CHAPTER 18. ARRAYS CHAPTER 18. ARRAYS
Listing 18.3: Non-optimizing GCC 4.9.1 (ARM64).LC0:
.string "a[%d]=%d\n"
main:
; save FP and LR in stack frame:
stp x29, x30, [sp, -112]!
; set stack frame (FP=SP)
add x29, sp, 0
; setting initial counter variable at 0 (WZR is the register always holding zero):
str wzr, [x29,108]
; jump to loop condition checking code:
b .L2
.L3:
; load value of "i" variable:
ldr w0, [x29,108]
; multiplicate it by 2:
lsl w2, w0, 1
; find a place of an array in local stack:
add x0, x29, 24
; load 32-bit integer from local stack and sign extend it to 64-bit one:
ldrsw x1, [x29,108]
; calculate address of element (X0+X1<<2=array address+i4) and store W2 (i2) there:
str w2, [x0,x1,lsl 2]
; increment counter (i):
ldr w0, [x29,108]
add w0, w0, 1
str w0, [x29,108]
.L2:
; check if we finished:
ldr w0, [x29,108]
cmp w0, 19
; jump to L3 (loop body begin) if not:
ble .L3
; second part of the function begins here.
; setting initial counter variable at 0.
; by the way, the same place in the local stack was used for counter,
; because the same local variable (i) is being used as counter.
str wzr, [x29,108]
b .L4
.L5:
; calculate array address:
add x0, x29, 24
; load "i" value:
ldrsw x1, [x29,108]
; load value from the array at the address (X0+X1<<2 = address of array + i*4)
ldr w2, [x0,x1,lsl 2]
; load address of the "a[%d]=%d\n" string:
adrp x0, .LC0
add x0, x0, :lo12:.LC0
; load "i" variable to W1 and pass it to printf() as second argument:
ldr w1, [x29,108]
; W2 still contains the value of array element which was just loaded.
; call printf():
bl printf
; increment "i" variable:
ldr w0, [x29,108]
add w0, w0, 1
str w0, [x29,108]
.L4:
; are we finished?
ldr w0, [x29,108]
cmp w0, 19
; jump to the loop body begin if not:
ble .L5
; return 0
mov w0, 0
; restore FP and LR:
ldp x29, x30, [sp], 112
ret