Question

Write the LEGv8 assembly program for following c program. Assume array1, array2 and array3 are stored in memory with base addresses at 1000, 2000 and 3000 respectively.

int absoluteDifference(int x, int y) {
int r;
if (x > y)
r = x - y;
else
r = y - x;

return r;
}

int main() {
for (int i=0; i < 10; i++) {
int a = array1[i];
int b = array2[i];
int c = absoluteDifference(a, b);
array3[i] = c;
}
}

Answer 1

absoluteDifference:

        str     fp, [sp, #-4]!

        add     fp, sp, #0

        sub     sp, sp, #20

        str     r0, [fp, #-16]

        str     r1, [fp, #-20]

        ldr     r2, [fp, #-16]

        ldr     r3, [fp, #-20]

        cmp     r2, r3

        ble     .L2

        ldr     r2, [fp, #-16]

        ldr     r3, [fp, #-20]

        sub     r3, r2, r3

        str     r3, [fp, #-8]

        b       .L3

.L2:

        ldr     r2, [fp, #-20]

        ldr     r3, [fp, #-16]

        sub     r3, r2, r3

        str     r3, [fp, #-8]

.L3:

        ldr     r3, [fp, #-8]

        mov     r0, r3

        add     sp, fp, #0

        ldr     fp, [sp], #4

        bx      lr

.LC0:

        .word   1

        .word   2

        .word   3

        .word   4

        .word   5

        .word   6

        .word   7

        .word   8

        .word   9

        .word   0

main:

        push    {fp, lr}

        add     fp, sp, #4

        sub     sp, sp, #136

        ldr     r3, .L9

        sub     ip, fp, #60

        mov     lr, r3

        ldmia   lr!, {r0, r1, r2, r3}

        stmia   ip!, {r0, r1, r2, r3}

        ldmia   lr!, {r0, r1, r2, r3}

        stmia   ip!, {r0, r1, r2, r3}

        ldm     lr, {r0, r1}

        stm     ip, {r0, r1}

        ldr     r3, .L9

        sub     ip, fp, #100

        mov     lr, r3

        ldmia   lr!, {r0, r1, r2, r3}

        stmia   ip!, {r0, r1, r2, r3}

        ldmia   lr!, {r0, r1, r2, r3}

        stmia   ip!, {r0, r1, r2, r3}

        ldm     lr, {r0, r1}

        stm     ip, {r0, r1}

        ldr     r3, .L9

        sub     ip, fp, #140

        mov     lr, r3

        ldmia   lr!, {r0, r1, r2, r3}

        stmia   ip!, {r0, r1, r2, r3}

        ldmia   lr!, {r0, r1, r2, r3}

        stmia   ip!, {r0, r1, r2, r3}

        ldm     lr, {r0, r1}

        stm     ip, {r0, r1}

        mov     r3, #0

        str     r3, [fp, #-8]

        b       .L6

.L7:

        ldr     r3, [fp, #-8]

        lsl     r3, r3, #2

        sub     r2, fp, #4

        add     r3, r2, r3

        ldr     r3, [r3, #-56]

        str     r3, [fp, #-12]

        ldr     r3, [fp, #-8]

        lsl     r3, r3, #2

        sub     r2, fp, #4

        add     r3, r2, r3

        ldr     r3, [r3, #-96]

        str     r3, [fp, #-16]

        ldr     r1, [fp, #-16]

        ldr     r0, [fp, #-12]

        bl      absoluteDifference

        str     r0, [fp, #-20]

        ldr     r3, [fp, #-8]

        lsl     r3, r3, #2

        sub     r2, fp, #4

        add     r3, r2, r3

        ldr     r2, [fp, #-20]

        str     r2, [r3, #-136]

        ldr     r3, [fp, #-8]

        add     r3, r3, #1

        str     r3, [fp, #-8]

.L6:

        ldr     r3, [fp, #-8]

        cmp     r3, #9

        ble     .L7

        mov     r3, #0

        mov     r0, r3

        sub     sp, fp, #4

        pop     {fp, pc}

.L9:

        .word   .LC0