Re: [avr-gcc-list] efficiency of assigning bits

[Jesper Hansen]

> Actually, it's more efficient to code it as two separate statements, 
> *if* the register is one of the first 32 in the chip's register space. 
> Two statements will result in 2 sbi or cbi instructions, while a single 
> statement will result in a read-modify-write sequence of 3 instructions.
>
> If the register is outside that range, then yes, it is more efficient to 
> do it as a single statement, because sbi/cbi don't work and you're back 
> to a read-modify-write sequence for each of the two statements, 
> resulting in 6 instructions.
>
> I went around on this several times while trying to shave one 
> instruction at a time on a project that's filled the 1024-byte code 
> space  (512 instructions) of an ATtiny15 to 99+%.  Waiting for ATtiny45 
> samples......

An often overlooked solution, especially with ports that changes in 
a determined way, is to read the contents of the port at an early stage,
then use that as a base for manipulation. 

 register uint8_t pval = PORTA;
- 
- 
PORTA = pval | 0x23;
- 
PORTA = pval;

e.t.c., you get the idea
This will result in one or two cycle (and instuction) sequnces.
Can save quite a few bytes in the right code.

/Jesper