Re: [avr-gcc-list] Avr-gcc Produces Incorrect Code with -Os

[Paulo Marques]

Thomas D. Dean wrote:
> > From 'info gcc',
>  ...
>
>    You can prevent an `asm' instruction from being deleted, moved
> significantly, or combined, by writing the keyword `volatile' after the
> `asm'. 

This only means that they are not reordered with respect to other 
"volatile asm" statements. Note the "moved significantly" part: if the 
other instruction is not "significant", then it might be moved over it.

Yes, I do agree that the outcome might be somewhat surprising, though.

> Changing the code fragment to
>   ...
>   dtostrf(cos_rad,6,3,&line4[14]);
>   line3[20] = ' ';
>   line4[20] = ' ';
>   asm volatile("sbi 0x1b,4" : :);	  
>   atan_rad = atan2(cos_rad,sin_rad);  
>   asm volatile("cbi 0x1b,4" : :);	  
>   dtostrf(atan_rad,6,3,&line4[26]);	
>   ...
>
> does not alter the result.

The PORTx accessors are already volatile, so this doesn't change much.

The main problem here is probably that the atan2 function is declared to 
be __attribute__((const)), which gives teh compiler more leeway to move 
it around...

--
Paulo Marques
Software Development Department - Grupo PIE, S.A.
Phone: +351 252 290600, Fax: +351 252 290601
Web: www.grupopie.com

"Power corrupts. Absolute power is kind of neat." John Lehman