Bignum addition in inline assembly

add.cpp

// See: https://stackoverflow.com/a/48812277/82294

template <int N>
void add(unsigned long long *dest, unsigned long long *src) {
  __asm__(
      "movq (%1), %%rax"
      "\n\taddq %%rax, (%0)"
      "\n.local add_offset"
      "\n.set add_offset,0"
      "\n.rept %P2"
      "\n.set add_offset,add_offset+8"
      "\n\tmovq add_offset(%1), %%rax"
      "\n\tadcq %%rax, add_offset(%0)"
      "\n.endr"
      :
      : "r"(dest), "r"(src), "n"(N-1)
      : "cc", "memory", "rax");
}   

#include <cstdio>

void print(const char *name, unsigned long long *n) {
  std::fputs(name, stdout);
  for (int i = 0; i < 4; i++) {
    std::printf(" %016llx", n[i]);
  }
  std::fputc('\n', stdout);
}

int main() {
  unsigned long long x[4] = {
    0x0123456789abcdefull,
    0x8000000000000000ull,
    0x9999000000000000ull,
    0x9999000000000000ull,
  };
  unsigned long long y[4] = {
    0x3333333333333333ull,
    0x8123456789abcdefull,
    0x9999000000000000ull,
    0x9999000000000000ull,
  };
  print("x:     ", x);
  print("y:     ", y);
  add<4>(x, y);
  print("x + y: ", x);
  return 0;
}

Googlers are not in the same programming category 🥇 ^_^