Re: [avr-gcc-list] Optimiser bloats code

[Wouter van Gulik]

Paulo Marques schreef:
>  Not really a better idea for 3 bits, but it would be for 4:
>
> prog_uint8_t inv_table[8]={0,4,2,6,1,5,3,7};
>
> unsigned char inv_test(void)
> {
>   return pgm_read_byte(&inv_table[PORTB & 0x3]);
> }

Ah yes, of course a table!

> The output from gcc 4.2.0:
>
> byte inv_test(void)
> {
>         return pgm_read_byte(&inv_table[PORTB & 0x3]);
>   96:   e8 b3           in      r30, 0x18       ; 24
>   98:   ff 27           eor     r31, r31
>   9a:   e3 70           andi    r30, 0x03       ; 3
>   9c:   f0 70           andi    r31, 0x00       ; 0
>   9e:   ec 5a           subi    r30, 0xAC       ; 172
>   a0:   ff 4f           sbci    r31, 0xFF       ; 255
>   a2:   e4 91           lpm     r30, Z
> }
>   a4:   8e 2f           mov     r24, r30
>   a6:   99 27           eor     r25, r25
>   a8:   08 95           ret
>
> If not for the redundant "andi r31, 0x00" (when r31 has just been zeroed
> by the "eor r31,r31") it would give the same number of instructions as
> your code.
>
> The nice thing about this approach is that it works the same for 4 or
> more bits (up to 8).

Yes but the table would grow large on 8 bits inversion :D. And I'm more 
afraid of running out of space, then running out of time.

Thanks for the help,

Wouter