Page 153 - ARM 64 Bit Assembly Language

Structured programming 139

Listing 5.31 shows how the reverse function can be implemented using recursion in AArch64
assembly. Line 8 saves the link register to the stack and decrements the stack pointer. Lines
10 and 11 test for the base case. If the current case is the base case, then the function simply
returns (restoring the stack as it goes). Otherwise, the first and last characters are swapped in
lines 12 through 15 and a recursive call is made in lines 17 through 19.

Listing 5.31 AArch64 assembly implementation of the reverse function.
1 .data // Declare static data
2 str: .string "\nThis is the string to reverse\n"
3
4 .text
5 .type reverse, %function
6 .global reverse
7 reverse:
8 stp x29, x30, [sp, #-16]! // push FP and LR
9 // if( left >= right) goto endif
10 cmp x1, x2
11 bge endif
12 ldrb w3, [x0, x1] // w3 = a[left]
13 ldrb w4, [x0, x2] // w4 = a[right]
14 strb w4, [x0, x1] // a[left] = w4
15 strb w3, [x0, x2] // a[right] = w3
16 // reverse(a, left+1, right-1)
17 add w1, w1, #1 // left += 1
18 sub w2, w2, #1 // right -= 1
19 bl reverse
20 endif: ldp x29, x30, [sp], #16 // pop FP and LR
21 ret
22 .size reverse, (. - reverse)
23
24 .type main, %function
25 .global main
26 main: stp x29, x30, [sp, #-16]!
27 // printf(str)
28 adr x0, str
29 bl printf
30 // reverse(str, 0, strlen(str)-1)
31 adr x0, str
32 bl strlen
33 add w0, w0, #-1
34 mov w2, w0
35 mov w1, wzr
36 adr x0, str
37 bl reverse
38 // printf(str)
39 adr x0, str
40 bl printf