string concat in x86-64 assembly AT&T. run with `gcc string-concat.s -o concat && ./concat`

string-concat.s

.data
.stringfmt:
      .string "'%s'\n"
.s1:
      .asciz "ab"
.s2:
      .asciz "cd"

.text
.global main
main:
      PUSHQ   %rbp
      MOVQ    %rsp, %rbp

      LEA     .s1(%rip), %rdi
      LEA     .s2(%rip), %rsi
      CALL    concat

      MOVQ    %rax, %rsi
      LEA     .stringfmt(%rip), %rdi
      XOR     %rax, %rax
      CALL    printf

      LEAVE
      RET

concat:
     PUSHQ    %rbp
     MOVQ     %rsp, %rbp

     # calc strlen of s1
     CALL     strlen                   # s1 is already in rdi
     MOVQ     %rax, %r8                # index in s1

     XOR      %r10, %r10               # tmp character to copy
     XOR      %rbx, %rbx               # index in s2

     # copy character by character
concat_copy:                           
     MOVB     0(%rsi, %rbx, 1), %r10b  # r10b=s2[rbx]
     CMP      $0, %r10b
     JE       concat_end               # end if s2[rbx] is 0
     MOVB     %r10b, 0(%rdi, %r8, 1)   # s1[r8]=s2[rbx]
     INC      %rbx                     # rbx++
     INC      %r8                      # r8++
     JMP      concat_copy
concat_end:
     MOVB     $0, 0(%rdi, %r8, 1)      # terminate s1 with 0
     MOVQ     %rdi, %rax

     LEAVE
     RET

strlen:
     PUSHQ    %rbp
     MOVQ     %rsp, %rbp

     XOR      %r10, %r10               # tmp character
     XOR      %rbx, %rbx               # counter
strlen_loop:
     MOVB     0(%rdi, %rbx, 1), %r10b
     CMP      $0, %r10b
     JE       strlen_end
     INC      %rbx
     JMP      strlen_loop
strlen_end:
     MOVQ     %rbx, %rax

     LEAVE
     RET