3.15. STRINGS TRIMMING
Apparently, GCC notices that the str_len-1 statement is used twice.
So it’s better to allocate a variable which always holds a value that’s smaller than the current string length
by one, and decrement it (this is the same effect as decrementing the str_len variable).
3.15.4 ARM64: Non-optimizing GCC (Linaro) 4.9
This implementation is straightforward:
Listing 3.65: Non-optimizing GCC (Linaro) 4.9str_trim:
stp x29, x30, [sp, -48]!
add x29, sp, 0
str x0, [x29,24] ; copy input argument into local stack
ldr x0, [x29,24] ; s
bl strlen
str x0, [x29,40] ; str_len variable in local stack
b .L2
; main loop begin
.L5:
ldrb w0, [x29,39]
; W0=c
cmp w0, 13 ; is it '\r'?
beq .L3
ldrb w0, [x29,39]
; W0=c
cmp w0, 10 ; is it '\n'?
bne .L4 ; goto exit if it is not
.L3:
ldr x0, [x29,40]
; X0=str_len
sub x0, x0, #1
; X0=str_len-1
ldr x1, [x29,24]
; X1=s
add x0, x1, x0
; X0=s+str_len-1
strb wzr, [x0] ; write byte at s+str_len-1
; decrement str_len:
ldr x0, [x29,40]
; X0=str_len
sub x0, x0, #1
; X0=str_len-1
str x0, [x29,40]
; save X0 (or str_len-1) to local stack
.L2:
ldr x0, [x29,40]
; str_len==0?
cmp x0, xzr
; goto exit then
beq .L4
ldr x0, [x29,40]
; X0=str_len
sub x0, x0, #1
; X0=str_len-1
ldr x1, [x29,24]
; X1=s
add x0, x1, x0
; X0=s+str_len-1
; load byte at address s+str_len-1 to W0
ldrb w0, [x0]
strb w0, [x29,39] ; store loaded byte to "c"
ldrb w0, [x29,39] ; reload it
; is it zero byte?
cmp w0, wzr
; goto exit, if its zero or to L5 if its not
bne .L5
.L4: