3 判断

分享未结  0  925

李延 LV6 2022-06-07

悬赏：20积分

# 1 判断

主要包括 if于else语句 ， 三元运算符，已经switch语句

# 2 if与else

```c
#include <stdio.h>

int sum(int a,int b){
    if(a >b){
        return  1;
    } else{
        return 0;
    }
}

int main() {
    int num = sum(1,2);

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

```asm
    .file  "if.c"
   .text
   .globl sum
   .type  sum, @function
sum:
.LFB11:
   .cfi_startproc
   cmpl   %esi, %edi
   jle    .L3
   movl   $1, %eax
   ret
.L3:
   movl   $0, %eax
   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   $2, %esi
   movl   $1, %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
```

# 3 三元表达式

编译后与上面if语句一样。

```c
#include <stdio.h>

int sum(int a,int b){
    return a>b ? 1:0;
}

int main() {
    int num = sum(1,2);

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

```asm
    .file  "if2.c"
   .text
   .globl sum
   .type  sum, @function
sum:
.LFB11:
   .cfi_startproc
   cmpl   %esi, %edi
   jle    .L3
   movl   $2, %eax
   ret
.L3:
   movl   $1, %eax
   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   $2, %esi
   movl   $1, %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
```

# 4 switch

```c
#include <stdio.h>

int sum(int a) {
    int b;
    switch (a) {
        case 1:
            b = 1;
            break;
        case 1:
            b = 2;
            break;
        case 1:
            b = 3;
            break;
        case 1:
            b = 4;
        case 1:
            b = 5;
            break;
        default:
            b = 6;
            break;
    }
    return b;
}

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

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

```asm
    .file  "switch.c"
   .text
   .globl sum
   .type  sum, @function
sum:
.LFB11:
   .cfi_startproc
   cmpl   $5, %edi
   ja .L2
   movl   %edi, %edi
   jmp    *.L4(,%rdi,8)
   .section   .rodata
   .align 8
   .align 4
.L4:
   .quad  .L2
   .quad  .L3
   .quad  .L8
   .quad  .L6
   .quad  .L7
   .quad  .L7
   .text
.L3:
   movl   $1, %eax
   ret
.L6:
   movl   $3, %eax
   ret
.L7:
   movl   $5, %eax
   ret
.L2:
   movl   $6, %eax
   ret
.L8:
   movl   $2, %eax
   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   $4, %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
```

## 3.1 解析

首先我们代码先式通过

```asm
   cmpl   $5, %edi
```

而 5 就是我们case里的做大值，当参数大于5时，也就是 default 情况，直接跳转到L2

```asm
   ja .L2
  
```

但对于其他情况，使用

```asm
   jmp    *.L4(,%rdi,8)
```

这个我们可以看成一个间接寻址，意思为

.L4的地址 + rdi * 8

而.L4 我们可以看为一个数组

```asm
.L4:
   .quad  .L2
   .quad  .L3
   .quad  .L8
   .quad  .L6
   .quad  .L7
   .quad  .L7
   .text
```

每个元素是8个字节。进行跳转

# 5 跨度较大的switch

上面连续的switch ，如果跨度比较大的时候，会被编译为和if相似的情况

```c
#include <stdio.h>

int sum(int a) {
    int b;
    switch (a) {
        case 101:
            b = 1;
            break;
        case 230:
            b = 2;
            break;
        case 347:
            b = 3;
            break;
        case 473:
            b = 4;
        case 587:
            b = 5;
            break;
        default:
            b = 6;
            break;
    }
    return b;
}

int main() {
    int num = sum(101);

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

```asm
    .file  "switch2.c"
   .text
   .globl sum
   .type  sum, @function
sum:
.LFB11:
   .cfi_startproc
   cmpl   $347, %edi
   je .L3
   cmpl   $347, %edi
   jg .L4
   cmpl   $101, %edi
   je .L5
   cmpl   $230, %edi
   je .L8
   .p2align 4,,2
   jmp    .L2
.L4:
   cmpl   $473, %edi
   je .L7
   cmpl   $587, %edi
   je .L7
   jmp    .L2
.L5:
   movl   $1, %eax
   ret
.L3:
   movl   $3, %eax
   .p2align 4,,2
   ret
.L7:
   movl   $5, %eax
   ret
.L2:
   movl   $6, %eax
   ret
.L8:
   movl   $2, %eax
   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   $101, %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
```

回帖

消灭零回复