Question

#!bash
_main
        PUSH    {R4,R5,LR}
; allocate place for 20 int variables + one more variable
        SUB     SP, SP, #0x54
; first loop
        MOVS    R0, #0                  ; i
        MOV     R5, SP                  ; pointer to first array element
loc_1CE
        LSLS    R1, R0, #1              ; R1=i<<1 (same as i*2)
        LSLS    R2, R0, #2              ; R2=i<<2 (same as i*4)
        ADDS    R0, R0, #1              ; i=i+1
        CMP     R0, #20                 ; i<20?
        STR     R1, [R5,R2]             ; store R1 to *(R5+R2) (same R5+i*4)
        BLT     loc_1CE                 ; yes, i<20, run loop body again
; second loop
        MOVS    R4, #0                  ; i=0
loc_1DC
        LSLS    R0, R4, #2              ; R0=i<<2 (same as i*4)
        LDR     R2, [R5,R0]             ; load from *(R5+R0) (same as R5+i*4)
        MOVS    R1, R4
        ADR     R0, aADD                ; "a[%d]=%d
"
        BL      __2printf
        ADDS    R4, R4, #1              ; i=i+1
        CMP     R4, #20                 ; i<20?
        BLT     loc_1DC                 ; yes, i<20, run loop body again
        MOVS    R0, #0                  ; value to return
; deallocate place, allocated for 20 int variables + one more variable
        ADD     SP, SP, #0x54
        POP     {R4,R5,PC}

Thumb 代码也是非常类似的。Thumb 模式计算数组偏移的移位操作使用特定的指令 LSLS。 编译器在堆栈中申请的数组空间更大，但是最后 4 个字节的空间未使用。