Remember that -Os is much much more aggressive in GCC than it is in LLVM, with LLVM you need to use -Oz to get the same result.

GCC has historically been a bit of a pig about -Os, my favorite example is on x86 where it will emit a runtime divide for a division by a compile time constant power of two because it saves a byte of text!

  int fn(int n)
  {
    return n / 8;
  }

..becomes:

  0000000000000000 <fn>:
   0:   89 f8                   mov    %edi,%eax
   2:   b9 08 00 00 00          mov    $0x8,%ecx
   7:   99                      cltd
   8:   f7 f9                   idiv   %ecx
   a:   c3                      ret

..versus:

  0000000000000000 <fn>:
   0:   8d 47 07                lea    0x7(%rdi),%eax
   3:   85 ff                   test   %edi,%edi
   5:   0f 49 c7                cmovns %edi,%eax
   8:   c1 f8 03                sar    $0x3,%eax
   b:   c3                      ret