AVR-Chat

Having a "the complete reference C++" by Herbert Schildt, third edition, 
I tackled his definition of volatile. He gives it 2 paragraphs. His 
opening statement is: "The modifier volatile tells the compiler that a 
variable's value may be changed in ways not explicitly specified by the 
program."

The point being "optimization" sometimes needs to be curbed, or not 
performed, in order to keep the compiler from assuming the variable 
hasn't changed. Volatile modifies the variable and keeps the compiler 
from making optimization assumptions. As your example illustrates.

My friend says "If you are going to modify a global variable in your ISR 
**or you are going to modify it in your main loop and then use it in 
your ISR, you need to declare that variable as volatile**, to tell the 
compiler to not use the optimized value and assume that it hasn't changed."

So I understand him to be saying Volatile tells the compiler 1) not to 
use an optimized value 2) not to assume that the value hasn't changed

rep

On 4/19/2012 9:31 AM, Gregory N wrote:
>
> > Do you have a suggestion on how we can achieve clarity?
>
> I haven't been following every message in this list, so pardon me if I 
> am repeating something.
>
> It seems to me that the lack of clarity is because people are 
> attempting to define the volatile storage class based on when and how 
> to use it, not what it is.
>
> If you define volatile by what it is, then its usage follows rather 
> naturally. The storage class volatile simply instructs the compile to 
> avoid optimization of any operations involving the thing that is 
> marked volatile.
>
> The name volatile is itself misleading because it implies one usage of 
> the storage class rather that what it is. nooptimize would be a more 
> meaningful name.
>
> [This, by the way, is a classic job interview C question. "What does 
> the volatile storage class mean?" If you give any anser other than the 
> above answer, you would probably fail the interview question.]
>
> Now how does that tie into the rest of this discussion. Well, the 
> optimizer does things that you may not want it to do. For example, if 
> you have a load from a symbolic name within a loop but never modify 
> the value within the loop, the optimizer is free to move the load 
> outside of the loop. This would be catastrophic if the thing you are 
> loading is a register or is a value that could be modified by an 
> interrupt or another thread.
>
> volatile says don't optimize... don't move the load outside the loop.
>
> Basically, anytime the compiler thinks it knows the value of 
> something, it can suppress re-loading it as part of the optimization.
>
> Another case is where the optimizer notices that a variable read but 
> is never used, in that case the optimizer says, "the variable is never 
> used I can eliminate it". But if you are reading a machine register 
> you probably want to read the value even though you don't use the 
> value that you read.
>
> volatile says don't optimize... read the value even though you don't 
> use it.
>
> Here is a complete unrelated use of volatile. Suppose I have a timing 
> loop like this:
>
> void wait_a_bit(void)
> {
> int i;
> for (i = 0; i < 1000; i++)
> ;
> }
>
> What can go wrong with that? Well, first if you are doing really 
> aggressive optimization, the compiler might decide that function does 
> nothing and eliminate it completely.
>
> But suppose it doesn't do that. That is still a problem... the problem 
> is that the variable i will be a memory location if optimization is 
> OFF but will be a register value if the optimization is ON. So if I 
> calibrated my timing loop with optimization OFF, it will be broken 
> when optimization is turned on. The fix?
>
> volatile says don't optimize... don't convert the variable i to a 
> register variable.
>
> void wait_a_bit(void)
> {
> volatile int i;
> for (i = 0; i < 1000; i++)
> ;
> }
>
> will behave approximately the same whether optimization is enabled or not.
>
> volatile can help in many different ways and is easy to understand 
> when to use it if you remember that all it does is turn optimization off.
>
> Greg
>
>