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

usually compiler assumes that a program is single threaded, therefore it has complete knowledge of what's happening with variable values. a smart compiler can then prove that the program can be transformed into another program with equivalent semantics but better performance. for example

x = y+y+y+y+y;

can be transformed to

x = y*5;

however, if a variable can be changed outside the thread, compiler doesn't have a complete knowledge of what's going on by simply examining this piece of code. it can no longer make optimizations like above. (edit: it probably can in this case; we need more sophisticated examples)

by default, for performance optimization, single thread access is assumed. this assumption is usually true. unless programmer explicitly instruct otherwise with the volatile keyword.