I have noticed this on SH, maybe it also applies to other targets (checked on 4.9 r208241).

The following simple loop (simple strlen implementation):

unsigned int test (const char* s0)
{
  const char* s1 = s0;

  while (*s1)
    s1++;

  return s1 - s0;
}

With -O2 -m4 gets compiled to:

        mov.b   @r4,r1
        tst     r1,r1
        bt/s    .L4
        mov     r4,r1
        add     #1,r1
	.align 2
.L3:
        mov     r1,r0
        mov.b   @r0,r2
        tst     r2,r2
        bf/s    .L3
        add     #1,r1
        rts
        sub     r4,r0
        .align 1
.L4:
        rts
        mov     #0,r0


With -Os -m4 it is basically just the inner loop:
        mov	r4,r1
.L2:
        mov     r1,r0
        mov.b   @r0,r2
        tst     r2,r2
        bf/s    .L2
        add     #1,r1
        rts
        sub     r4,r0


The additional loop test in the loop header in the -O2 version seems a bit pointless.  If the loop exists at the first iteration, it simply falls through.  The additional test and jump around the loop doesn't gain anything in this case but just increases code size unnecessarily.