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

[E. Weddington]

Erik Walthinsen wrote:

> E. Weddington wrote:
> > If you don't care, then you're
>
> > free to have them as two statements, but it would also be inefficient 
> > in terms of code space. Your choice.
>
>
> 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.
Oh, sure! *Very* good point.

> 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......
And for everyone else: this is why it pays to understand what your 
compiler is doing; take a look at the resulting assembly of your C code 
and never assume anything.

Eric