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技巧:把选项传给汇编器
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xingmingjie committed Jun 30, 2014
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# 汇编程序
* [生成有详细信息的汇编文件](verbose-asm.md)
* [把选项传给汇编器](pass-options-to-assembler.md)

# 常见错误
* [error: cast from ... to ... loses precision](cast-lose-precision.md)
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# 把选项传给汇编器

## 例子

#include <stdio.h>

int main(void)
{
int i;

for (i = 0; i < 10; i++)
printf("%d ", i);
putchar ('\n');

return 0;
}

## 技巧

使用`-Wa,option`可以将选项`option`传递给汇编器。

注意,逗号和选项之间不能有空格。例如:

$ gcc -c -Wa,-L foo.c
$ objdump -d foo.o

foo.o: file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <main>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 48 83 ec 10 sub $0x10,%rsp
8: c7 45 fc 00 00 00 00 movl $0x0,-0x4(%rbp)
f: eb 1b jmp 2c <.L2>

0000000000000011 <.L3>:
11: b8 00 00 00 00 mov $0x0,%eax
16: 8b 55 fc mov -0x4(%rbp),%edx
19: 89 d6 mov %edx,%esi
1b: 48 89 c7 mov %rax,%rdi
1e: b8 00 00 00 00 mov $0x0,%eax
23: e8 00 00 00 00 callq 28 <.L3+0x17>
28: 83 45 fc 01 addl $0x1,-0x4(%rbp)

000000000000002c <.L2>:
2c: 83 7d fc 09 cmpl $0x9,-0x4(%rbp)
30: 7e df jle 11 <.L3>
32: bf 0a 00 00 00 mov $0xa,%edi
37: e8 00 00 00 00 callq 3c <.L2+0x10>
3c: b8 00 00 00 00 mov $0x0,%eax
41: c9 leaveq
42: c3 retq

这里的`-L`是汇编器as的选项,用于在目标文件中保留局部符号(local symbol)。可以看到,反汇编代码中给出了每个局部符号。

如果此时你使用`oprofile`来统计性能事件,那么获得的结果将不是以函数为单位了,而是以这些符号所划分的代码块为单位。

详情参见[gcc手册](https://gcc.gnu.org/onlinedocs/gcc-4.3.0/gcc/Assembler-Options.html#Assembler-Options)[as手册](https://sourceware.org/binutils/docs-2.24/as/L.html#L)

## 贡献者

xmj

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