3.15. STRINGS TRIMMING
str_trim:
push rbp
mov rbp, rsp
sub rsp, 32
mov QWORD PTR [rbp-24], rdi
; for() first part begins here
mov rax, QWORD PTR [rbp-24]
mov rdi, rax
call strlen
mov QWORD PTR [rbp-8], rax ; str_len
; for() first part ends here
jmp .L2
; for() body begins here
.L5:
cmp BYTE PTR [rbp-9], 13 ; c=='\r'?
je .L3
cmp BYTE PTR [rbp-9], 10 ; c=='\n'?
jne .L4
.L3:
mov rax, QWORD PTR [rbp-8] ; str_len
lea rdx, [rax-1] ; EDX=str_len-1
mov rax, QWORD PTR [rbp-24] ; s
add rax, rdx ; RAX=s+str_len-1
mov BYTE PTR [rax], 0 ; s[str_len-1]=0
; for() body ends here
; for() third part begins here
sub QWORD PTR [rbp-8], 1 ; str_len--
; for() third part ends here
.L2:
; for() second part begins here
cmp QWORD PTR [rbp-8], 0 ; str_len==0?
je .L4 ; exit then
; check second clause, and load "c"
mov rax, QWORD PTR [rbp-8] ; RAX=str_len
lea rdx, [rax-1] ; RDX=str_len-1
mov rax, QWORD PTR [rbp-24] ; RAX=s
add rax, rdx ; RAX=s+str_len-1
movzx eax, BYTE PTR [rax] ; AL=s[str_len-1]
mov BYTE PTR [rbp-9], al ; store loaded char into "c"
cmp BYTE PTR [rbp-9], 0 ; is it zero?
jne .L5 ; yes? exit then
; for() second part ends here
.L4:
; return "s"
mov rax, QWORD PTR [rbp-24]
leave
ret
Comments are added by the author of the book.
After the execution ofstrlen(), the control is passed to the L2 label, and there two clauses are checked,
one after another.
The second will never be checked, if the first one (str_len==0) is false (this is “short-circuit”).
Now let’s see this function in short form:
- First for() part (call tostrlen())
- goto L2
- L5: for() body. goto exit, if needed
- for() third part (decrement of str_len)
- L2: for() second part: check first clause, then second. goto loop body begin or exit.
- L4: // exit
- return s