CHAPTER 47. STRINGS TRIMMING CHAPTER 47. STRINGS TRIMMING
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
47.6 ARM: Optimizing Keil 6/2013 (ARM mode).
And again, the compiler took advantage of ARM mode’s conditional instructions, so the code is much more compact.
Listing 47.6: Optimizing Keil 6/2013 (ARM mode)str_trim PROC
PUSH {r4,lr}
; R0=s
MOV r4,r0
; R4=s
BL strlen ; strlen() takes "s" value from R0
; R0=str_len
MOV r3,#0
; R3 will always hold 0
|L0.16|
CMP r0,#0 ; str_len==0?
ADDNE r2,r4,r0 ; (if str_len!=0) R2=R4+R0=s+str_len
LDRBNE r1,[r2,#-1] ; (if str_len!=0) R1=load byte at address R2-1=s+str_len-1
CMPNE r1,#0 ; (if str_len!=0) compare loaded byte against 0
BEQ |L0.56| ; jump to exit if str_len==0 or loaded byte is 0
CMP r1,#0xd ; is loaded byte '\r'?
CMPNE r1,#0xa ; (if loaded byte is not '\r') is loaded byte '\r'?
SUBEQ r0,r0,#1 ; (if loaded byte is '\r' or '\n') R0-- or str_len--
STRBEQ r3,[r2,#-1] ; (if loaded byte is '\r' or '\n') store R3 (zero) at
address R2-1=s+str_len-1
BEQ |L0.16| ; jump to loop begin if loaded byte was '\r' or '\n'
|L0.56|
; return "s"
MOV r0,r4
POP {r4,pc}
ENDP
47.7 ARM: Optimizing Keil 6/2013 (Thumb mode)
There are less conditional instructions in Thumb mode, so the code is simpler. But there are is really weird thing with
the 0x20 and 0x19 offsets. Why did the Keil compiler do so? Honestly, it’s hard to say. Probably, this is a quirk of Keil’s
optimization process. Nevertheless, the code works correctly.
Listing 47.7: Optimizing Keil 6/2013 (Thumb mode)str_trim PROC
PUSH {r4,lr}
MOVS r4,r0
; R4=s
BL strlen ; strlen() takes "s" value from R0
; R0=str_len
MOVS r3,#0
; R3 will always hold 0
B |L0.24|
|L0.12|
CMP r1,#0xd ; is loaded byte '\r'?
BEQ |L0.20|
CMP r1,#0xa ; is loaded byte '\n'?