Free bits in pointers (tested on amazon graviton processors)

test.c

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

int main() {
  int x = 10;
  int* y = &x;
  printf("Hey! %p (size=%zu)\n", y, sizeof(y));

  uint64_t mask = 0;
  for (int j = 63; j > 55; j--) {
    mask |= 1ull << j;
  }
  
  int* newPtr = (uint64_t)y ^ mask;
  printf("Mask => %llu\n", mask);
  printf("new ptr => %p (%d)\n", newPtr, *newPtr);
}

Well, it looks like Amazon's arm64 doesn't care about most significant byte in address i.e. you can fill it with your own information