When Watching a Variable, the Debugger says Optimized away

Most compilers today are what is known as an optimizing compiler. This means that the compiler will employ a number of tricks to reduce the size of your program or speed it up.
Note: This behavior is usually controlled by the -On switches.
The cause of this error is usually trying to debug parts of the code that does nothing. Trying to watch the variable a in the following example may cause this behavior.
int main() {
   int a = 0;
   while (a < 42) {
       a += 2;

The reason for a to be optimized away is obvious as the incrementation of a does not affect any other part of our code. This example of a busy wait loop is a prime example of unexpected behavior if you are unaware of this fact.

To fix this, either lower the optimization level used during compilation or preferably declare a as volatile. Other situations where a variable should be declared volatile is if some variable is shared between the code and an ISR1.

For a thorough walkthrough of this issue, have a look at Cliff Lawsons excellent tutorial on this issue.


Interrupt Service Routine