What kinds of optimizations does 'volatile' prevent in C++?

Question

I was looking up the keyword volatile and what it's for, and the answer I got was pretty much:

It's used to prevent the compiler from optimizing away code.

There were some examples, such as when polling memory-mapped hardware: without volatile the polling loop would be removed as the compiler might recognize that the condition value is never changed. But since there only were one example or maybe two, it got me thinking: Are there other situations where we need to use volatile in terms of avoiding unwanted optimization? Are condition variables the only place where volatile is needed?

I imagine that optimization is compiler-specific and therefore is not specified in the C++ specification. Does that mean we have to go by gut feeling, saying Hm, I suspect my compiler will do away with this if I don't declare that variable as volatile or are there any clear rules to go by?

Answer 1

Basically, volatile announces that a value might change behind your program's back. That prevents compilers from caching the value (in a CPU register) and from optimizing away accesses to that value when they seem unnecessary from the POV of your program.

What should trigger usage of volatile is when a value changes despite the fact that your program hasn't written to it, and when no other memory barriers (like mutexes as used for multi-threaded programs) are present.