The following source

char c;

void bar_c (int x)
{
    c = x ? x/2 : c+1;
}

reveals two issues

== 1 == 

Compiled with

$ avr-gcc -S -Os

there is a call to libgcc's division routine. RTX costs of division has to be checked and costs of loading the constant added to the costs of division.

bar_c:
	sbiw r24,0
	breq .L2
	ldi r22,lo8(2)
	clr r23
	rcall __divmodhi4
	rjmp .L3
.L2:
	lds r22,c
	subi r22,lo8(-(1))
.L3:
	sts c,r22
	ret

== 1 == 

Compiled with

$ avr-gcc -S -O2

the code is happily jumping around to L7 and then back to L3.

bar_c:
	sbiw r24,0
	brne .L6
	lds r24,c
	subi r24,lo8(-(1))
	sts c,r24
	ret
.L6:
	sbrc r25,7
	rjmp .L7
.L3:
	asr r25
	ror r24
	sts c,r24
	ret
.L7:
	adiw r24,1
	rjmp .L3

Presumably, the cause is 
	#define BRANCH_COST 0
in avr.h, and the sequence could be instead:

	...
.L6:
	sbrc r25,7
	adiw r24,1
.L3:
	asr r25
	ror r24
	sts c,r24
	ret