cltq.

hello,
what does cltq do?

0x0000000000400500 <my_m+4>:    movl   $0x0,0xfffffffffffffffc(%rbp)
0x0000000000400507 <my_m+11>:   jmp    0x40051b <my_m+31>
0x0000000000400509 <my_m+13>:   mov    0xfffffffffffffffc(%rbp),%eax
0x000000000040050c <my_m+16>:   mov    $0x4004e8,%edx
0x0000000000400511 <my_m+21>:   cltq
0x0000000000400513 <my_m+23>:   mov    %rdx,0xffffffffffffffa0(%rbp,%rax,8)
0x0000000000400518 <my_m+28>:   incl   0xfffffffffffffffc(%rbp)
0x000000000040051b <my_m+31>:   cmpl   $0xc,0xfffffffffffffffc(%rbp)
0x000000000040051f <my_m+35>:   jle    0x400509 <my_m+13>

what compares?
Thanks in advance,

-- 
noone is alone.