x86 - What does cltq do in assembly?

0x0000000000400553 :   mov    -0x4(%rbp),%eax

0x0000000000400556 :   cltq   
0x0000000000400558 :   shl    $0x3,%rax
0x000000000040055c :   mov    %rax,%rdx

In fact my programe is as simple as :

5   int main(int argc, char *argv[]) { 
6     int i = 0;
7     while(environ[i]) {

8       printf("%s\n", environ[i++]);
9     }
10    return 0;

But the assembly output is pretty long:

Dump of assembler code for function main:
0x0000000000400518 :    push   %rbp
0x0000000000400519 :    mov    %rsp,%rbp

0x000000000040051c :    sub    $0x20,%rsp
0x0000000000400520 :    mov    %edi,-0x14(%rbp)
0x0000000000400523 :   mov    %rsi,-0x20(%rbp)
0x0000000000400527 :   movl   $0x0,-0x4(%rbp)
0x000000000040052e :   jmp    0x400553 
0x0000000000400530 :   mov    -0x4(%rbp),%eax
0x0000000000400533 :   cltq   
0x0000000000400535 :   shl    $0x3,%rax
0x0000000000400539 :   mov    %rax,%rdx
0x000000000040053c :   mov    0x2003e5(%rip),%rax        # 0x600928 

0x0000000000400543 :   lea    (%rdx,%rax,1),%rax
0x0000000000400547 :   mov    (%rax),%rdi
0x000000000040054a :   addl   $0x1,-0x4(%rbp)
0x000000000040054e :   callq  0x400418 
0x0000000000400553 :   mov    -0x4(%rbp),%eax
0x0000000000400556 :   cltq   
0x0000000000400558 :   shl    $0x3,%rax
0x000000000040055c :   mov    %rax,%rdx
0x000000000040055f :   mov    0x2003c2(%rip),%rax        # 0x600928 
0x0000000000400566 :   lea    (%rdx,%rax,1),%rax

0x000000000040056a :   mov    (%rax),%rax
0x000000000040056d :   test   %rax,%rax
0x0000000000400570 :   jne    0x400530 
0x0000000000400572 :   mov    $0x0,%eax
0x0000000000400577 :   leaveq 
0x0000000000400578 :   retq   
End of assembler dump.

What I don't understand is this block:

0x000000000040052e :   jmp    0x400553 
0x0000000000400530 :   mov    -0x4(%rbp),%eax
0x0000000000400533 :   cltq   
0x0000000000400535 :   shl    $0x3,%rax
0x0000000000400539 :   mov    %rax,%rdx
0x000000000040053c :   mov    0x2003e5(%rip),%rax        # 0x600928 
0x0000000000400543 :   lea    (%rdx,%rax,1),%rax
0x0000000000400547 :   mov    (%rax),%rdi
0x000000000040054a :   addl   $0x1,-0x4(%rbp)

0x000000000040054e :   callq  0x400418 
0x0000000000400553 :   mov    -0x4(%rbp),%eax
0x0000000000400556 :   cltq   
0x0000000000400558 :   shl    $0x3,%rax
0x000000000040055c :   mov    %rax,%rdx
0x000000000040055f :   mov    0x2003c2(%rip),%rax        # 0x600928 
0x0000000000400566 :   lea    (%rdx,%rax,1),%rax
0x000000000040056a :   mov    (%rax),%rax
0x000000000040056d :   test   %rax,%rax
0x0000000000400570 :   jne    0x400530