Clang doesn't pass value via register as the code specified-CodePudding

I have the below snippet of code, and here I want to pass the value of variable num by register rax, but it seems in the assembly code, clang doesn't use rax directly. Rather, it saves the value of num on the stack and then passes the value on the stack to the rsi register which the behavior is different from GCC. Why? BTW, I didn't use any optimization flags in the compilation command.

C Code:

#include <stdio.h>
int main(void) {
  register long num asm("rax") = 0x100000000;
  printf("%ld\n", num);
  return 0;
}

Assembly Code (Clang):

main:                                   # @main
        pushq   %rbp
        movq    %rsp, %rbp
        subq    $16, %rsp
        movl    $0, -4(%rbp)
        movabsq $4294967296, %rax               # imm = 0x100000000
        movq    %rax, -16(%rbp)
        movq    -16(%rbp), %rsi  # <- here, for calling printf, the parameter is not passed directly from register, -
                                 # but indirectly from a stack location. Why?
        movabsq $.L.str, %rdi
        movb    $0, %al
        callq   printf
        xorl