Re: [avr-gcc-list] gcc bug

[Troy]

Thanks to all four who came up with the "volatile" reply.

Now it seems very obvious. 

Thanks for being a great help.


Troy










> 
> Hello
> 
> Please try the following
> 
> struct smotor { uint16_t count; };
> volatile struct smotor motor;
> 
> This should generate the correct code. 
> Volatile indicates that the variable is changed by the interrupt function.
> It's not possible for gcc to guess that the variable is changed by another
> function when you don't call the function directly. That is the case with
> interrupt functions.
> 
> Cheers
> Rune
> 
> 
> > -----Original Message-----
> > From: address@hidden 
> > [mailto:address@hidden On Behalf Of Troy
> > Sent: Wednesday, June 23, 2004 8:38 PM
> > To: address@hidden
> > Subject: [avr-gcc-list] gcc bug
> > 
> > 
> > Hi,
> > 
> > I am writing on behalf of a friend who has a problem
> > with output from win-avr-gcc (I am writing on his behalf 
> > because of language issues).
> > 
> > The program has a global variable called "motor"
> > which is a struct containing a uint16_t variable
> > called "count."
> > 
> > struct smotor { uint16_t count; } motor;
> > 
> > main() {
> >     // code to set up a timer which sets up a 
> >     // signal handler for SIG_OVERFLOW0.
> >     ... 
> >     func();
> >     // never returns from func().
> > }
> > 
> > func() {
> >     motor.count = 10;
> >     while(motor.count > 0) {
> >             ;
> >     }
> > }
> > 
> > signal(SIG_OVERFLOW0) {
> >     if (motor.count) {
> >             motor.count--;
> >     }
> > }
> > 
> > 
> > The assembly code produced by gcc uses register R24 for "motor.count."
> > signal() stores R24 properly in data space, altering its 
> > value to 0, as it should.
> > func() has the problem. It doesn't read R24 from data space 
> > before each comparison, but instead treats it as a local 
> > variable. This causes the 
> > busy loop in func() to never exit.
> > 
> > Optimizer options used were the default values for the 
> > Makefile which came with the latest win-avr-gcc (downloaded 
> > one month ago).
> > 
> > If the code is altered to include further steps inside func()'s 
> > while loop, the generated assembly code is different and 
> > works properly. This is, however, not desirable.
> > 
> > It seems like an error that the R24 variable's value is not 
> > read from data space before each comparison, since it is a 
> > global variable.
> > 
> > 
> > 
> > 
> > Thank you.
> > 
> > 
> > 
> > Troy (on behalf of Timo)
> > 
> >  
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > _______________________________________________
> > avr-gcc-list mailing list
> > address@hidden 
> > http://www.avr1.org/mailman/listinfo/avr-gcc-list
> > 
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