Re: [avr-gcc-list] Question about code size

[Dave Hylands]

Hi Eric (and Erik and Paulo),

> Can you determine where that difference is? Like you did with the inlined
> code.

I wrote the following C function:

unsigned char getch(void);
void getNch(unigned char count)
{
   unsigned char i;
   for(i=0;i<count;i++)
       getch();
}

3.4.4 avr-gcc -Os -mshort-calls -mmcu=atmega168

00000000 <getNch>:
  0:   cf 93           push    r28
  2:   88 23           and     r24, r24
  4:   21 f0           breq    .+8             ; 0xe <getNch+0xe>
  6:   c8 2f           mov     r28, r24
  8:   00 d0           rcall   .+0             ; 0xa <getNch+0xa>
  a:   c1 50           subi    r28, 0x01       ; 1
  c:   e9 f7           brne    .-6             ; 0x8 <getNch+0x8>
  e:   cf 91           pop     r28
 10:   08 95           ret

4.2.0 produced:

00000000 <getNch>:
  0:   0f 93           push    r16
  2:   1f 93           push    r17
  4:   08 2f           mov     r16, r24
  6:   10 e0           ldi     r17, 0x00       ; 0
  8:   00 c0           rjmp    .+0             ; 0xa <getNch+0xa>
  a:   00 d0           rcall   .+0             ; 0xc <getNch+0xc>
  c:   1f 5f           subi    r17, 0xFF       ; 255
  e:   10 17           cp      r17, r16
 10:   01 f4           brne    .+0             ; 0x12 <getNch+0x12>
 12:   1f 91           pop     r17
 14:   0f 91           pop     r16
 16:   08 95           ret

I also did a comparison of the following optimization flags:

     -O0 -O1 -O2 -O3 -Os
3.4.4  60  26  18  18  18
4.2.0  60  26  26  26  24

The numbers are the size in bytes of the .text section for the
function above. It appears that the compiler is promoting the unsigned
char's to ints.

If the integer promotion is removed, then you'd loose, a push, a pop,
a load of zero and the cp instruction, resulting in a 2 byte smaller
function - that would be nice :)

Aside: I've just moved my development platform from Windows to linux.
Where would I go to get the latest patches for gcc/binutils/et al.?

I'm currently using Marc Bodmer's build_avr_toolchain.sh script which
is the latest version I've found so far. It's using gcc-4.2-20060819

--
Dave Hylands
Vancouver, BC, Canada
http://www.DaveHylands.com/