How to compute 2⁶⁴/n in C?

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I’ll use uint64_t here (which needs the <stdint.h> include) so as not to require your assumption about the size of unsigned long.

phuclv’s idea of using -n is clever, but can be made much simpler. As unsigned 64-bit integers, we have -n = 264-n, then (-n)/n = 264/n – 1, and we can simply add back the 1.

uint64_t divide_two_to_the_64(uint64_t n) {
  return (-n)/n + 1;
}

The generated code is just what you would expect (gcc 8.3 on x86-64 via godbolt):

    mov     rax, rdi
    xor     edx, edx
    neg     rax
    div     rdi
    add     rax, 1
    ret

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