What do I mean by "not every CPU allows to make unaligned 32-bit or 64-bit writes"? Let's test the code (as of commit eac67b6) on a Raspberry Pi 400:

  pi@rasppi400:~/memset_benchmark $ uname -a
  Linux rasppi400 5.10.63-v8+ #1459 SMP PREEMPT Wed Oct 6 16:42:49 BST 2021 aarch64 GNU/Linux
  
  pi@rasppi400:~/memset_benchmark $ ./bench_memset
  size, alignment, offset, libc, local
  0, 16, 0, 1237452, 834116, 1.483549,
  1, 16, 0, 1612697, 945325, 1.705971,
  2, 16, 0, 1779538, 945320, 1.882472,
  3, 16, 0, 1557081, 945324, 1.647140,
  4, 16, 0, 1779527, 889736, 2.000062,
  5, 16, 0, 1557103, 1000940, 1.555641,
  6, 16, 0, 1779551, 1000944, 1.777873,
  7, 16, 0, 1557111, 1000945, 1.555641,
  8, 16, 0, 1334654, 889723, 1.500078,
  Bus error
  
  pi@rasppi400:~/memset_benchmark $ gdb ./bench_memset
  [...]
  (gdb) run
  Starting program: /home/pi/memset_benchmark/bench_memset
  size, alignment, offset, libc, local
  0, 16, 0, 1557105, 722928, 2.153887,
  1, 16, 0, 1557103, 889797, 1.749953,
  2, 16, 0, 1557107, 889849, 1.749855,
  3, 16, 0, 1557108, 889759, 1.750033,
  4, 16, 0, 1557117, 889789, 1.749985,
  5, 16, 0, 1557110, 889745, 1.750063,
  6, 16, 0, 1557116, 889754, 1.750052,
  7, 16, 0, 1557110, 889758, 1.750038,
  8, 16, 0, 1557109, 889803, 1.749948,
  
  Program received signal SIGBUS, Bus error.
  
  small_memset (n=<optimized out>, c=<optimized out>, s=0x29690)
      at /home/pi/memset_benchmark/src/lib.c:33
  33          *((uint64_t *)last) = val8;