I've been reading about div and mul assembly operations, and I decided to see them in action by writing a simple program in C:

File division.c

#include <stdlib.h>
#include <stdio.h>

int main()
{
    size_t i = 9;
    size_t j = i / 5;
    printf("%zu
",j);
    return 0;
}

And then generating assembly language code with:

gcc -S division.c -O0 -masm=intel

But looking at generated division.s file, it doesn't contain any div operations! Instead, it does some kind of black magic with bit shifting and magic numbers. Here's a code snippet that computes i/5:

mov     rax, QWORD PTR [rbp-16]   ; Move i (=9) to RAX
movabs  rdx, -3689348814741910323 ; Move some magic number to RDX (?)
mul     rdx                       ; Multiply 9 by magic number
mov     rax, rdx                  ; Take only the upper 64 bits of the result
shr     rax, 2                    ; Shift these bits 2 places to the right (?)
mov     QWORD PTR [rbp-8], rax    ; Magically, RAX contains 9/5=1 now, 
                                  ; so we can assign it to j

What's going on here? Why doesn't GCC use div at all? How does it generate this magic number and why does everything work?