Re: [avr-gcc-list] A Question Of Optmization

[Graham Davies]

David Brown wrote:
> In fact, the compiler will *always* do *exactly*
> what your program code asks ...

Although the general thrust of your argument is correct, this statement may 
not be true, depending on how the reader interprets "what your program code 
asks".  The language definition requires the compiler to produce code which 
*has the same final effect* as what you have written, but in the interests 
of optimization, it is permitted to deviate from doing each and every thing 
you have asked it to do.  It is thinking that the compiler must do exactly 
what you have asked that leads people to omit the volatile keyword in the 
first place.

The volatile keyword tells the compiler that things happen to or are caused 
by a variable that would otherwise be unknown to the compiler when it 
translates a unit of code.  This is an indication that the compiler cannot 
evaluate the final effect of access to that variable during the translation. 
Therefore, it translates each access exactly as it is written.  In fact, 
then, you can only be sure that "the compiler will *always* do *exactly* 
what your program code asks" if your variables are volatile.

You are right that fiddling with optimization flags is not required to have 
the same effect and is not the way to fix the problem.  I think the 
important thing here is to read the manual for your compiler as this won't 
be specified by the language standard.

Graham.