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

[Erik Walthinsen]

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.

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......