Assembly Language for Beginners

3.15. STRINGS TRIMMING

; return s
ldr x0, [x29,24]
ldp x29, x30, [sp], 48
ret

3.15.5 ARM64: Optimizing GCC (Linaro) 4.9.

This is a more advanced optimization.

The first character is loaded at the beginning, and compared against 10 (theLFcharacter).

Characters are also loaded in the main loop, for the characters after first one.

This is somewhat similar to the3.15.3 on page 529example.

Listing 3.66: Optimizing GCC (Linaro) 4.9

str_trim:
stp x29, x30, [sp, -32]!
add x29, sp, 0
str x19, [sp,16]
mov x19, x0
; X19 will always hold value of "s"
bl strlen
; X0=str_len
cbz x0, .L9 ; goto L9 (exit) if str_len==0
sub x1, x0, #1
; X1=X0-1=str_len-1
add x3, x19, x1
; X3=X19+X1=s+str_len-1
ldrb w2, [x19,x1] ; load byte at address X19+X1=s+str_len-1
; W2=loaded character
cbz w2, .L9 ; is it zero? jump to exit then
cmp w2, 10 ; is it '\n'?
bne .L15
.L12:
; main loop body. loaded character is always 10 or 13 at this moment!
sub x2, x1, x0
; X2=X1-X0=str_len-1-str_len=-1
add x2, x3, x2
; X2=X3+X2=s+str_len-1+(-1)=s+str_len-2
strb wzr, [x2,1] ; store zero byte at address s+str_len-2+1=s+str_len-1
cbz x1, .L9 ; str_len-1==0? goto exit, if so
sub x1, x1, #1 ; str_len--
ldrb w2, [x19,x1] ; load next character at address X19+X1=s+str_len-1
cmp w2, 10 ; is it '\n'?
cbz w2, .L9 ; jump to exit, if its zero
beq .L12 ; jump to begin loop, if its '\n'
.L15:
cmp w2, 13 ; is it '\r'?
beq .L12 ; yes, jump to the loop body begin
.L9:
; return "s"
mov x0, x19
ldr x19, [sp,16]
ldp x29, x30, [sp], 32
ret

3.15.6 ARM: Optimizing Keil 6/2013 (ARM mode).

Andagain, the compilertook advantageof ARMmode’s conditionalinstructions, so thecode is muchmore
compact.

Listing 3.67: Optimizing Keil 6/2013 (ARM mode)

str_trim PROC
PUSH {r4,lr}