C 语言和 ARM 汇编 - 7 循环

while

实例代码

int a = 4;
int b = 8;
int d = 0;
while(b > a)
{
    d = a + 2;
    b--;
}

汇编代码

    sub sp, sp, #16
    movs    r3, #4
    str r3, [sp, #8]
    movs    r3, #8
    str r3, [sp, #12]
    movs    r3, #0
    str r3, [sp, #4]
# 跳转到 while 比较大小
    b   .L2
.L3:
    ldr r3, [sp, #8]
    adds    r3, r3, #2
    str r3, [sp, #4]
    ldr r3, [sp, #12]
    subs    r3, r3, #1
    str r3, [sp, #12]
.L2:
# 比较 b > a
    ldr r2, [sp, #12]
    ldr r3, [sp, #8]
    cmp r2, r3
    bgt .L3
    add sp, sp, #16
    @ sp needed
    bx  lr

for

int a = 4;
int b = 8;
int d = 0;
for(b = 9; b > a; b--)
{
    d = a + 2;
}

汇编代码

    sub sp, sp, #16
    movs    r3, #4
    str r3, [sp, #8]
    movs    r3, #8
    str r3, [sp, #12]
    movs    r3, #0
    str r3, [sp, #4]
# 条件初始化 b = 9
    movs    r3, #9
    str r3, [sp, #12]
# 条件判断 b > a
    b   .L2
.L3:
# tmp = sp[8] + 2
    ldr r3, [sp, #8]
    adds    r3, r3, #2
# sp[4] = tmp
    str r3, [sp, #4]
# 条件更新 b--
    ldr r3, [sp, #12]
    subs    r3, r3, #1
    str r3, [sp, #12]
.L2:
    ldr r2, [sp, #12]
    ldr r3, [sp, #8]
    cmp r2, r3
    bgt .L3
    add sp, sp, #16
    @ sp needed
    bx  lr

生成的汇编与 while 类似，有2点需要注意