CHAPTER 7. SCANF() CHAPTER 7. SCANF()
Listing 7.14: Non-optimizing GCC 4.9.1 ARM64
1 .LC0:
2 .string "Enter X:"
3 .LC1:
4 .string "%d"
5 .LC2:
6 .string "You entered %d...\n"
7 .LC3:
8 .string "What you entered? Huh?"
9 f6:
10 ; save FP and LR in stack frame:
11 stp x29, x30, [sp, -32]!
12 ; set stack frame (FP=SP)
13 add x29, sp, 0
14 ; load pointer to the "Enter X:" string:
15 adrp x0, .LC0
16 add x0, x0, :lo12:.LC0
17 bl puts
18 ; load pointer to the "%d" string:
19 adrp x0, .LC1
20 add x0, x0, :lo12:.LC1
21 ; calculate address of x variable in the local stack
22 add x1, x29, 28
23 bl __isoc99_scanf
24 ; scanf() returned result in W0.
25 ; check it:
26 cmp w0, 1
27 ; BNE is Branch if Not Equal
28 ; so if W0<>0, jump to L2 will be occurred
29 bne .L2
30 ; at this moment W0=1, meaning no error
31 ; load x value from the local stack
32 ldr w1, [x29,28]
33 ; load pointer to the "You entered %d...\n" string:
34 adrp x0, .LC2
35 add x0, x0, :lo12:.LC2
36 bl printf
37 ; skip the code, which print the "What you entered? Huh?" string:
38 b .L3
39 .L2:
40 ; load pointer to the "What you entered? Huh?" string:
41 adrp x0, .LC3
42 add x0, x0, :lo12:.LC3
43 bl puts
44 .L3:
45 ; return 0
46 mov w0, 0
47 ; restore FP and LR:
48 ldp x29, x30, [sp], 32
49 ret
Code flow in this case forks with the use of CMP/BNE (Branch if Not Equal) instructions pair.7.3.7 MIPS.
Listing 7.15: Optimizing GCC 4.4.5 (IDA)
.text:004006A0 main:
.text:004006A0
.text:004006A0 var_18 = -0x18
.text:004006A0 var_10 = -0x10
.text:004006A0 var_4 = -4
.text:004006A0
.text:004006A0 lui $gp, 0x42
.text:004006A4 addiu $sp, -0x28
.text:004006A8 li $gp, 0x418960
.text:004006AC sw $ra, 0x28+var_4($sp)
.text:004006B0 sw $gp, 0x28+var_18($sp)