 0赞

### 背景 ###

在开发一个项目时，使用了非常多的第三方.a静态库文件，导致编译出的可执行文件非常大。这样一是占用ROM空间，二是会导致程序启动加载速度变慢（项目对启动时间非常敏感）。其实，这些静态库中的函数，并非所有都有调用，项目只使用了其中小部分。这种情况下，gcc的“-Wl,–gc-sections”参数，就非常有用。

### 参数说明 ###

英文原版的-Wl,–gc-sections[解释说明][Link 1]如下：

> 6.3.3.2 Compilation options  
> The operation of eliminating the unused code and data from the final executable is directly performed by the linker.
> 
> In order to do this, it has to work with objects compiled with the following options: -ffunction-sections -fdata-sections.
> 
> These options are usable with C and Ada files. They will place respectively each function or data in a separate section in the resulting object file.
> 
> Once the objects and static libraries are created with these options, the linker can perform the dead code elimination. You can do this by setting the -Wl,–gc-sections option to gcc command or in the -largs section of gnatmake. This will perform a garbage collection of code and data never referenced.
> 
> If the linker performs a partial link (-r linker option), then you will need to provide the entry point using the -e / --entry linker option.
> 
> Note that objects compiled without the -ffunction-sections and -fdata-sections options can still be linked with the executable. However, no dead code elimination will be performed on those objects (they will be linked as is).
> 
> The GNAT static library is now compiled with -ffunction-sections and -fdata-sections on some platforms. This allows you to eliminate the unused code and data of the GNAT library from your executable.

大致说明如下：

1.  在编译C、Ada源文件（C++也可以），在gcc/g++编译选项中增加`-ffunction-sections`、`-fdata-sections`，在编译生成的.o目标文件中，会将每个函数或数据段，放在各种单独独立的`section`中；
2.  在链接生成最终可执行文件时，如果带有`-Wl,--gc-sections`参数，并且之前编译目标文件时带有`-ffunction-sections`、`-fdata-sections`参数，则链接器ld不会链接未使用的函数，从而减小可执行文件大小；
3.  如果使用了`-r`的链接参数，来产生重定位的输出，需要显示的调用`-e`参数来指定程序入口。否则`-Wl,--gc-sections`不会生效。

关于GNU ld更多参数的说明，可以参考[官网][Link 2]。

### 使用示例 ###

测试程序section\_test.c如下：

#include <stdio.h>
    
    void func_0(void) {
        printf("%s\n\r", __func__);
    }
    
    void func_1(void) {
        printf("%s\n\r", __func__);
    }
    
    void func_2(void) {
        printf("%s\n\r", __func__);
    }
    
    void func_3(void) {
        printf("%s\n\r", __func__);
    }
    
    void func_4(void) {
        printf("%s\n\r", __func__);
    }
    
    void func_5(void) {
        printf("%s\n\r", __func__);
    }
    
    void func_6(void) {
        printf("%s\n\r", __func__);
    }
    
    void func_7(void) {
        printf("%s\n\r", __func__);
    }
    
    int main(void) {
        func_0();
        return 0;
    }

Makefile如下：

.PHONY: test_sections test_normal
    
    all: test_sections test_normal
    
    test_sections:
    	gcc -ffunction-sections -fdata-sections -c section_test.c
    	gcc -Wl,--gc-sections -o $@ section_test.o
    	mv section_test.o with_section.o
    
    test_normal:
    	gcc -c section_test.c
    	gcc -o $@ section_test.o
    	mv section_test.o no_section.o
    
    clean:
    	rm -rf *.o main_sections main_normal

执行`make`后，编译生成2个.o文件和2个可执行程序：`no_section.o`、`with_section.o`、`test_normal`、`test_sections`。

**首先**，通过`ls -l`查看可执行文件大小。可以看到，没有链接未使用函数的`test_sections`稍微小一点。如果工程中使用的静态库.a多，且未使用的函数也多的话，效果会更显著：

-rwxr-xr-x 1 user user 8864 9月  10 14:40 test_normal
    -rwxr-xr-x 1 user user 8272 9月  10 14:40 test_sections

**然后**，查看.o目标文件，在gcc编译选项`-ffunction-sections -fdata-sections`是否带来区别：

*  没有`-ffunction-sections -fdata-sections`选项的目标文件，只有1个`test`和`rodata`；
 *  带有`-ffunction-sections -fdata-sections`选项的目标文件，分别有`text.func_0`到`text.func_7`，以及8个`rodata.__func__`。说明gcc在编译生成目标文件时，将每个函数或数据段，放在各种单独独立的`section`；

$ readelf -t no_section.o 
    There are 13 section headers, starting at offset 0xa08:
    
    Section Headers:
      [Nr] Name
           Type              Address          Offset            Link
           Size              EntSize          Info              Align
           Flags
      [ 0] 
           NULL                   NULL             0000000000000000  0000000000000000  0
      [ 1] .text
           PROGBITS               PROGBITS         0000000000000000  0000000000000040  0
      [ 2] .rela.text
           RELA                   RELA             0000000000000000  0000000000000670  10
      [ 3] .data
           PROGBITS               PROGBITS         0000000000000000  0000000000000148  0
      [ 4] .bss
           NOBITS                 NOBITS           0000000000000000  0000000000000148  0
      [ 5] .rodata
           PROGBITS               PROGBITS         0000000000000000  0000000000000148  0

$ readelf -t with_section.o 
    There are 38 section headers, starting at offset 0xce8:
    
    Section Headers:
      [Nr] Name
           Type              Address          Offset            Link
           Size              EntSize          Info              Align
           Flags
      [ 0] 
           NULL                   NULL             0000000000000000  0000000000000000  0
      [ 1] .text
           PROGBITS               PROGBITS         0000000000000000  0000000000000040  0
      [ 2] .data
           PROGBITS               PROGBITS         0000000000000000  0000000000000040  0
      [ 3] .bss
           NOBITS                 NOBITS           0000000000000000  0000000000000040  0
      [ 4] .rodata
           PROGBITS               PROGBITS         0000000000000000  0000000000000040  0
      [ 5] .text.func_0
           PROGBITS               PROGBITS         0000000000000000  0000000000000045  0
      [ 6] .rela.text.func_0
           RELA                   RELA             0000000000000000  0000000000000808  35
      [ 7] .text.func_1
           PROGBITS               PROGBITS         0000000000000000  0000000000000064  0
      [ 8] .rela.text.func_1
           RELA                   RELA             0000000000000000  0000000000000850  35
      [ 9] .text.func_2
           PROGBITS               PROGBITS         0000000000000000  0000000000000083  0
      [10] .rela.text.func_2
           RELA                   RELA             0000000000000000  0000000000000898  35
      [11] .text.func_3
           PROGBITS               PROGBITS         0000000000000000  00000000000000a2  0
      [12] .rela.text.func_3
           RELA                   RELA             0000000000000000  00000000000008e0  35
      [13] .text.func_4
           PROGBITS               PROGBITS         0000000000000000  00000000000000c1  0
      [14] .rela.text.func_4
           RELA                   RELA             0000000000000000  0000000000000928  35
      [15] .text.func_5
           PROGBITS               PROGBITS         0000000000000000  00000000000000e0  0
      [16] .rela.text.func_5
           RELA                   RELA             0000000000000000  0000000000000970  35
      [17] .text.func_6
           PROGBITS               PROGBITS         0000000000000000  00000000000000ff  0
      [18] .rela.text.func_6
           RELA                   RELA             0000000000000000  00000000000009b8  35
      [19] .text.func_7
           PROGBITS               PROGBITS         0000000000000000  000000000000011e  0
      [20] .rela.text.func_7
           RELA                   RELA             0000000000000000  0000000000000a00  35
      [21] .text.main
           PROGBITS               PROGBITS         0000000000000000  000000000000013d  0
      [22] .rela.text.main
           RELA                   RELA             0000000000000000  0000000000000a48  35
      [23] .rodata.__func__.2250
           PROGBITS               PROGBITS         0000000000000000  000000000000014d  0
      [24] .rodata.__func__.2254
           PROGBITS               PROGBITS         0000000000000000  0000000000000154  0
      [25] .rodata.__func__.2258
           PROGBITS               PROGBITS         0000000000000000  000000000000015b  0
      [26] .rodata.__func__.2262
           PROGBITS               PROGBITS         0000000000000000  0000000000000162  0
      [27] .rodata.__func__.2266
           PROGBITS               PROGBITS         0000000000000000  0000000000000169  0
      [28] .rodata.__func__.2270
           PROGBITS               PROGBITS         0000000000000000  0000000000000170  0
      [29] .rodata.__func__.2274
           PROGBITS               PROGBITS         0000000000000000  0000000000000177  0
      [30] .rodata.__func__.2278
           PROGBITS               PROGBITS         0000000000000000  000000000000017e  0

**最后**，再看一下，不带`-Wl,–gc-sections`参数生成的可执行文件，与带`-Wl,–gc-sections`参数生成的可执行文件的差别。很明显，在链接过程中，带`-Wl,–gc-sections`参数后，可执行文件仅联编了调用的函数。

$ readelf -a test_normal | grep func_
        55: 00000000000006c6    31 FUNC    GLOBAL DEFAULT   14 func_4
        57: 000000000000064a    31 FUNC    GLOBAL DEFAULT   14 func_0
        58: 0000000000000688    31 FUNC    GLOBAL DEFAULT   14 func_2
        60: 0000000000000704    31 FUNC    GLOBAL DEFAULT   14 func_6
        71: 00000000000006a7    31 FUNC    GLOBAL DEFAULT   14 func_3
        72: 0000000000000669    31 FUNC    GLOBAL DEFAULT   14 func_1
        73: 0000000000000723    31 FUNC    GLOBAL DEFAULT   14 func_7
        74: 00000000000006e5    31 FUNC    GLOBAL DEFAULT   14 func_5

readelf -a test_sections | grep func_
        48: 000000000000064a    31 FUNC    GLOBAL DEFAULT   14 func_0

[Link 1]: https://gcc.gnu.org/onlinedocs/gnat_ugn/Compilation-options.html
[Link 2]: https://ftp.gnu.org/old-gnu/Manuals/ld-2.9.1/html_node/ld_3.html#SEC3