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 o

Answer 1

Remember that the "as if rule" means that the compiler can, and should, do whatever it wants, as long as the behaviour as seen from outside the program as a whole is the same. In particular, while a variable conceptually names an area in memory, there is no reason why it actually should be in memory.

It could be in a register:

Its value could be calculated away, e.g. in:

int x = 2;
int y = x + 7;
return y + 1;

Need not have an x and y at all, but could just be replaced with:

return 10;

And another example, is that any code that doesn't affect state from the outside could be removed entirely. E.g. if you zeroise sensitive data, the compiler can see this as a wasted exercise ("why are you writing to what won't be read?") and remove it. volatile can be used to stop that happening.

volatile can be thought of as meaning "the state of this variable must be considered part of the outwardly visible state, and not messed with". Optimisations that would use it other than literally following the source code are not allowed.

(A note C#. A lot I've seen of late on volatile suggests that people are reading about C++ volatile and applying it to C#, and reading about it in C# and applying it to C++. Really though, volatile behaves so differently between the two as to not be useful to consider them related).