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

[Anatoly Sokolov]

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. 

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"

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?

Anatoly.