4 循环

分享未结  0  871

李延 LV6 2022-06-07

悬赏：20积分

# 1 while

```c
#include <stdio.h>

int sum(int n) {
    int q = 0;
    while (n > 0) {
        q += n;
        n--;
    }
    return q;
}

int main() {
    int num = sum(20);

printf("sum is %d!\n", num);
    return 0;
}
```

```asm
    .file  "while.c"
   .text
   .globl sum
   .type  sum, @function
sum:
.LFB11:
   .cfi_startproc
   movl   $0, %eax
   jmp    .L2
.L3:
   addl   %edi, %eax
   subl   $1, %edi
.L2:
   testl  %edi, %edi
   jg .L3
   rep ret
   .cfi_endproc
.LFE11:
   .size  sum, .-sum
   .section   .rodata.str1.1,"aMS",@progbits,1
.LC0:
   .string    "sum is %d!\n"
   .text
   .globl main
   .type  main, @function
main:
.LFB12:
   .cfi_startproc
   subq   $8, %rsp
   .cfi_def_cfa_offset 16
   movl   $20, %edi
   call   sum
   movl   %eax, %esi
   movl   $.LC0, %edi
   movl   $0, %eax
   call   printf
   movl   $0, %eax
   addq   $8, %rsp
   .cfi_def_cfa_offset 8
   ret
   .cfi_endproc
.LFE12:
   .size  main, .-main
   .ident "GCC: (GNU) 4.8.5 20150623 (Red Hat 4.8.5-44)"
   .section   .note.GNU-stack,"",@progbits
```

上面逻辑还是比较清晰的，也是通过jmp命令控制跳转。只不过在最后通过

```asm
   testl  %edi, %edi
   jg .L3
```

判断n为负时，退出循环

# 2 for

```c
#include <stdio.h>

int sum(int n) {
    int q = 0;
    for(;n >0; n--){
        q += n;
    }
    return q;
}

int main() {
    int num = sum(20);

printf("sum is %d!\n", num);
    return 0;
}
```

```asm
    .file  "for.c"
   .text
   .globl sum
   .type  sum, @function
sum:
.LFB11:
   .cfi_startproc
   movl   $0, %eax
   jmp    .L2
.L3:
   addl   %edi, %eax
   subl   $1, %edi
.L2:
   testl  %edi, %edi
   jg .L3
   rep ret
   .cfi_endproc
.LFE11:
   .size  sum, .-sum
   .section   .rodata.str1.1,"aMS",@progbits,1
.LC0:
   .string    "sum is %d!\n"
   .text
   .globl main
   .type  main, @function
main:
.LFB12:
   .cfi_startproc
   subq   $8, %rsp
   .cfi_def_cfa_offset 16
   movl   $20, %edi
   call   sum
   movl   %eax, %esi
   movl   $.LC0, %edi
   movl   $0, %eax
   call   printf
   movl   $0, %eax
   addq   $8, %rsp
   .cfi_def_cfa_offset 8
   ret
   .cfi_endproc
.LFE12:
   .size  main, .-main
   .ident "GCC: (GNU) 4.8.5 20150623 (Red Hat 4.8.5-44)"
   .section   .note.GNU-stack,"",@progbits
```

在for循环里，编译后和while 是一样的逻辑

回帖

消灭零回复