Re: [avr-gcc-list] possible compiler/optimizer problem (?)

[Radu Ciurlea]

You problem is you are using plain wrong C syntax. `value!=value' is an 
expression, not an assignment. It basically says `value is not equal to 
value'. This is not quite useful since it will always evaluate to false 
and do nothing else. However, if you write
`value = !value' you change `value' to true (1) if it was false (0) and 
change it to false if it was true. So the difference is pretty huge.
Cheers,

Radu Ciurlea
radu at ploiesti dot roedu dot net

Gary Douglas wrote:
> Hi all,
>
> Not sure if this is a bug, or if what I'm trying to do is just incorrect 
> C syntax.  Regardless, the compiler doesn't warn about the following:
>
> void led_blinky()
> {
>     static uint8_t value= 1;
>     value != value;//This is the problem line - always true   
>     if(value)
>     {
>         PORTD &= ~(1<<7);
>     }
>     else
>     {
>         PORTD |= (1<<7);
>     }
> }
>
> If I change it to this it works:
>
> void led_blinky()
> {
>     static uint8_t value= 1;
>     value = (!value); //This works...
>     if(value)
>     {
>         PORTD &= ~(1<<7);
>     }
>     else
>     {
>         PORTD |= (1<<7);
>     }
> }
>
> Are these not fundamentally the same?
>
> FYI: Using avr-gcc 3.4.3 on FC1 Linux.
> Configured with: ../../source/gcc-3.4.3/configure -v --target=avr 
> --disable-nls --prefix=/tools/avr --with-gnu-ld --with-gnu-as 
> --enable-languages=c,c++ --quiet --with-dwarf2
> Thread model: single
> gcc version 3.4.3
>
> Regards,
>
> Gary Douglas