main.c

#include <stdio.h>
#include <stdint.h>

#include "sum.h"

int main(void) {
    uint64_t x = sum(7, 1, 2, 3, 4, 5, 6, 7);

    printf("%llu\n", x);

    return 0;
}

sum.asm

global sum

segment .text

sum:
    xor     rax, rax
    xchg    rcx, rdi
    jrcxz   _0_register_args
    dec     rcx
    jrcxz   _1_register_args
    dec     rcx
    jrcxz   _2_register_args
    dec     rcx
    jrcxz   _3_register_args
    dec     rcx
    jrcxz   _4_register_args
    dec     rcx
    jrcxz   _5_register_args
_stack_args:
    add     rax, [rcx*8+rsp]
    loop    _stack_args
_5_register_args:
    add     rax, r9
_4_register_args:
    add     rax, r8
_3_register_args:
    add     rax, rdi
_2_register_args:
    add     rax, rdx
_1_register_args:
    add     rax, rsi
_0_register_args:
    ret

sum.h

#ifndef __SUM_H__
#define __SUM_H__

#include <stdint.h>

extern uint64_t sum(uint64_t count, ...);

#endif