Re: [avr-gcc-list] Add builtins in avr target.

[Wouter van Gulik]

Anatoly Sokolov schreef:
> Hi.
>
> >   I wish to add:
> ..
> >     3. builtin similarly to IAR '__delay_cycles';
> ..
>
> This unfinished patch add '__builtin_avr_delay_cycles(long delay)'  builtin to 
> the avr backend. The 'delay' parameter should be constant.
>
> If 'delay' is 1 or 2 then one or two 'nop' instructions is generated. 

For a 2 cycles delays an rjmp can be used. Saves an instruction!

> If  'delay' is from 3 to 756 then code:
>    ldi rX, (delay/3)
>   1:dec rX
>     brne 1b
> is generated. 'ldi' instruction can be removed by optimizer.
>
> For  'delay'  from 757 to 196605 loop is:  
>  1:sbiw Rx,1
>     brne 1b"
>
> For  'delay' from 196606 to 83886075 loop is:  
>  "1:subi %0,1
>      sbci %B0,0
>      sbci %C0,0
>      brne 1b"
>
> And for  'delay' from 83886076 to 0xFFFFFFFF loop is:
> "1:subi %0,1
>     sbci %B0,0
>     sbci %C0,0
>     sbci %D0,0
>     brne 1b"

That is a high registers usage. 4 register used just for burning cycles? 
On the other hand burning cycles this way will probably never be used in 
"real" code.

> Adding '__builtin_avr_delay_cycles' builtin will allow to remove restrictions 
> on max possible values of parameter for '_delay_us' macro and reduce code size 
> for long delay of  '_delay_ms' macro.
>
> Also it will simplify porting code from IAR C, if define  '__delay_cycles' as 
> '__builtin_avr_delay_cycles'.
>
> As you consider, this builtin will be useful?

Yes it is useful, especially the really short ones.

Wouter