Re: [avr-gcc-list] Problem with delay loop

[Paulo Marques]

David Brown wrote:
> [...]
> it could perhaps reason 
> that since there is no way for anything outside the program to find out 
> where the local volatile variable resides, there is no way for anything 
> else to influence or use the variable, and therefore the "volatile" 
> qualifier can be ignored.

This sentence makes no sense at all. The "volatile" is precisely to warn 
the compiler that it should not "reason" anything about this variable.

If you give a volatile qualifier to a local variable, it's obvious that 
you want it to behave differently from regular local variables, so I 
think gcc is doing the best it can, from the weakly defined volatile 
semantics.

This makes as much sense as saying that any volatile is futile, since 
you can compile a program with "-combine -whole-program" and so the 
compiler can always "reason" that any variable will not be accessed 
outside of its control.

Note that I'm not disputing that the compiler could in theory use a 
register instead. I personally don't think that would be a good idea, 
but it might be allowed by the specs. What I'm disputing is that the 
compiler _can not_ ignore the volatile and optimize the loop away 
entirely. That would be a compiler bug, for sure.

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

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