What does the compiler do in assembly when optimizing code? ie -O2 flag

Question

So when you add an optimization flag when compiling your C++, it runs faster, but how does this work? Could someone explain what really goes on in the assembly?

Answer 1

Most of the optimization happens in the compiler's intermediate representation before the assembly is generated. You should definitely check out Agner Fog's Software optimization resources. Chapter 8 of the 1st manual describes optimizations performed by the compiler with examples.