concat string in dynamic memory using malloc from x86-64 assembly with at&t syntax

strcat-malloc.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

     # store s2 and s1 on stack while doing prepration ops
     PUSHQ    %rsi
     PUSHQ    %rdi

     # calc strlen of s2
     MOVQ     %rsi, %rdi
     CALL     strlen
     MOVQ     %rax, %r9                # length of s2
     
     # allocate len(s1) + len(s2) + 1
     ADDQ     %r8, %r9
     INC      %r9                      # terminating space
     MOVQ     %r9, %rdi
     CALL     malloc
     MOVQ     %rax, %rbx                # rbx now points to the address of the allocated memory

     MOVQ     %rbx, %rdi                # newly allocated memory
     POPQ     %rsi                     # pop s1 back from stack
     CALL     strcpy                   # copy 1

     MOVQ     %rax, %rdi               # result of prev strcpy points to end of string
     POPQ     %rsi                     # pop s2 from stack
     CALL     strcat                   # append s2

     MOVQ     %rbx, %rax

     LEAVE
     RET