deadcode strip

• 死代码剥离, clang在链接过程中deadcode strip默认就是生效的

• dead_strip

    -dead_strip
  
    Remove functions and data that are unreachable by the entry point or exported symbols.
  

• 区分于Other Linker Flags (OTHER_LDFLAGS), Other Linker Flags (OTHER_LDFLAGS)是仅限于静态库的

dead_strip与其他链接参数对比

• 准备test.m文件

  • 定义了全局函数: global_function() 从符号角度讲是导出符号
  • 定义了静态函数: static_function() 从符号角度讲是本地符号

  • 没有使用global_function()全局函数

    #import <Foundation/Foundation.h>
    
    // 全局符号 导出符号
    void global_function() {
    
    }
    
    // 本地符号
    static void static_function() {
    
    }
    int main() {
        // global_function();
        NSLog(@"testApp ------");
        return 0;
    }
    

• 脚本生成可执行文件, 并查看__TEXT, 对比-dead_strip作用

  • 加上-dead_strip

    SYSROOT=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk
    
    # 链接OCTest到test.m中生成.o文件
    clang -x objective-c \
    -target x86_64-apple-macos11.1 \
    -fobjc-arc \
    -isysroot $SYSROOT \
    -I./OCStaticLib \
    -c test.m \
    -o test.o
    
    # 将.o生成执行文件
    clang -target x86_64-apple-macos11.1 \
    -fobjc-arc \
    -isysroot $SYSROOT \
    -Xlinker -dead_strip \
    -L./OCStaticLib \
    -lOCTest \
    test.o -o test
    

    test:
    (__TEXT,__text) section
    _main:
    100003f50:    55    pushq    %rbp
    100003f51:    48 89 e5    movq    %rsp, %rbp
    100003f54:    48 83 ec 10    subq    $16, %rsp
    100003f58:    48 8d 05 a9 00 00 00    leaq    169(%rip), %rax ## Objc cfstring ref: @"testApp ------"
    100003f5f:    c7 45 fc 00 00 00 00    movl    $0, -4(%rbp)
    100003f66:    48 89 c7    movq    %rax, %rdi
    100003f69:    b0 00    movb    $0, %al
    100003f6b:    e8 08 00 00 00    callq    0x100003f78 ## symbol stub for: _NSLog
    100003f70:    31 c0    xorl    %eax, %eax
    100003f72:    48 83 c4 10    addq    $16, %rsp
    100003f76:    5d    popq    %rbp
    100003f77:    c3    retq
    

  • 不加-dead_strip

    SYSROOT=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk
    
    # 链接OCTest到test.m中生成.o文件
    clang -x objective-c \
    -target x86_64-apple-macos11.1 \
    -fobjc-arc \
    -isysroot $SYSROOT \
    -I./OCStaticLib \
    -c test.m \
    -o test.o
    
    # 将.o生成执行文件
    clang -target x86_64-apple-macos11.1 \
    -fobjc-arc \
    -isysroot $SYSROOT \
    -L./OCStaticLib \
    -lOCTest \
    test.o -o test
    

    test:
    (__TEXT,__text) section
    _global_function:
    100003f40:    55    pushq    %rbp
    100003f41:    48 89 e5    movq    %rsp, %rbp
    100003f44:    5d    popq    %rbp
    100003f45:    c3    retq
    100003f46:    66 2e 0f 1f 84 00 00 00 00 00    nopw    %cs:(%rax,%rax)
    _main:
    100003f50:    55    pushq    %rbp
    100003f51:    48 89 e5    movq    %rsp, %rbp
    100003f54:    48 83 ec 10    subq    $16, %rsp
    100003f58:    48 8d 05 a9 00 00 00    leaq    169(%rip), %rax ## Objc cfstring ref: @"testApp ------"
    100003f5f:    c7 45 fc 00 00 00 00    movl    $0, -4(%rbp)
    100003f66:    48 89 c7    movq    %rax, %rdi
    100003f69:    b0 00    movb    $0, %al
    100003f6b:    e8 08 00 00 00    callq    0x100003f78 ## symbol stub for: _NSLog
    100003f70:    31 c0    xorl    %eax, %eax
    100003f72:    48 83 c4 10    addq    $16, %rsp
    100003f76:    5d    popq    %rbp
    100003f77:    c3    retq
    

在没有使用global_function()全局函数情况下, 增加了-dead_strip参数后, 被优化掉了

• 满足两个条件

  1. 没有被入口点使用
  2. 没有被导出符号使用

OC中的-dead_strip

1. 准备代码

    #import <Foundation/Foundation.h>
    #import "OCTest.h"
   
    int main() {
        OCTest *oc = [OCTest new];
        [oc test:nil];
        NSLog(@"testApp ------");
        return 0;
    }
   

2. 脚本

    SYSROOT=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk
   
    # 链接OCTest到test.m中生成.o文件
    clang -x objective-c \
    -target x86_64-apple-macos11.1 \
    -fobjc-arc \
    -isysroot $SYSROOT \
    -I./OCStaticLib \
    -c test.m \
    -o test.o
   
    # 将.o生成执行文件
    clang -target x86_64-apple-macos11.1 \
    -fobjc-arc \
    -isysroot $SYSROOT \
    -Xlinker -dead_strip \
    -Xlinker -all_load \
    -L./OCStaticLib \
    -lOCTest \
    test.o -o test
   

   在连接过程中默认参数noall_load, 避免Other Linker Flags将代码剥离掉, 所以设置为all_load, 来观察dead_strip的作用效果

3. 查看__TEXT

    test:
    (__TEXT,__text) section
    -[OCTest test:]:
    100003470:    55    pushq    %rbp
    100003471:    48 89 e5    movq    %rsp, %rbp
    100003474:    48 83 ec 20    subq    $32, %rsp
    100003478:    48 89 7d f8    movq    %rdi, -8(%rbp)
    10000347c:    48 89 75 f0    movq    %rsi, -16(%rbp)
    100003480:    48 c7 45 e8 00 00 00 00    movq    $0, -24(%rbp)
    100003488:    48 8d 7d e8    leaq    -24(%rbp), %rdi
    10000348c:    48 89 d6    movq    %rdx, %rsi
    10000348f:    e8 28 09 00 00    callq    0x100003dbc ## symbol stub for: _objc_storeStrong
    100003494:    48 8d 05 75 0b 00 00    leaq    2933(%rip), %rax ## Objc cfstring ref: @"__TestExample"
    10000349b:    48 89 c7    movq    %rax, %rdi
    10000349e:    b0 00    movb    $0, %al
    1000034a0:    e8 0b 09 00 00    callq    0x100003db0 ## symbol stub for: _NSLog
    1000034a5:    31 c9    xorl    %ecx, %ecx
    1000034a7:    89 ce    movl    %ecx, %esi
    1000034a9:    48 8d 7d e8    leaq    -24(%rbp), %rdi
    1000034ad:    e8 0a 09 00 00    callq    0x100003dbc ## symbol stub for: _objc_storeStrong
    1000034b2:    48 83 c4 20    addq    $32, %rsp
    1000034b6:    5d    popq    %rbp
    ...
    省略
   

   在OC中即便没有使用, 静态库中的符号也不会被剥离掉, 因为OC是动态语言