RE: [avr-gcc-list] Gcc bug??

[Brian Cuthie]

I'll do some tests and let you know. I know I did forget to add volatile to
at least one variable that I'm using in my interrupt handler, and I fixed
this already. Since I don't recall which one I'll need to explicitly test
this theory. I'll be a little surprised (although happy) if this turns out
the be the cause. It's not quite the code I'd expect the compiler to emit,
even without the volatile keyword. I mean, 0 is still 0.

Thanks

-brian  

> -----Original Message-----
> From: Peter Hierholzer [mailto:address@hidden 
> Sent: Wednesday, March 03, 2004 5:52 AM
> To: address@hidden
> Cc: address@hidden
> Subject: AW: [avr-gcc-list] Gcc bug??
> 
> > I'm having some problems with an app that uses AvrX and gcc 
> (v. 3.3.1) 
> > on an ATmega16. In the interrupt routine for the TWI  I 
> write a static 
> > with a code that indicates its current state. In an effort 
> to debug a 
> > different problem I compiled the code without any optimization.
> > 
> > While I did discover the original problem I had been hunting, I 
> > started to see some very unreliable behavior with the TWI. 
> After some 
> > investigation I determined that the state variable 
> (mentioned above) 
> > was getting written with an unreasonable value (not a valid state). 
> > The correct state value that should have been written into the 
> > variable is "0". Further experiments revealed that memory 
> corruption 
> > (at least directly into the variable's memory location) was not the 
> > problem.
> > 
> > What I then determined is that the "0" register (r1) contained the 
> > value 8!
> > This, not coincidentally (and looking at the disassembled code) is 
> > exactly the value that's being incorrectly written into the 
> static state variable.
> > Interestingly, turning on any level of optimization seems 
> to fix it, 
> > at least as observed by the TWI port handler not hanging.
> > 
> > I was wondering if anyone else has seen anything like this, 
> or has any 
> > advice or suggestions.
> > 
> > Thanks
> > 
> > -brian
> 
> 
> Hi Brian,
> 
> Did you declare the state variable as volatile?
> If you access a variable in your normal program AND in the 
> interrupt routine you must declare it as volatile. Otherwise 
> the compiler is not aware that the variable is modified in 
> the interrupt routine and generates wrong code.
> 
> Peter
> 
> 


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